Look through `extend require` in an interface or named constraint in name lookup (#6630)

Add the required facet type as an extended scope of the containing
interface/named constraint, and teach name lookup to look for extended
scopes in named constraints.

This makes name lookup work properly when the facet type does not have a
specific that involves `Self`. Support for `Self` needs further work in
another PR.

Note that when an _interface_ requires another interface, this PR lets
us find the name, but we still fail to find a witness for the interface
named through `extend require`, and this is future work. For a named
constraint, things work correctly as the identified facet type chases
through the named constraint and includes the required interface, so
impl lookup is able to provide a witness.

toolchain/check/handle_require.cpp

@@ -301,6 +301,21 @@ auto HandleParseNode(Context& context, Parse::RequireDeclId node_id) -> bool {
             })) {
       return true;
     }
+
+    // The extended scope instruction must be part of the enclosing scope (and
+    // generic). A specific for the enclosing scope will be applied to it when
+    // using the instruction later. To do so, we wrap the constraint facet type
+    // it in a SpecificConstant, which preserves the require declaration's
+    // specific along with the facet type.
+    auto constraint_id_in_self_specific = AddTypeInst<SemIR::SpecificConstant>(
+        context, node_id,
+        {.type_id = SemIR::TypeType::TypeId,
+         .inst_id = constraint_inst_id,
+         .specific_id = context.generics().GetSelfSpecific(
+             context.require_impls().Get(require_impls_id).generic_id)});
+    auto enclosing_scope_id = context.scope_stack().PeekNameScopeId();
+    auto& enclosing_scope = context.name_scopes().Get(enclosing_scope_id);
+    enclosing_scope.AddExtendedScope(constraint_id_in_self_specific);
   }
 
   context.require_impls_stack().AppendToTop(require_impls_id);

toolchain/check/impl.cpp

@@ -238,6 +238,11 @@ static auto ApplyExtendImplAs(Context& context, SemIR::LocId loc_id,
   if (!impl.generic_id.has_value()) {
     parent_scope.AddExtendedScope(impl.constraint_id);
   } else {
+    // The extended scope instruction must be part of the enclosing scope (and
+    // generic). A specific for the enclosing scope will be applied to it when
+    // using the instruction later. To do so, we wrap the constraint facet type
+    // it in a SpecificConstant, which preserves the impl declaration's
+    // specific along with the facet type.
     auto constraint_id_in_self_specific = AddTypeInst<SemIR::SpecificConstant>(
         context, SemIR::LocId(impl.constraint_id),
         {.type_id = SemIR::TypeType::TypeId,

toolchain/check/import_ref.cpp

@@ -2779,9 +2779,6 @@ static auto AddInterfaceDefinition(ImportContext& context,
   new_interface.body_block_id =
       context.local_context().inst_block_stack().Pop();
   new_interface.self_param_id = self_param_id;
-
-  CARBON_CHECK(import_scope.extended_scopes().empty(),
-               "Interfaces don't currently have extended scopes to support.");
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
@@ -2930,10 +2927,6 @@ static auto AddNamedConstraintDefinition(
       context.local_context().inst_block_stack().Pop();
   new_named_constraint.self_param_id = self_param_id;
   new_named_constraint.complete = import_named_constraint.complete;
-
-  CARBON_CHECK(
-      import_scope.extended_scopes().empty(),
-      "Named constraints don't currently have extended scopes to support.");
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,

toolchain/check/name_lookup.cpp

@@ -328,11 +328,16 @@ auto AppendLookupScopesForConstant(Context& context, SemIR::LocId loc_id,
       auto facet_type_info =
           context.facet_types().Get(facet_type->facet_type_id);
       // Name lookup into "extend" constraints but not "self impls" constraints.
-      // TODO: Include named constraints, once they are supported.
-      for (const auto& interface : facet_type_info.extend_constraints) {
-        auto& interface_info = context.interfaces().Get(interface.interface_id);
-        scopes->push_back({.name_scope_id = interface_info.scope_id,
-                           .specific_id = interface.specific_id});
+      for (const auto& extend : facet_type_info.extend_constraints) {
+        auto& interface = context.interfaces().Get(extend.interface_id);
+        scopes->push_back({.name_scope_id = interface.scope_id,
+                           .specific_id = extend.specific_id});
+      }
+      for (const auto& extend : facet_type_info.extend_named_constraints) {
+        auto& constraint =
+            context.named_constraints().Get(extend.named_constraint_id);
+        scopes->push_back({.name_scope_id = constraint.scope_id,
+                           .specific_id = extend.specific_id});
       }
     } else {
       // Lookup into this scope should fail without producing an error since

