Allow checking to continue after 'impl as' outside class (#4937)

Currently it returns false which just ends typechecking. Instead handle
the error state later and avoid firing overlapping diagnostics in
'extend impl as'.

---------

Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>

toolchain/check/handle_impl.cpp

@@ -99,7 +99,6 @@ auto HandleParseNode(Context& context, Parse::DefaultSelfImplAsId node_id)
                       "`impl as` can only be used in a class");
     context.emitter().Emit(node_id, ImplAsOutsideClass);
     self_type_id = SemIR::ErrorInst::SingletonTypeId;
-    return false;
   }
 
   // Build the implicit access to the enclosing `Self`.
@@ -141,14 +140,14 @@ static auto ExtendImpl(Context& context, Parse::NodeId extend_node,
     DiagnoseExtendImplOutsideClass(context, node_id);
     return false;
   }
-  auto& parent_scope = context.name_scopes().Get(parent_scope_id);
-
   // TODO: This is also valid in a mixin.
   if (!TryAsClassScope(context, parent_scope_id)) {
     DiagnoseExtendImplOutsideClass(context, node_id);
     return false;
   }
 
+  auto& parent_scope = context.name_scopes().Get(parent_scope_id);
+
   if (params_node.has_value()) {
     CARBON_DIAGNOSTIC(ExtendImplForall, Error,
                       "cannot `extend` a parameterized `impl`");
@@ -387,7 +386,8 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
   ReplaceInstBeforeConstantUse(context, impl_decl_id, impl_decl);
 
   // For an `extend impl` declaration, mark the impl as extending this `impl`.
-  if (introducer.modifier_set.HasAnyOf(KeywordModifierSet::Extend)) {
+  if (self_type_id != SemIR::ErrorInst::SingletonTypeId &&
+      introducer.modifier_set.HasAnyOf(KeywordModifierSet::Extend)) {
     auto extend_node = introducer.modifier_node_id(ModifierOrder::Decl);
     if (impl_info.generic_id.has_value()) {
       SemIR::TypeId type_id = context.insts().Get(constraint_inst_id).type_id();

toolchain/check/impl.cpp

@@ -86,6 +86,11 @@ auto ImplWitnessForDeclaration(Context& context, const SemIR::Impl& impl)
     -> SemIR::InstId {
   CARBON_CHECK(!impl.has_definition_started());
 
+  auto self_type_id = context.types().GetTypeIdForTypeInstId(impl.self_id);
+  if (self_type_id == SemIR::ErrorInst::SingletonTypeId) {
+    // When 'impl as' is invalid, the self type is an error.
+    return SemIR::ErrorInst::SingletonInstId;
+  }
   auto facet_type_id =
       context.types().GetTypeIdForTypeInstId(impl.constraint_id);
   if (facet_type_id == SemIR::ErrorInst::SingletonTypeId) {

toolchain/check/impl_lookup.cpp

@@ -145,14 +145,11 @@ auto LookupImplWitness(Context& context, SemIR::LocId loc_id,
     // constraint, the unique `interface` declaration.)
 
     auto specific_id = SemIR::SpecificId::None;
-    // This check comes first to avoid deduction when the witness_id is missing.
-    // We use an error value to indicate an error during creation of the impl,
-    // such as a recursive impl which will cause deduction to recurse
-    // infinitely.
-    if (!impl.witness_id.has_value() ||
-        impl.witness_id == SemIR::ErrorInst::SingletonInstId) {
-      // TODO: Diagnose if the impl isn't defined yet?
-      return SemIR::InstId::None;
+    // This check comes first to avoid deduction with an invalid impl. We use an
+    // error value to indicate an error during creation of the impl, such as a
+    // recursive impl which will cause deduction to recurse infinitely.
+    if (impl.witness_id == SemIR::ErrorInst::SingletonInstId) {
+      continue;
     }
     if (impl.generic_id.has_value()) {
       specific_id = DeduceImplArguments(context, loc_id, impl, type_const_id,
@@ -175,6 +172,10 @@ auto LookupImplWitness(Context& context, SemIR::LocId loc_id,
       // the constraint's interfaces.
       continue;
     }
+    if (!impl.witness_id.has_value()) {
+      // TODO: Diagnose if the impl isn't defined yet?
+      return SemIR::InstId::None;
+    }
     LoadImportRef(context, impl.witness_id);
     if (specific_id.has_value()) {
       // We need a definition of the specific `impl` so we can access its

toolchain/check/testdata/function/declaration/no_prelude/name_poisoning.carbon

@@ -301,24 +301,6 @@ class N.X {
   }
 }
 
-// --- fail_using_poisoned_name_in_impl_outside_class.carbon
-
-library "[[@TEST_NAME]]";
-
-interface A {
-  fn B();
-}
-class X {
-  extend impl as A {
-    fn F() { return; }
-    // CHECK:STDERR: fail_using_poisoned_name_in_impl_outside_class.carbon:[[@LINE+4]]:10: error: `impl as` can only be used in a class [ImplAsOutsideClass]
-    // CHECK:STDERR:     impl as B {}
-    // CHECK:STDERR:          ^~
-    // CHECK:STDERR:
-    impl as B {}
-  }
-}
-
 // --- fail_poison_when_lookup_fails.carbon
 
 library "[[@TEST_NAME]]";
@@ -1200,50 +1182,6 @@ fn F() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F1(%x.param_patt: %C.type);
 // CHECK:STDOUT:
-// CHECK:STDOUT: --- fail_using_poisoned_name_in_impl_outside_class.carbon
-// CHECK:STDOUT:
-// CHECK:STDOUT: constants {
-// CHECK:STDOUT:   %A.type: type = facet_type <@A> [concrete]
-// CHECK:STDOUT:   %Self: %A.type = bind_symbolic_name Self, 0 [symbolic]
-// CHECK:STDOUT:   %B.type: type = fn_type @B [concrete]
-// CHECK:STDOUT:   %B: %B.type = struct_value () [concrete]
-// CHECK:STDOUT:   %A.assoc_type: type = assoc_entity_type %A.type [concrete]
-// CHECK:STDOUT:   %assoc0: %A.assoc_type = assoc_entity element0, @A.%B.decl [concrete]
-// CHECK:STDOUT:   %X: type = class_type @X [concrete]
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: file {}
-// CHECK:STDOUT:
-// CHECK:STDOUT: interface @A {
-// CHECK:STDOUT:   %Self: %A.type = bind_symbolic_name Self, 0 [symbolic = constants.%Self]
-// CHECK:STDOUT:   %B.decl: %B.type = fn_decl @B [concrete = constants.%B] {} {}
-// CHECK:STDOUT:   %assoc0: %A.assoc_type = assoc_entity element0, %B.decl [concrete = constants.%assoc0]
-// CHECK:STDOUT:
-// CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .Self = %Self
-// CHECK:STDOUT:   .B = %assoc0
-// CHECK:STDOUT:   witness = (%B.decl)
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: impl @impl: <unexpected>.inst26.loc8_15 as <unexpected>.inst27.loc8_18;
-// CHECK:STDOUT:
-// CHECK:STDOUT: class @X {
-// CHECK:STDOUT:   complete_type_witness = invalid
-// CHECK:STDOUT:
-// CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .Self = constants.%X
-// CHECK:STDOUT:   .A = <poisoned>
-// CHECK:STDOUT:   extend <unexpected>.inst27.loc8_18
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: generic fn @B(@A.%Self: %A.type) {
-// CHECK:STDOUT:   fn();
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: fn @F();
-// CHECK:STDOUT:
-// CHECK:STDOUT: specific @B(constants.%Self) {}
-// CHECK:STDOUT:
 // CHECK:STDOUT: --- fail_poison_when_lookup_fails.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {

toolchain/check/testdata/impl/fail_impl_as_scope.carbon

@@ -47,17 +47,37 @@ fn Function() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %Simple.type: type = facet_type <@Simple> [concrete]
 // CHECK:STDOUT:   %Self: %Simple.type = bind_symbolic_name Self, 0 [symbolic]
-// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
-// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
+// CHECK:STDOUT:   %F.type.e2e: type = fn_type @F.1 [concrete]
+// CHECK:STDOUT:   %F.df8: %F.type.e2e = struct_value () [concrete]
 // CHECK:STDOUT:   %Simple.assoc_type: type = assoc_entity_type %Simple.type [concrete]
 // CHECK:STDOUT:   %assoc0: %Simple.assoc_type = assoc_entity element0, @Simple.%F.decl [concrete]
+// CHECK:STDOUT:   %F.type.fe5: type = fn_type @F.2 [concrete]
+// CHECK:STDOUT:   %F.32d: %F.type.fe5 = struct_value () [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: file {}
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Core = imports.%Core
+// CHECK:STDOUT:     .Simple = %Simple.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT:   %Simple.decl: type = interface_decl @Simple [concrete = constants.%Simple.type] {} {}
+// CHECK:STDOUT:   impl_decl @impl [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, <error> [concrete = <error>]
+// CHECK:STDOUT:     %Simple.ref: type = name_ref Simple, file.%Simple.decl [concrete = constants.%Simple.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Simple {
 // CHECK:STDOUT:   %Self: %Simple.type = bind_symbolic_name Self, 0 [symbolic = constants.%Self]
-// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {} {}
+// CHECK:STDOUT:   %F.decl: %F.type.e2e = fn_decl @F.1 [concrete = constants.%F.df8] {} {}
 // CHECK:STDOUT:   %assoc0: %Simple.assoc_type = assoc_entity element0, %F.decl [concrete = constants.%assoc0]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
@@ -66,28 +86,61 @@ fn Function() {
 // CHECK:STDOUT:   witness = (%F.decl)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: generic fn @F(@Simple.%Self: %Simple.type) {
+// CHECK:STDOUT: impl @impl: %Self.ref as %Simple.ref {
+// CHECK:STDOUT:   %F.decl: %F.type.fe5 = fn_decl @F.2 [concrete = constants.%F.32d] {} {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .F = %F.decl
+// CHECK:STDOUT:   witness = <error>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @F.1(@Simple.%Self: %Simple.type) {
 // CHECK:STDOUT:   fn();
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: specific @F(constants.%Self) {}
+// CHECK:STDOUT: fn @F.2() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @F.1(constants.%Self) {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: --- fail_function.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %Simple.type: type = facet_type <@Simple> [concrete]
 // CHECK:STDOUT:   %Self: %Simple.type = bind_symbolic_name Self, 0 [symbolic]
-// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
-// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
+// CHECK:STDOUT:   %F.type.e2e: type = fn_type @F.1 [concrete]
+// CHECK:STDOUT:   %F.df8: %F.type.e2e = struct_value () [concrete]
 // CHECK:STDOUT:   %Simple.assoc_type: type = assoc_entity_type %Simple.type [concrete]
 // CHECK:STDOUT:   %assoc0: %Simple.assoc_type = assoc_entity element0, @Simple.%F.decl [concrete]
+// CHECK:STDOUT:   %Function.type: type = fn_type @Function [concrete]
+// CHECK:STDOUT:   %Function: %Function.type = struct_value () [concrete]
+// CHECK:STDOUT:   %F.type.bc7: type = fn_type @F.2 [concrete]
+// CHECK:STDOUT:   %F.0a2: %F.type.bc7 = struct_value () [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: file {}
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Core = imports.%Core
+// CHECK:STDOUT:     .Simple = %Simple.decl
+// CHECK:STDOUT:     .Function = %Function.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT:   %Simple.decl: type = interface_decl @Simple [concrete = constants.%Simple.type] {} {}
+// CHECK:STDOUT:   %Function.decl: %Function.type = fn_decl @Function [concrete = constants.%Function] {} {}
+// CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Simple {
 // CHECK:STDOUT:   %Self: %Simple.type = bind_symbolic_name Self, 0 [symbolic = constants.%Self]
-// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {} {}
+// CHECK:STDOUT:   %F.decl: %F.type.e2e = fn_decl @F.1 [concrete = constants.%F.df8] {} {}
 // CHECK:STDOUT:   %assoc0: %Simple.assoc_type = assoc_entity element0, %F.decl [concrete = constants.%assoc0]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
@@ -96,11 +149,31 @@ fn Function() {
 // CHECK:STDOUT:   witness = (%F.decl)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: generic fn @F(@Simple.%Self: %Simple.type) {
+// CHECK:STDOUT: impl @impl: %Self.ref as %Simple.ref {
+// CHECK:STDOUT:   %F.decl: %F.type.bc7 = fn_decl @F.2 [concrete = constants.%F.0a2] {} {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .F = %F.decl
+// CHECK:STDOUT:   witness = <error>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @F.1(@Simple.%Self: %Simple.type) {
 // CHECK:STDOUT:   fn();
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @Function();
+// CHECK:STDOUT: fn @Function() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   impl_decl @impl [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, <error> [concrete = <error>]
+// CHECK:STDOUT:     %Simple.ref: type = name_ref Simple, file.%Simple.decl [concrete = constants.%Simple.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F.2() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: specific @F(constants.%Self) {}
+// CHECK:STDOUT: specific @F.1(constants.%Self) {}
 // CHECK:STDOUT:

toolchain/check/testdata/impl/no_prelude/fail_extend_impl_scope.carbon

@@ -36,24 +36,20 @@ fn F() {
 library "[[@TEST_NAME]]";
 
 interface Z {
-  fn Zero() -> Self;
+  fn Zero();
 
   // CHECK:STDERR: fail_extend_impl_self_interface.carbon:[[@LINE+4]]:15: error: `impl as` can only be used in a class [ImplAsOutsideClass]
   // CHECK:STDERR:   extend impl as Z {
   // CHECK:STDERR:               ^~
   // CHECK:STDERR:
   extend impl as Z {
-    fn Zero() -> Self {
-      return {};
-    }
+    fn Zero() {}
   }
 }
 
 class Point {
   extend impl as Z {
-    fn Zero() -> Self {
-      return {};
-    }
+    fn Zero() {}
   }
 }
 
@@ -148,27 +144,136 @@ fn F() {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %Z.type: type = facet_type <@Z> [concrete]
 // CHECK:STDOUT:   %Self: %Z.type = bind_symbolic_name Self, 0 [symbolic]
-// CHECK:STDOUT:   %Self.as_type: type = facet_access_type %Self [symbolic]
+// CHECK:STDOUT:   %Zero.type.822: type = fn_type @Zero.1 [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %Zero.d14: %Zero.type.822 = struct_value () [concrete]
+// CHECK:STDOUT:   %Z.assoc_type: type = assoc_entity_type %Z.type [concrete]
+// CHECK:STDOUT:   %assoc0: %Z.assoc_type = assoc_entity element0, @Z.%Zero.decl [concrete]
+// CHECK:STDOUT:   %Zero.type.db4: type = fn_type @Zero.2, @impl.1(%Self) [symbolic]
+// CHECK:STDOUT:   %Zero.8fb: %Zero.type.db4 = struct_value () [symbolic]
+// CHECK:STDOUT:   %Point: type = class_type @Point [concrete]
+// CHECK:STDOUT:   %impl_witness: <witness> = impl_witness (@impl.2.%Zero.decl) [concrete]
+// CHECK:STDOUT:   %Zero.type.e33: type = fn_type @Zero.3 [concrete]
+// CHECK:STDOUT:   %Zero.dec: %Zero.type.e33 = struct_value () [concrete]
+// CHECK:STDOUT:   %Z.facet: %Z.type = facet_value %Point, %impl_witness [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete]
+// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
+// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Point.val: %Point = struct_value () [concrete]
+// CHECK:STDOUT:   %.c37: type = fn_type_with_self_type %Zero.type.822, %Z.facet [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: file {}
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Z = %Z.decl
+// CHECK:STDOUT:     .Point = %Point.decl
+// CHECK:STDOUT:     .F = %F.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Z.decl: type = interface_decl @Z [concrete = constants.%Z.type] {} {}
+// CHECK:STDOUT:   %Point.decl: type = class_decl @Point [concrete = constants.%Point] {} {}
+// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {} {}
+// CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Z {
+// CHECK:STDOUT:   %Self: %Z.type = bind_symbolic_name Self, 0 [symbolic = constants.%Self]
+// CHECK:STDOUT:   %Zero.decl: %Zero.type.822 = fn_decl @Zero.1 [concrete = constants.%Zero.d14] {} {}
+// CHECK:STDOUT:   %assoc0: %Z.assoc_type = assoc_entity element0, %Zero.decl [concrete = constants.%assoc0]
+// CHECK:STDOUT:   impl_decl @impl.1 [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, <error> [concrete = <error>]
+// CHECK:STDOUT:     %Z.ref: type = name_ref Z, file.%Z.decl [concrete = constants.%Z.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .Self = <unexpected>.inst15
-// CHECK:STDOUT:   .Zero = <unexpected>.inst32.loc4_20
-// CHECK:STDOUT:   witness = invalid
+// CHECK:STDOUT:   .Self = %Self
+// CHECK:STDOUT:   .Zero = %assoc0
+// CHECK:STDOUT:   .Z = <poisoned>
+// CHECK:STDOUT:   witness = (%Zero.decl)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: generic fn @Zero(<unexpected>.inst15: %Z.type) {
+// CHECK:STDOUT: generic impl @impl.1(@Z.%Self: %Z.type) {
+// CHECK:STDOUT: !definition:
 // CHECK:STDOUT:   %Self: %Z.type = bind_symbolic_name Self, 0 [symbolic = %Self (constants.%Self)]