toolchain/check/testdata/facet/require_import.carbon

@@ -58,6 +58,7 @@ fn F(A:! X, B:! Y) {}
 // CHECK:STDOUT:   %Main.import_ref.926bc7.1: type = import_ref Main//a, loc10_24, loaded [concrete = constants.%Z.type]
 // CHECK:STDOUT:   %Main.import_ref.e33: %X.type = import_ref Main//a, loc9_14, loaded [symbolic = constants.%Self.f45]
 // CHECK:STDOUT:   %Main.import_ref.65b = import_ref Main//a, loc9_14, unloaded
+// CHECK:STDOUT:   %Main.import_ref.1c5 = import_ref Main//a, loc10_25, unloaded
 // CHECK:STDOUT:   %Main.import_ref.7bb: type = import_ref Main//a, loc6_11, loaded [symbolic = @Y.Self.binding.as_type.impls.Z.type.require.%Self.binding.as_type (constants.%Self.binding.as_type.63e)]
 // CHECK:STDOUT:   %Main.import_ref.926bc7.2: type = import_ref Main//a, loc6_17, loaded [concrete = constants.%Z.type]
 // CHECK:STDOUT:   %Main.import_ref.8c7: %Y.type = import_ref Main//a, loc5_13, loaded [symbolic = constants.%Self.d4d]
@@ -112,6 +113,7 @@ fn F(A:! X, B:! Y) {}
 // CHECK:STDOUT: constraint @X [from "a.carbon"] {
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Self = imports.%Main.import_ref.65b
+// CHECK:STDOUT:   extend imports.%Main.import_ref.1c5
 // CHECK:STDOUT:
 // CHECK:STDOUT: !requires:
 // CHECK:STDOUT:   @X.Self.binding.as_type.impls.Z.type.require {