-// CHECK:STDOUT:   %Self.as_type: type = facet_access_type %Self [symbolic = %Self.as_type (constants.%Self.as_type)]
+// CHECK:STDOUT:   %Zero.type: type = fn_type @Zero.2, @impl.1(%Self) [symbolic = %Zero.type (constants.%Zero.type.db4)]
+// CHECK:STDOUT:   %Zero: @impl.1.%Zero.type (%Zero.type.db4) = struct_value () [symbolic = %Zero (constants.%Zero.8fb)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   impl: %Self.ref as %Z.ref {
+// CHECK:STDOUT:     %Zero.decl: @impl.1.%Zero.type (%Zero.type.db4) = fn_decl @Zero.2 [symbolic = @impl.1.%Zero (constants.%Zero.8fb)] {} {}
 // CHECK:STDOUT:
-// CHECK:STDOUT:   fn() -> @Zero.%Self.as_type (%Self.as_type);
+// CHECK:STDOUT:   !members:
+// CHECK:STDOUT:     .Zero = %Zero.decl
+// CHECK:STDOUT:     witness = <error>
+// CHECK:STDOUT:   }
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: specific @Zero(constants.%Self) {
+// CHECK:STDOUT: impl @impl.2: %Self.ref as %Z.ref {
+// CHECK:STDOUT:   %Zero.decl: %Zero.type.e33 = fn_decl @Zero.3 [concrete = constants.%Zero.dec] {} {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Zero = %Zero.decl
+// CHECK:STDOUT:   witness = @Point.%impl_witness
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @Point {
+// CHECK:STDOUT:   impl_decl @impl.2 [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, constants.%Point [concrete = constants.%Point]
+// CHECK:STDOUT:     %Z.ref: type = name_ref Z, file.%Z.decl [concrete = constants.%Z.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %impl_witness: <witness> = impl_witness (@impl.2.%Zero.decl) [concrete = constants.%impl_witness]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete = constants.%complete_type]
+// CHECK:STDOUT:   complete_type_witness = %complete_type
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%Point
+// CHECK:STDOUT:   .Z = <poisoned>
+// CHECK:STDOUT:   .Zero = <poisoned>
+// CHECK:STDOUT:   extend @impl.2.%Z.ref
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @Zero.1(@Z.%Self: %Z.type) {
+// CHECK:STDOUT:   fn();
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @Zero.2(@Z.%Self: %Z.type) {
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn() {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     return
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @Zero.3() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %.loc25_5.1: %empty_struct_type = struct_literal ()
+// CHECK:STDOUT:   %Point.ref: type = name_ref Point, file.%Point.decl [concrete = constants.%Point]
+// CHECK:STDOUT:   %.loc25_5.2: ref %Point = temporary_storage
+// CHECK:STDOUT:   %.loc25_5.3: init %Point = class_init (), %.loc25_5.2 [concrete = constants.%Point.val]
+// CHECK:STDOUT:   %.loc25_5.4: ref %Point = temporary %.loc25_5.2, %.loc25_5.3
+// CHECK:STDOUT:   %.loc25_7: ref %Point = converted %.loc25_5.1, %.loc25_5.4
+// CHECK:STDOUT:   %Zero.ref: %Z.assoc_type = name_ref Zero, @Z.%assoc0 [concrete = constants.%assoc0]
+// CHECK:STDOUT:   %impl.elem0: %.c37 = impl_witness_access constants.%impl_witness, element0 [concrete = constants.%Zero.dec]
+// CHECK:STDOUT:   %Zero.call: init %empty_tuple.type = call %impl.elem0()
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @Zero.1(constants.%Self) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @impl.1(constants.%Self) {
+// CHECK:STDOUT: !definition:
 // CHECK:STDOUT:   %Self => constants.%Self
-// CHECK:STDOUT:   %Self.as_type => constants.%Self.as_type
+// CHECK:STDOUT:   %Zero.type => constants.%Zero.type.db4
+// CHECK:STDOUT:   %Zero => constants.%Zero.8fb
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: specific @Zero.2(constants.%Self) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @impl.1(%Self) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @Zero.1(constants.%Z.facet) {}
+// CHECK:STDOUT:

toolchain/check/testdata/impl/no_prelude/fail_impl_as_scope.carbon

@@ -0,0 +1,444 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/impl/no_prelude/fail_impl_as_scope.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/impl/no_prelude/fail_impl_as_scope.carbon
+
+// --- fail_impl_as_file_scope.carbon
+library "[[@TEST_NAME]]";
+
+interface I {}
+
+fn G() {}
+
+// CHECK:STDERR: fail_impl_as_file_scope.carbon:[[@LINE+4]]:6: error: `impl as` can only be used in a class [ImplAsOutsideClass]
+// CHECK:STDERR: impl as I {}
+// CHECK:STDERR:      ^~
+// CHECK:STDERR:
+impl as I {}
+
+fn F() { G(); }
+
+// --- fail_impl_as_function_scope.carbon
+library "[[@TEST_NAME]]";
+
+interface J {}
+
+fn G() {}
+
+fn F() {
+  // CHECK:STDERR: fail_impl_as_function_scope.carbon:[[@LINE+4]]:8: error: `impl as` can only be used in a class [ImplAsOutsideClass]
+  // CHECK:STDERR:   impl as J {}
+  // CHECK:STDERR:        ^~
+  // CHECK:STDERR:
+  impl as J {}
+  G();
+}
+
+// --- fail_impl_as_self_interface.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {
+  fn Zero();
+
+   // CHECK:STDERR: fail_impl_as_self_interface.carbon:[[@LINE+4]]:9: error: `impl as` can only be used in a class [ImplAsOutsideClass]
+   // CHECK:STDERR:    impl as Z {
+   // CHECK:STDERR:         ^~
+   // CHECK:STDERR:
+   impl as Z {
+    fn Zero() {}
+  }
+}
+
+class Point {
+   impl as Z {
+    fn Zero() {}
+  }
+}
+
+fn F() {
+  // Even if the `impl` is diagnosed above, we must not add the impl of the
+  // interface to itself in a way that allows it to be used during impl lookup,
+  // or we end up with infinite impl lookup recursion here.
+  ({} as Point).(Z.Zero)();
+}
+
+// --- fail_impl_as_other_interface.carbon
+
+library "[[@TEST_NAME]]";
+
+interface A {
+  fn B();
+}
+interface C {}
+
+fn G() {}
+
+class X {
+  impl as A {
+    // CHECK:STDERR: fail_impl_as_other_interface.carbon:[[@LINE+4]]:10: error: `impl as` can only be used in a class [ImplAsOutsideClass]
+    // CHECK:STDERR:     impl as C {}
+    // CHECK:STDERR:          ^~
+    // CHECK:STDERR:
+    impl as C {}
+    fn B() {
+      G();
+    }
+  }
+}
+
+// CHECK:STDOUT: --- fail_impl_as_file_scope.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %I.type: type = facet_type <@I> [concrete]
+// CHECK:STDOUT:   %Self: %I.type = bind_symbolic_name Self, 0 [symbolic]
+// CHECK:STDOUT:   %G.type: type = fn_type @G [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %G: %G.type = struct_value () [concrete]
+// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
+// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .I = %I.decl
+// CHECK:STDOUT:     .G = %G.decl
+// CHECK:STDOUT:     .F = %F.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %I.decl: type = interface_decl @I [concrete = constants.%I.type] {} {}
+// CHECK:STDOUT:   %G.decl: %G.type = fn_decl @G [concrete = constants.%G] {} {}
+// CHECK:STDOUT:   impl_decl @impl [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, <error> [concrete = <error>]
+// CHECK:STDOUT:     %I.ref: type = name_ref I, file.%I.decl [concrete = constants.%I.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @I {
+// CHECK:STDOUT:   %Self: %I.type = bind_symbolic_name Self, 0 [symbolic = constants.%Self]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = %Self
+// CHECK:STDOUT:   witness = ()
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: impl @impl: %Self.ref as %I.ref {
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = <error>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @G() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %G.ref: %G.type = name_ref G, file.%G.decl [concrete = constants.%G]
+// CHECK:STDOUT:   %G.call: init %empty_tuple.type = call %G.ref()
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_impl_as_function_scope.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %J.type: type = facet_type <@J> [concrete]
+// CHECK:STDOUT:   %Self: %J.type = bind_symbolic_name Self, 0 [symbolic]
+// CHECK:STDOUT:   %G.type: type = fn_type @G [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %G: %G.type = struct_value () [concrete]
+// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
+// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .J = %J.decl
+// CHECK:STDOUT:     .G = %G.decl
+// CHECK:STDOUT:     .F = %F.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %J.decl: type = interface_decl @J [concrete = constants.%J.type] {} {}
+// CHECK:STDOUT:   %G.decl: %G.type = fn_decl @G [concrete = constants.%G] {} {}
+// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @J {
+// CHECK:STDOUT:   %Self: %J.type = bind_symbolic_name Self, 0 [symbolic = constants.%Self]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = %Self
+// CHECK:STDOUT:   witness = ()
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: impl @impl: %Self.ref as %J.ref {
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = <error>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @G() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   impl_decl @impl [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, <error> [concrete = <error>]
+// CHECK:STDOUT:     %J.ref: type = name_ref J, file.%J.decl [concrete = constants.%J.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %G.ref: %G.type = name_ref G, file.%G.decl [concrete = constants.%G]
+// CHECK:STDOUT:   %G.call: init %empty_tuple.type = call %G.ref()