toolchain/check/testdata/impl/import_generic.carbon

@@ -260,11 +260,13 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:       %T.ref: type = name_ref T, @J.%T.loc4_13.2 [symbolic = %T (constants.%T)]
 // CHECK:STDOUT:       %I.type.loc5_27.1: type = facet_type <@I, @I(constants.%T)> [symbolic = %I.type.loc5_27.2 (constants.%I.type.1ab)]
 // CHECK:STDOUT:     }
+// CHECK:STDOUT:     %.loc5: type = specific_constant @J.Self.binding.as_type.impls.I.type.require.%I.type.loc5_27.1, @J.Self.binding.as_type.impls.I.type.require(constants.%T, constants.%Self.a60) [symbolic = %I.type (constants.%I.type.1ab)]
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !members:
 // CHECK:STDOUT:     .Self = %Self.loc4_23.1
 // CHECK:STDOUT:     .I = <poisoned>
 // CHECK:STDOUT:     .T = <poisoned>
+// CHECK:STDOUT:     extend %.loc5
 // CHECK:STDOUT:     witness = ()
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !requires:
@@ -341,6 +343,7 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:   %Main.import_ref.b3bc94.2: type = import_ref Main//basic_import_generic_interface, loc4_13, loaded [symbolic = @J.%T (constants.%T)]
 // CHECK:STDOUT:   %Main.import_ref.99f: @J.%J.type (%J.type.04e) = import_ref Main//basic_import_generic_interface, loc4_23, loaded [symbolic = @J.%Self (constants.%Self.c25)]
 // CHECK:STDOUT:   %Main.import_ref.e9b = import_ref Main//basic_import_generic_interface, loc4_23, unloaded
+// CHECK:STDOUT:   %Main.import_ref.b55 = import_ref Main//basic_import_generic_interface, loc5_28, unloaded
 // CHECK:STDOUT:   %Main.import_ref.b3bc94.3: type = import_ref Main//basic_import_generic_interface, loc4_13, loaded [symbolic = @J.%T (constants.%T)]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -397,6 +400,7 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:   interface {
 // CHECK:STDOUT:   !members:
 // CHECK:STDOUT:     .Self = imports.%Main.import_ref.e9b
+// CHECK:STDOUT:     extend imports.%Main.import_ref.b55
 // CHECK:STDOUT:     witness = ()
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !requires:
@@ -559,11 +563,13 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:       %T.ref: type = name_ref T, @J.%T.loc4_14.2 [symbolic = %T (constants.%T)]
 // CHECK:STDOUT:       %I.type.loc5_27.1: type = facet_type <@I, @I(constants.%T)> [symbolic = %I.type.loc5_27.2 (constants.%I.type.1ab)]
 // CHECK:STDOUT:     }
+// CHECK:STDOUT:     %.loc5: type = specific_constant @J.Self.binding.as_type.impls.I.type.require.%I.type.loc5_27.1, @J.Self.binding.as_type.impls.I.type.require(constants.%T, constants.%Self.31c) [symbolic = %I.type (constants.%I.type.1ab)]
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !members:
 // CHECK:STDOUT:     .Self = %Self.loc4_24.1
 // CHECK:STDOUT:     .I = <poisoned>
 // CHECK:STDOUT:     .T = <poisoned>
+// CHECK:STDOUT:     extend %.loc5
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !requires:
 // CHECK:STDOUT:     @J.Self.binding.as_type.impls.I.type.require {
@@ -636,6 +642,7 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:   %Main.import_ref.b3bc94.2: type = import_ref Main//basic_import_generic_constraint, loc4_14, loaded [symbolic = @J.%T (constants.%T)]
 // CHECK:STDOUT:   %Main.import_ref.2fc: @J.%J.type (%J.type.d682d6.1) = import_ref Main//basic_import_generic_constraint, loc4_24, loaded [symbolic = @J.%Self (constants.%Self.e1f642.1)]
 // CHECK:STDOUT:   %Main.import_ref.830 = import_ref Main//basic_import_generic_constraint, loc4_24, unloaded
+// CHECK:STDOUT:   %Main.import_ref.b41 = import_ref Main//basic_import_generic_constraint, loc5_28, unloaded
 // CHECK:STDOUT:   %Main.import_ref.b3bc94.3: type = import_ref Main//basic_import_generic_constraint, loc4_14, loaded [symbolic = @J.%T (constants.%T)]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -685,6 +692,7 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:   constraint {
 // CHECK:STDOUT:   !members:
 // CHECK:STDOUT:     .Self = imports.%Main.import_ref.830
+// CHECK:STDOUT:     extend imports.%Main.import_ref.b41
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !requires:
 // CHECK:STDOUT:     @J.Self.binding.as_type.impls.I.type.require {
@@ -880,11 +888,13 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:       %T.ref: type = name_ref T, @N.%T.loc14_14.2 [symbolic = %T (constants.%T)]
 // CHECK:STDOUT:       %I.type.loc15_27.1: type = facet_type <@I, @I(constants.%T)> [symbolic = %I.type.loc15_27.2 (constants.%I.type.1ab)]
 // CHECK:STDOUT:     }
+// CHECK:STDOUT:     %.loc15: type = specific_constant @N.Self.binding.as_type.impls.I.type.require.%I.type.loc15_27.1, @N.Self.binding.as_type.impls.I.type.require(constants.%T, constants.%Self.31c) [symbolic = %I.type (constants.%I.type.1ab)]
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !members:
 // CHECK:STDOUT:     .Self = %Self.loc14_24.1
 // CHECK:STDOUT:     .I = <poisoned>
 // CHECK:STDOUT:     .T = <poisoned>
+// CHECK:STDOUT:     extend %.loc15
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !requires:
 // CHECK:STDOUT:     @N.Self.binding.as_type.impls.I.type.require {