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_impl_as_self_interface.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %Z.type: type = facet_type <@Z> [concrete]
+// CHECK:STDOUT:   %Self: %Z.type = bind_symbolic_name Self, 0 [symbolic]
+// CHECK:STDOUT:   %Zero.type.822: type = fn_type @Zero.1 [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %Zero.d14: %Zero.type.822 = struct_value () [concrete]
+// CHECK:STDOUT:   %Z.assoc_type: type = assoc_entity_type %Z.type [concrete]
+// CHECK:STDOUT:   %assoc0: %Z.assoc_type = assoc_entity element0, @Z.%Zero.decl [concrete]
+// CHECK:STDOUT:   %Zero.type.db4: type = fn_type @Zero.2, @impl.1(%Self) [symbolic]
+// CHECK:STDOUT:   %Zero.8fb: %Zero.type.db4 = struct_value () [symbolic]
+// CHECK:STDOUT:   %Point: type = class_type @Point [concrete]
+// CHECK:STDOUT:   %impl_witness: <witness> = impl_witness (@impl.2.%Zero.decl) [concrete]
+// CHECK:STDOUT:   %Zero.type.e33: type = fn_type @Zero.3 [concrete]
+// CHECK:STDOUT:   %Zero.dec: %Zero.type.e33 = struct_value () [concrete]
+// CHECK:STDOUT:   %Z.facet: %Z.type = facet_value %Point, %impl_witness [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete]
+// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
+// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Point.val: %Point = struct_value () [concrete]
+// CHECK:STDOUT:   %.c37: type = fn_type_with_self_type %Zero.type.822, %Z.facet [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Z = %Z.decl
+// CHECK:STDOUT:     .Point = %Point.decl
+// CHECK:STDOUT:     .F = %F.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Z.decl: type = interface_decl @Z [concrete = constants.%Z.type] {} {}
+// CHECK:STDOUT:   %Point.decl: type = class_decl @Point [concrete = constants.%Point] {} {}
+// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @Z {
+// CHECK:STDOUT:   %Self: %Z.type = bind_symbolic_name Self, 0 [symbolic = constants.%Self]
+// CHECK:STDOUT:   %Zero.decl: %Zero.type.822 = fn_decl @Zero.1 [concrete = constants.%Zero.d14] {} {}
+// CHECK:STDOUT:   %assoc0: %Z.assoc_type = assoc_entity element0, %Zero.decl [concrete = constants.%assoc0]
+// CHECK:STDOUT:   impl_decl @impl.1 [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, <error> [concrete = <error>]
+// CHECK:STDOUT:     %Z.ref: type = name_ref Z, file.%Z.decl [concrete = constants.%Z.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = %Self
+// CHECK:STDOUT:   .Zero = %assoc0
+// CHECK:STDOUT:   .Z = <poisoned>
+// CHECK:STDOUT:   witness = (%Zero.decl)
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic impl @impl.1(@Z.%Self: %Z.type) {
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %Self: %Z.type = bind_symbolic_name Self, 0 [symbolic = %Self (constants.%Self)]
+// CHECK:STDOUT:   %Zero.type: type = fn_type @Zero.2, @impl.1(%Self) [symbolic = %Zero.type (constants.%Zero.type.db4)]
+// CHECK:STDOUT:   %Zero: @impl.1.%Zero.type (%Zero.type.db4) = struct_value () [symbolic = %Zero (constants.%Zero.8fb)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   impl: %Self.ref as %Z.ref {
+// CHECK:STDOUT:     %Zero.decl: @impl.1.%Zero.type (%Zero.type.db4) = fn_decl @Zero.2 [symbolic = @impl.1.%Zero (constants.%Zero.8fb)] {} {}
+// CHECK:STDOUT:
+// CHECK:STDOUT:   !members:
+// CHECK:STDOUT:     .Zero = %Zero.decl
+// CHECK:STDOUT:     witness = <error>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: impl @impl.2: %Self.ref as %Z.ref {
+// CHECK:STDOUT:   %Zero.decl: %Zero.type.e33 = fn_decl @Zero.3 [concrete = constants.%Zero.dec] {} {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Zero = %Zero.decl
+// CHECK:STDOUT:   witness = @Point.%impl_witness
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @Point {
+// CHECK:STDOUT:   impl_decl @impl.2 [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, constants.%Point [concrete = constants.%Point]
+// CHECK:STDOUT:     %Z.ref: type = name_ref Z, file.%Z.decl [concrete = constants.%Z.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %impl_witness: <witness> = impl_witness (@impl.2.%Zero.decl) [concrete = constants.%impl_witness]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete = constants.%complete_type]
+// CHECK:STDOUT:   complete_type_witness = %complete_type
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%Point
+// CHECK:STDOUT:   .Z = <poisoned>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @Zero.1(@Z.%Self: %Z.type) {
+// CHECK:STDOUT:   fn();
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @Zero.2(@Z.%Self: %Z.type) {
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn() {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     return
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @Zero.3() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %.loc25_5.1: %empty_struct_type = struct_literal ()
+// CHECK:STDOUT:   %Point.ref: type = name_ref Point, file.%Point.decl [concrete = constants.%Point]
+// CHECK:STDOUT:   %.loc25_5.2: ref %Point = temporary_storage
+// CHECK:STDOUT:   %.loc25_5.3: init %Point = class_init (), %.loc25_5.2 [concrete = constants.%Point.val]
+// CHECK:STDOUT:   %.loc25_5.4: ref %Point = temporary %.loc25_5.2, %.loc25_5.3
+// CHECK:STDOUT:   %.loc25_7: ref %Point = converted %.loc25_5.1, %.loc25_5.4
+// CHECK:STDOUT:   %Z.ref: type = name_ref Z, file.%Z.decl [concrete = constants.%Z.type]