@@ -1038,6 +1048,7 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:   %Main.import_ref.b3bc94.4: type = import_ref Main//import_generic, loc14_14, loaded [symbolic = @N.%T (constants.%T)]
 // CHECK:STDOUT:   %Main.import_ref.2fc: @N.%N.type (%N.type.d682d6.1) = import_ref Main//import_generic, loc14_24, loaded [symbolic = @N.%Self (constants.%Self.e1f)]
 // CHECK:STDOUT:   %Main.import_ref.830 = import_ref Main//import_generic, loc14_24, unloaded
+// CHECK:STDOUT:   %Main.import_ref.b41 = import_ref Main//import_generic, loc15_28, unloaded
 // CHECK:STDOUT:   %Main.import_ref.b3bc94.5: type = import_ref Main//import_generic, loc14_14, loaded [symbolic = @N.%T (constants.%T)]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -1142,6 +1153,7 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:   constraint {
 // CHECK:STDOUT:   !members:
 // CHECK:STDOUT:     .Self = imports.%Main.import_ref.830
+// CHECK:STDOUT:     extend imports.%Main.import_ref.b41
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !requires:
 // CHECK:STDOUT:     @N.Self.binding.as_type.impls.I.type.require {
@@ -1434,11 +1446,13 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:       %T.ref: type = name_ref T, @N.%T.loc9_14.2 [symbolic = %T (constants.%T)]
 // CHECK:STDOUT:       %I.type.loc10_27.1: type = facet_type <@I, @I(constants.%T)> [symbolic = %I.type.loc10_27.2 (constants.%I.type.1ab)]
 // CHECK:STDOUT:     }
+// CHECK:STDOUT:     %.loc10: type = specific_constant @N.Self.binding.as_type.impls.I.type.require.%I.type.loc10_27.1, @N.Self.binding.as_type.impls.I.type.require(constants.%T, constants.%Self.31c) [symbolic = %I.type (constants.%I.type.1ab)]
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !members:
 // CHECK:STDOUT:     .Self = %Self.loc9_24.1
 // CHECK:STDOUT:     .I = <poisoned>
 // CHECK:STDOUT:     .T = <poisoned>
+// CHECK:STDOUT:     extend %.loc10
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !requires:
 // CHECK:STDOUT:     @N.Self.binding.as_type.impls.I.type.require {
@@ -1560,6 +1574,7 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:   %Main.import_ref.b3bc94.3: type = import_ref Main//import_generic_with_different_specific, loc9_14, loaded [symbolic = @N.%T (constants.%T)]
 // CHECK:STDOUT:   %Main.import_ref.2fc: @N.%N.type (%N.type.d682d6.1) = import_ref Main//import_generic_with_different_specific, loc9_24, loaded [symbolic = @N.%Self (constants.%Self.e1f642.1)]
 // CHECK:STDOUT:   %Main.import_ref.830 = import_ref Main//import_generic_with_different_specific, loc9_24, unloaded
+// CHECK:STDOUT:   %Main.import_ref.b41 = import_ref Main//import_generic_with_different_specific, loc10_28, unloaded
 // CHECK:STDOUT:   %Main.import_ref.b3bc94.4: type = import_ref Main//import_generic_with_different_specific, loc9_14, loaded [symbolic = @N.%T (constants.%T)]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -1624,6 +1639,7 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:   constraint {
 // CHECK:STDOUT:   !members:
 // CHECK:STDOUT:     .Self = imports.%Main.import_ref.830
+// CHECK:STDOUT:     extend imports.%Main.import_ref.b41
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !requires:
 // CHECK:STDOUT:     @N.Self.binding.as_type.impls.I.type.require {
@@ -1874,11 +1890,13 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:       %T.ref: type = name_ref T, @N.%T.loc7_14.2 [symbolic = %T (constants.%T)]
 // CHECK:STDOUT:       %J.type.loc8_27.1: type = facet_type <@J, @J(constants.%T)> [symbolic = %J.type.loc8_27.2 (constants.%J.type.04e)]
 // CHECK:STDOUT:     }
+// CHECK:STDOUT:     %.loc8: type = specific_constant @N.Self.binding.as_type.impls.J.type.require.%J.type.loc8_27.1, @N.Self.binding.as_type.impls.J.type.require(constants.%T, constants.%Self.31c) [symbolic = %J.type (constants.%J.type.04e)]
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !members:
 // CHECK:STDOUT:     .Self = %Self.loc7_24.1
 // CHECK:STDOUT:     .J = <poisoned>
 // CHECK:STDOUT:     .T = <poisoned>
+// CHECK:STDOUT:     extend %.loc8
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !requires:
 // CHECK:STDOUT:     @N.Self.binding.as_type.impls.J.type.require {
@@ -2006,6 +2024,7 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:   %Main.import_ref.b3bc94.4: type = import_ref Main//import_generic_decl, loc7_14, loaded [symbolic = @N.%T (constants.%T)]
 // CHECK:STDOUT:   %Main.import_ref.2fc: @N.%N.type (%N.type.d682d6.1) = import_ref Main//import_generic_decl, loc7_24, loaded [symbolic = @N.%Self (constants.%Self.e1f)]
 // CHECK:STDOUT:   %Main.import_ref.830 = import_ref Main//import_generic_decl, loc7_24, unloaded
+// CHECK:STDOUT:   %Main.import_ref.105 = import_ref Main//import_generic_decl, loc8_28, unloaded
 // CHECK:STDOUT:   %Main.import_ref.b3bc94.5: type = import_ref Main//import_generic_decl, loc7_14, loaded [symbolic = @N.%T (constants.%T)]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -2110,6 +2129,7 @@ impl forall [T:! type] D as N(T*) {}
 // CHECK:STDOUT:   constraint {
 // CHECK:STDOUT:   !members:
 // CHECK:STDOUT:     .Self = imports.%Main.import_ref.830
+// CHECK:STDOUT:     extend imports.%Main.import_ref.105
 // CHECK:STDOUT:
 // CHECK:STDOUT:   !requires:
 // CHECK:STDOUT:     @N.Self.binding.as_type.impls.J.type.require {

toolchain/check/testdata/interface/require.carbon

@@ -23,8 +23,12 @@ interface Z {
 }
 //@dump-sem-ir-end
 
-fn F(T:! Z) {
-  // CHECK:STDERR: fail_todo_extend.carbon:[[@LINE+4]]:3: error: member name `YY` not found in `Z` [MemberNameNotFoundInInstScope]
+fn F(T:! Z, t:! T) {
+  // TODO: We find the name `YY`, but then fail to do impl lookup for `Y` in
+  // `T:! Z` since the identified facet type doesn't include `Y`. The
+  // identified facet type needs to include extends inside interfaces?
+
+  // CHECK:STDERR: fail_todo_extend.carbon:[[@LINE+4]]:3: error: cannot access member of interface `Y` in type `T` that does not implement that interface [MissingImplInMemberAccess]
   // CHECK:STDERR:   T.YY();
   // CHECK:STDERR:   ^~~~
   // CHECK:STDERR:
@@ -34,6 +38,12 @@ fn F(T:! Z) {
   // CHECK:STDERR:   ^~~~~~~~
   // CHECK:STDERR:
   T.(Y.YY)();
+
+  // CHECK:STDERR: fail_todo_extend.carbon:[[@LINE+4]]:3: error: cannot access member of interface `Y` in type `T` that does not implement that interface [MissingImplInMemberAccess]
+  // CHECK:STDERR:   t.YY();
+  // CHECK:STDERR:   ^~~~
+  // CHECK:STDERR:
+  t.YY();
 }
 
 // --- fail_todo_implicit_self_impls.carbon
@@ -293,11 +303,13 @@ interface Z(T:! type) {
 // CHECK:STDOUT:     %Self.as_type: type = facet_access_type @Z.%Self [symbolic = %Self.binding.as_type (constants.%Self.binding.as_type)]
 // CHECK:STDOUT:     %Y.ref: type = name_ref Y, file.%Y.decl [concrete = constants.%Y.type]
 // CHECK:STDOUT:   }
+// CHECK:STDOUT:   %.loc9: type = specific_constant @Z.Self.binding.as_type.impls.Y.type.require.%Y.ref, @Z.Self.binding.as_type.impls.Y.type.require(constants.%Self.c59) [concrete = constants.%Y.type]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Self = %Self
 // CHECK:STDOUT:   .Y = <poisoned>
 // CHECK:STDOUT:   .YY = <poisoned>
+// CHECK:STDOUT:   extend %.loc9
 // CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT:
 // CHECK:STDOUT: !requires:

toolchain/check/testdata/named_constraint/import_constraint_decl.carbon

@@ -99,6 +99,7 @@ impl C as B {}
 // CHECK:STDOUT:   %Main.import_ref.72a: type = import_ref Main//b, loc5_24, loaded [concrete = constants.%I.type]
 // CHECK:STDOUT:   %Main.import_ref.e33: %B.type = import_ref Main//b, loc4_14, loaded [symbolic = constants.%Self.f45]
 // CHECK:STDOUT:   %Main.import_ref.65b = import_ref Main//b, loc4_14, unloaded
+// CHECK:STDOUT:   %Main.import_ref.121 = import_ref Main//b, loc5_25, unloaded
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -131,6 +132,7 @@ impl C as B {}
 // CHECK:STDOUT: constraint @B [from "b.carbon"] {
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Self = imports.%Main.import_ref.65b
+// CHECK:STDOUT:   extend imports.%Main.import_ref.121
 // CHECK:STDOUT:
 // CHECK:STDOUT: !requires:
 // CHECK:STDOUT:   @B.Self.binding.as_type.impls.I.type.require {

toolchain/check/testdata/named_constraint/require.carbon

@@ -10,7 +10,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/named_constraint/require.carbon
 
-// --- fail_todo_extend.carbon
+// --- extend.carbon
 library "[[@TEST_NAME]]";
 
 interface Y {
@@ -23,13 +23,13 @@ constraint Z {
 }
 //@dump-sem-ir-end
 
-fn F(T:! Z) {
-  // CHECK:STDERR: fail_todo_extend.carbon:[[@LINE+4]]:3: error: member name `YY` not found [MemberNameNotFound]
-  // CHECK:STDERR:   T.YY();
-  // CHECK:STDERR:   ^~~~
-  // CHECK:STDERR:
+fn F(T:! Z, t: T) {
+  //@dump-sem-ir-begin
   T.YY();
   T.(Y.YY)();
+
+  t.YY();
+  //@dump-sem-ir-end
 }
 
 // --- implicit_self_impls.carbon
@@ -80,7 +80,7 @@ constraint Z {
 fn F(T:! Z) {
   // This should fail name lookup since Z does not extend Y.
   //
-  // CHECK:STDERR: fail_implicit_self_no_extend_name_lookup_fails.carbon:[[@LINE+4]]:3: error: member name `YY` not found [MemberNameNotFound]
+  // CHECK:STDERR: fail_implicit_self_no_extend_name_lookup_fails.carbon:[[@LINE+4]]:3: error: member name `YY` not found in `Z` [MemberNameNotFoundInInstScope]
   // CHECK:STDERR:   T.YY();
   // CHECK:STDERR:   ^~~~
   // CHECK:STDERR:
@@ -101,7 +101,7 @@ constraint Z {
 fn F(T:! Z) {
   // This should fail name lookup since Z does not extend Y.
   //
-  // CHECK:STDERR: fail_explicit_self_no_extend_name_lookup_fails.carbon:[[@LINE+4]]:3: error: member name `YY` not found [MemberNameNotFound]
+  // CHECK:STDERR: fail_explicit_self_no_extend_name_lookup_fails.carbon:[[@LINE+4]]:3: error: member name `YY` not found in `Z` [MemberNameNotFoundInInstScope]
   // CHECK:STDERR:   T.YY();
   // CHECK:STDERR:   ^~~~
   // CHECK:STDERR:
@@ -419,15 +419,25 @@ fn F() {
   () as N;
 }
 
-// CHECK:STDOUT: --- fail_todo_extend.carbon
+// CHECK:STDOUT: --- extend.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %Y.type: type = facet_type <@Y> [concrete]
+// CHECK:STDOUT:   %Y.YY.type: type = fn_type @Y.YY [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %Y.assoc_type: type = assoc_entity_type @Y [concrete]
+// CHECK:STDOUT:   %assoc0: %Y.assoc_type = assoc_entity element0, @Y.%Y.YY.decl [concrete]
 // CHECK:STDOUT:   %Z.type: type = facet_type <@Z> [concrete]
 // CHECK:STDOUT:   %Self.550: %Z.type = symbolic_binding Self, 0 [symbolic]
 // CHECK:STDOUT:   %Self.binding.as_type: type = symbolic_binding_type Self, 0, %Self.550 [symbolic]
 // CHECK:STDOUT:   %T: %Z.type = symbolic_binding T, 0 [symbolic]
 // CHECK:STDOUT:   %T.binding.as_type: type = symbolic_binding_type T, 0, %T [symbolic]
+// CHECK:STDOUT:   %pattern_type.349: type = pattern_type %T.binding.as_type [symbolic]
+// CHECK:STDOUT:   %Y.lookup_impl_witness: <witness> = lookup_impl_witness %T, @Y [symbolic]
+// CHECK:STDOUT:   %Y.facet: %Y.type = facet_value %T.binding.as_type, (%Y.lookup_impl_witness) [symbolic]
+// CHECK:STDOUT:   %.bb4: type = fn_type_with_self_type %Y.YY.type, %Y.facet [symbolic]
+// CHECK:STDOUT:   %impl.elem0: %.bb4 = impl_witness_access %Y.lookup_impl_witness, element0 [symbolic]
+// CHECK:STDOUT:   %specific_impl_fn: <specific function> = specific_impl_function %impl.elem0, @Y.YY(%Y.facet) [symbolic]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
@@ -445,10 +455,13 @@ fn F() {
 // CHECK:STDOUT:     %Self.as_type: type = facet_access_type @Z.%Self [symbolic = %Self.binding.as_type (constants.%Self.binding.as_type)]
 // CHECK:STDOUT:     %Y.ref: type = name_ref Y, file.%Y.decl [concrete = constants.%Y.type]
 // CHECK:STDOUT:   }
+// CHECK:STDOUT:   %.loc9: type = specific_constant @Z.Self.binding.as_type.impls.Y.type.require.%Y.ref, @Z.Self.binding.as_type.impls.Y.type.require(constants.%Self.550) [concrete = constants.%Y.type]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Self = %Self
 // CHECK:STDOUT:   .Y = <poisoned>
+// CHECK:STDOUT:   .YY = <poisoned>
+// CHECK:STDOUT:   extend %.loc9
 // CHECK:STDOUT:
 // CHECK:STDOUT: !requires:
 // CHECK:STDOUT:   @Z.Self.binding.as_type.impls.Y.type.require {
@@ -461,11 +474,55 @@ fn F() {
 // CHECK:STDOUT:   %Self.binding.as_type: type = symbolic_binding_type Self, 0, %Self [symbolic = %Self.binding.as_type (constants.%Self.binding.as_type)]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @F(%T.loc13_6.2: %Z.type) {
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT:   %Y.lookup_impl_witness: <witness> = lookup_impl_witness %T.loc13_6.1, @Y [symbolic = %Y.lookup_impl_witness (constants.%Y.lookup_impl_witness)]
+// CHECK:STDOUT:   %Y.facet.loc15: %Y.type = facet_value %T.binding.as_type, (%Y.lookup_impl_witness) [symbolic = %Y.facet.loc15 (constants.%Y.facet)]
+// CHECK:STDOUT:   %.loc15_4.2: type = fn_type_with_self_type constants.%Y.YY.type, %Y.facet.loc15 [symbolic = %.loc15_4.2 (constants.%.bb4)]
+// CHECK:STDOUT:   %impl.elem0.loc15_4.2: @F.%.loc15_4.2 (%.bb4) = impl_witness_access %Y.lookup_impl_witness, element0 [symbolic = %impl.elem0.loc15_4.2 (constants.%impl.elem0)]
+// CHECK:STDOUT:   %specific_impl_fn.loc15_4.2: <specific function> = specific_impl_function %impl.elem0.loc15_4.2, @Y.YY(%Y.facet.loc15) [symbolic = %specific_impl_fn.loc15_4.2 (constants.%specific_impl_fn)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn(%t.param: @F.%T.binding.as_type (%T.binding.as_type)) {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     %T.ref.loc15: %Z.type = name_ref T, %T.loc13_6.2 [symbolic = %T.loc13_6.1 (constants.%T)]
+// CHECK:STDOUT:     %T.as_type.loc15: type = facet_access_type %T.ref.loc15 [symbolic = %T.binding.as_type (constants.%T.binding.as_type)]
+// CHECK:STDOUT:     %.loc15_4.1: type = converted %T.ref.loc15, %T.as_type.loc15 [symbolic = %T.binding.as_type (constants.%T.binding.as_type)]
+// CHECK:STDOUT:     %YY.ref.loc15: %Y.assoc_type = name_ref YY, @Y.%assoc0 [concrete = constants.%assoc0]
+// CHECK:STDOUT:     %impl.elem0.loc15_4.1: @F.%.loc15_4.2 (%.bb4) = impl_witness_access constants.%Y.lookup_impl_witness, element0 [symbolic = %impl.elem0.loc15_4.2 (constants.%impl.elem0)]
+// CHECK:STDOUT:     %specific_impl_fn.loc15_4.1: <specific function> = specific_impl_function %impl.elem0.loc15_4.1, @Y.YY(constants.%Y.facet) [symbolic = %specific_impl_fn.loc15_4.2 (constants.%specific_impl_fn)]
+// CHECK:STDOUT:     %Y.YY.call.loc15: init %empty_tuple.type = call %specific_impl_fn.loc15_4.1()
+// CHECK:STDOUT:     %T.ref.loc16: %Z.type = name_ref T, %T.loc13_6.2 [symbolic = %T.loc13_6.1 (constants.%T)]
+// CHECK:STDOUT:     %Y.ref: type = name_ref Y, file.%Y.decl [concrete = constants.%Y.type]
+// CHECK:STDOUT:     %YY.ref.loc16: %Y.assoc_type = name_ref YY, @Y.%assoc0 [concrete = constants.%assoc0]
+// CHECK:STDOUT:     %T.as_type.loc16: type = facet_access_type %T.ref.loc16 [symbolic = %T.binding.as_type (constants.%T.binding.as_type)]
+// CHECK:STDOUT:     %Y.facet.loc16: %Y.type = facet_value %T.as_type.loc16, (constants.%Y.lookup_impl_witness) [symbolic = %Y.facet.loc15 (constants.%Y.facet)]
+// CHECK:STDOUT:     %.loc16: %Y.type = converted %T.ref.loc16, %Y.facet.loc16 [symbolic = %Y.facet.loc15 (constants.%Y.facet)]
+// CHECK:STDOUT:     %impl.elem0.loc16: @F.%.loc15_4.2 (%.bb4) = impl_witness_access constants.%Y.lookup_impl_witness, element0 [symbolic = %impl.elem0.loc15_4.2 (constants.%impl.elem0)]
+// CHECK:STDOUT:     %specific_impl_fn.loc16: <specific function> = specific_impl_function %impl.elem0.loc16, @Y.YY(constants.%Y.facet) [symbolic = %specific_impl_fn.loc15_4.2 (constants.%specific_impl_fn)]
+// CHECK:STDOUT:     %Y.YY.call.loc16: init %empty_tuple.type = call %specific_impl_fn.loc16()
+// CHECK:STDOUT:     %t.ref: @F.%T.binding.as_type (%T.binding.as_type) = name_ref t, %t
+// CHECK:STDOUT:     %YY.ref.loc18: %Y.assoc_type = name_ref YY, @Y.%assoc0 [concrete = constants.%assoc0]
+// CHECK:STDOUT:     %impl.elem0.loc18: @F.%.loc15_4.2 (%.bb4) = impl_witness_access constants.%Y.lookup_impl_witness, element0 [symbolic = %impl.elem0.loc15_4.2 (constants.%impl.elem0)]
+// CHECK:STDOUT:     %specific_impl_fn.loc18: <specific function> = specific_impl_function %impl.elem0.loc18, @Y.YY(constants.%Y.facet) [symbolic = %specific_impl_fn.loc15_4.2 (constants.%specific_impl_fn)]
+// CHECK:STDOUT:     %Y.YY.call.loc18: init %empty_tuple.type = call %specific_impl_fn.loc18()
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: specific @Z.Self.binding.as_type.impls.Y.type.require(constants.%Self.550) {
 // CHECK:STDOUT:   %Self => constants.%Self.550
 // CHECK:STDOUT:   %Self.binding.as_type => constants.%Self.binding.as_type
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: specific @F(constants.%T) {
+// CHECK:STDOUT:   %T.loc13_6.1 => constants.%T
+// CHECK:STDOUT:   %T.binding.as_type => constants.%T.binding.as_type
+// CHECK:STDOUT:   %pattern_type => constants.%pattern_type.349
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: specific @Z.Self.binding.as_type.impls.Y.type.require(constants.%T) {
 // CHECK:STDOUT:   %Self => constants.%T
 // CHECK:STDOUT:   %Self.binding.as_type => constants.%T.binding.as_type