+// CHECK:STDOUT:   %Zero.ref: %Z.assoc_type = name_ref Zero, @Z.%assoc0 [concrete = constants.%assoc0]
+// CHECK:STDOUT:   %impl.elem0: %.c37 = impl_witness_access constants.%impl_witness, element0 [concrete = constants.%Zero.dec]
+// CHECK:STDOUT:   %Zero.call: init %empty_tuple.type = call %impl.elem0()
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @Zero.1(constants.%Self) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @impl.1(constants.%Self) {
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %Self => constants.%Self
+// CHECK:STDOUT:   %Zero.type => constants.%Zero.type.db4
+// CHECK:STDOUT:   %Zero => constants.%Zero.8fb
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @Zero.2(constants.%Self) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @impl.1(%Self) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @Zero.1(constants.%Z.facet) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_impl_as_other_interface.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %A.type: type = facet_type <@A> [concrete]
+// CHECK:STDOUT:   %Self.31d: %A.type = bind_symbolic_name Self, 0 [symbolic]
+// CHECK:STDOUT:   %B.type.1c3: type = fn_type @B.1 [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %B.b08: %B.type.1c3 = struct_value () [concrete]
+// CHECK:STDOUT:   %A.assoc_type: type = assoc_entity_type %A.type [concrete]
+// CHECK:STDOUT:   %assoc0: %A.assoc_type = assoc_entity element0, @A.%B.decl [concrete]
+// CHECK:STDOUT:   %C.type: type = facet_type <@C> [concrete]
+// CHECK:STDOUT:   %Self.02e: %C.type = bind_symbolic_name Self, 0 [symbolic]
+// CHECK:STDOUT:   %G.type: type = fn_type @G [concrete]
+// CHECK:STDOUT:   %G: %G.type = struct_value () [concrete]
+// CHECK:STDOUT:   %X: type = class_type @X [concrete]
+// CHECK:STDOUT:   %impl_witness: <witness> = impl_witness (@impl.1.%B.decl) [concrete]
+// CHECK:STDOUT:   %B.type.d47: type = fn_type @B.2 [concrete]
+// CHECK:STDOUT:   %B.4af: %B.type.d47 = struct_value () [concrete]
+// CHECK:STDOUT:   %A.facet: %A.type = facet_value %X, %impl_witness [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .A = %A.decl
+// CHECK:STDOUT:     .C = %C.decl
+// CHECK:STDOUT:     .G = %G.decl
+// CHECK:STDOUT:     .X = %X.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %A.decl: type = interface_decl @A [concrete = constants.%A.type] {} {}
+// CHECK:STDOUT:   %C.decl: type = interface_decl @C [concrete = constants.%C.type] {} {}
+// CHECK:STDOUT:   %G.decl: %G.type = fn_decl @G [concrete = constants.%G] {} {}
+// CHECK:STDOUT:   %X.decl: type = class_decl @X [concrete = constants.%X] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @A {
+// CHECK:STDOUT:   %Self: %A.type = bind_symbolic_name Self, 0 [symbolic = constants.%Self.31d]
+// CHECK:STDOUT:   %B.decl: %B.type.1c3 = fn_decl @B.1 [concrete = constants.%B.b08] {} {}
+// CHECK:STDOUT:   %assoc0: %A.assoc_type = assoc_entity element0, %B.decl [concrete = constants.%assoc0]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = %Self
+// CHECK:STDOUT:   .B = %assoc0
+// CHECK:STDOUT:   witness = (%B.decl)
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @C {
+// CHECK:STDOUT:   %Self: %C.type = bind_symbolic_name Self, 0 [symbolic = constants.%Self.02e]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = %Self
+// CHECK:STDOUT:   witness = ()
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: impl @impl.1: %Self.ref as %A.ref {
+// CHECK:STDOUT:   impl_decl @impl.2 [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, <error> [concrete = <error>]
+// CHECK:STDOUT:     %C.ref: type = name_ref C, file.%C.decl [concrete = constants.%C.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %B.decl: %B.type.d47 = fn_decl @B.2 [concrete = constants.%B.4af] {} {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .C = <poisoned>
+// CHECK:STDOUT:   .B = %B.decl
+// CHECK:STDOUT:   .G = <poisoned>
+// CHECK:STDOUT:   witness = @X.%impl_witness
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: impl @impl.2: %Self.ref as %C.ref {
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = <error>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @X {
+// CHECK:STDOUT:   impl_decl @impl.1 [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, constants.%X [concrete = constants.%X]
+// CHECK:STDOUT:     %A.ref: type = name_ref A, file.%A.decl [concrete = constants.%A.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %impl_witness: <witness> = impl_witness (@impl.1.%B.decl) [concrete = constants.%impl_witness]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete = constants.%complete_type]
+// CHECK:STDOUT:   complete_type_witness = %complete_type
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%X
+// CHECK:STDOUT:   .A = <poisoned>
+// CHECK:STDOUT:   .C = <poisoned>
+// CHECK:STDOUT:   .G = <poisoned>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @B.1(@A.%Self: %A.type) {
+// CHECK:STDOUT:   fn();
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @G() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @B.2() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %G.ref: %G.type = name_ref G, file.%G.decl [concrete = constants.%G]
+// CHECK:STDOUT:   %G.call: init %empty_tuple.type = call %G.ref()
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @B.1(constants.%Self.31d) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @B.1(constants.%A.facet) {}
+// CHECK:STDOUT:

toolchain/check/testdata/impl/no_prelude/fail_impl_bad_type.carbon

@@ -22,7 +22,6 @@ impl true as I {}
 // CHECK:STDOUT:   %I.type: type = facet_type <@I> [concrete]
 // CHECK:STDOUT:   %Self: %I.type = bind_symbolic_name Self, 0 [symbolic]
 // CHECK:STDOUT:   %true: bool = bool_literal true [concrete]
-// CHECK:STDOUT:   %impl_witness: <witness> = impl_witness () [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -35,7 +34,6 @@ impl true as I {}
 // CHECK:STDOUT:     %.loc17: type = converted %true, <error> [concrete = <error>]
 // CHECK:STDOUT:     %I.ref: type = name_ref I, file.%I.decl [concrete = constants.%I.type]
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %impl_witness: <witness> = impl_witness () [concrete = constants.%impl_witness]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @I {
@@ -48,6 +46,6 @@ impl true as I {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: <error> as %I.ref {
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   witness = file.%impl_witness
+// CHECK:STDOUT:   witness = <error>
 // CHECK:STDOUT: }
 // CHECK:STDOUT:

toolchain/check/testdata/impl/no_prelude/todo_impl_with_unrelated_fn.carbon

@@ -0,0 +1,105 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/impl/no_prelude/todo_impl_with_unrelated_fn.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/impl/no_prelude/todo_impl_with_unrelated_fn.carbon
+
+interface A {
+  fn B();
+}
+
+class X {
+  impl as A {
+    fn B() {}
+    // TODO: This should be diagnosed as an error; there is no `A.F` to
+    // implement here.
+    fn F() {}
+  }
+}
+
+// CHECK:STDOUT: --- todo_impl_with_unrelated_fn.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %A.type: type = facet_type <@A> [concrete]
+// CHECK:STDOUT:   %Self: %A.type = bind_symbolic_name Self, 0 [symbolic]
+// CHECK:STDOUT:   %B.type.1c3: type = fn_type @B.1 [concrete]
+// CHECK:STDOUT:   %B.b08: %B.type.1c3 = struct_value () [concrete]
+// CHECK:STDOUT:   %A.assoc_type: type = assoc_entity_type %A.type [concrete]
+// CHECK:STDOUT:   %assoc0: %A.assoc_type = assoc_entity element0, @A.%B.decl [concrete]
+// CHECK:STDOUT:   %X: type = class_type @X [concrete]
+// CHECK:STDOUT:   %impl_witness: <witness> = impl_witness (@impl.%B.decl) [concrete]
+// CHECK:STDOUT:   %B.type.d47: type = fn_type @B.2 [concrete]
+// CHECK:STDOUT:   %B.4af: %B.type.d47 = struct_value () [concrete]
+// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
+// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
+// CHECK:STDOUT:   %A.facet: %A.type = facet_value %X, %impl_witness [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .A = %A.decl
+// CHECK:STDOUT:     .X = %X.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %A.decl: type = interface_decl @A [concrete = constants.%A.type] {} {}
+// CHECK:STDOUT:   %X.decl: type = class_decl @X [concrete = constants.%X] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @A {
+// CHECK:STDOUT:   %Self: %A.type = bind_symbolic_name Self, 0 [symbolic = constants.%Self]
+// CHECK:STDOUT:   %B.decl: %B.type.1c3 = fn_decl @B.1 [concrete = constants.%B.b08] {} {}
+// CHECK:STDOUT:   %assoc0: %A.assoc_type = assoc_entity element0, %B.decl [concrete = constants.%assoc0]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = %Self
+// CHECK:STDOUT:   .B = %assoc0
+// CHECK:STDOUT:   witness = (%B.decl)
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: impl @impl: %Self.ref as %A.ref {
+// CHECK:STDOUT:   %B.decl: %B.type.d47 = fn_decl @B.2 [concrete = constants.%B.4af] {} {}
+// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {} {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .B = %B.decl
+// CHECK:STDOUT:   .F = %F.decl
+// CHECK:STDOUT:   witness = @X.%impl_witness
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @X {
+// CHECK:STDOUT:   impl_decl @impl [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, constants.%X [concrete = constants.%X]
+// CHECK:STDOUT:     %A.ref: type = name_ref A, file.%A.decl [concrete = constants.%A.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %impl_witness: <witness> = impl_witness (@impl.%B.decl) [concrete = constants.%impl_witness]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete = constants.%complete_type]
+// CHECK:STDOUT:   complete_type_witness = %complete_type
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%X
+// CHECK:STDOUT:   .A = <poisoned>
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @B.1(@A.%Self: %A.type) {
+// CHECK:STDOUT:   fn();
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @B.2() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @B.1(constants.%Self) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @B.1(constants.%A.facet) {}
+// CHECK:STDOUT: