Diagnose using named constraint's name inside its definition (#6906)

Using a named constraint inside itself is problematic:
- If there were not require decls written above, it identifies as an
empty set. This makes `Z(Self)` essentially disappear in the identified
facet type, which produces "no use of Self" diagnostics while the user
can see a use of Self in the code.
- It won't include require decls that are written after, and so `require
T impls Z` won't actually enforce that `T` impls all of `Z`.

Previously this was an error because using the named constraint would
require it to be identified, and it's not identified until it is
complete. But this will change in proposal #6902. So that proposal also
includes changes to preserve diagnostics for incorrect use of a named
constraint before it's complete, which is implemented here.

Discussed in open discussion [on
2026-03-12](https://docs.google.com/document/d/1mjllGO3ZCL4qGt9uJHUtcxKoHAGEY7Y999ie4EtBWB8/edit?tab=t.0#heading=h.1dvbbrp5a6t3).

The new tests exposed a bug where we're not copying named constraints in
a facet type on the RHS of `where .Self impls` into the facet type on
the left, which is now fixed. The
`fail_require_impls_incomplete_self_in_period_self_impls.carbon` test
would not diagnose its error without this fix.

toolchain/check/eval.cpp

@@ -2494,6 +2494,10 @@ auto TryEvalTypedInst<SemIR::WhereExpr>(EvalContext& eval_context,
                                more_info.extend_constraints);
             llvm::append_range(info.self_impls_constraints,
                                more_info.self_impls_constraints);
+            llvm::append_range(info.self_impls_named_constraints,
+                               more_info.extend_named_constraints);
+            llvm::append_range(info.self_impls_named_constraints,
+                               more_info.self_impls_named_constraints);
             // Other requirements are copied in.
             llvm::append_range(info.rewrite_constraints,
                                more_info.rewrite_constraints);

toolchain/check/handle_require.cpp

@@ -17,6 +17,7 @@
 #include "toolchain/parse/node_ids.h"
 #include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/named_constraint.h"
+#include "toolchain/sem_ir/specific_named_constraint.h"
 #include "toolchain/sem_ir/type_iterator.h"
 #include "toolchain/sem_ir/typed_insts.h"
 
@@ -209,6 +210,36 @@ static auto ValidateRequire(Context& context, SemIR::LocId loc_id,
     return std::nullopt;
   }
 
+  if (auto named_constraint =
+          context.insts().TryGetAs<SemIR::NamedConstraintWithSelfDecl>(
+              scope_inst_id)) {
+    const auto& constraint_facet_type_info =
+        context.facet_types().Get(constraint_facet_type->facet_type_id);
+    // TODO: Handle other impls named constraints for the
+    // RequireImplsReferenceCycle diagnostic.
+    if (constraint_facet_type_info.other_requirements) {
+      context.TODO(constraint_inst_id,
+                   "facet type has constraints that we don't handle yet");
+      return std::nullopt;
+    }
+    auto named_constraints = llvm::concat<const SemIR::SpecificNamedConstraint>(
+        constraint_facet_type_info.extend_named_constraints,
+        constraint_facet_type_info.self_impls_named_constraints);
+    for (auto c : named_constraints) {
+      if (c.named_constraint_id == named_constraint->named_constraint_id) {
+        const auto& named_constraint =
+            context.named_constraints().Get(c.named_constraint_id);
+        CARBON_DIAGNOSTIC(RequireImplsReferenceCycle, Error,
+                          "facet type in `require` declaration refers to the "
+                          "named constraint `{0}` from within its definition",
+                          SemIR::NameId);
+        context.emitter().Emit(constraint_inst_id, RequireImplsReferenceCycle,
+                               named_constraint.name_id);
+        return std::nullopt;
+      }
+    }
+  }
+
   auto identified_facet_type_id = RequireIdentifiedFacetType(
       context, SemIR::LocId(constraint_inst_id), self_constant_value_id,
       *constraint_facet_type, [&](auto& builder) {

toolchain/check/testdata/named_constraint/require.carbon

@@ -175,34 +175,82 @@ constraint Z {
 // --- fail_require_impls_incomplete_self.carbon
 library "[[@TEST_NAME]]";
 
-//@dump-sem-ir-begin
 constraint Z {
-  // CHECK:STDERR: fail_require_impls_incomplete_self.carbon:[[@LINE+7]]:17: error: facet type `Z` cannot be identified in `require` declaration [RequireImplsUnidentifiedFacetType]
+  // CHECK:STDERR: fail_require_impls_incomplete_self.carbon:[[@LINE+4]]:17: error: facet type in `require` declaration refers to the named constraint `Z` from within its definition [RequireImplsReferenceCycle]
   // CHECK:STDERR:   require impls Z;
   // CHECK:STDERR:                 ^
-  // CHECK:STDERR: fail_require_impls_incomplete_self.carbon:[[@LINE-4]]:1: note: constraint is currently being defined [NamedConstraintIncompleteWithinDefinition]
-  // CHECK:STDERR: constraint Z {
-  // CHECK:STDERR: ^~~~~~~~~~~~~~
   // CHECK:STDERR:
   require impls Z;
 }
-//@dump-sem-ir-end
+
+// --- fail_require_impls_incomplete_self_forward_declared.carbon
+library "[[@TEST_NAME]]";
+
+constraint Z;
+
+constraint Z {
+  // CHECK:STDERR: fail_require_impls_incomplete_self_forward_declared.carbon:[[@LINE+4]]:17: error: facet type in `require` declaration refers to the named constraint `Z` from within its definition [RequireImplsReferenceCycle]
+  // CHECK:STDERR:   require impls Z;
+  // CHECK:STDERR:                 ^
+  // CHECK:STDERR:
+  require impls Z;
+}
+
+// --- fail_require_impls_incomplete_self_in_period_self_impls.carbon
+library "[[@TEST_NAME]]";
+
+constraint Z {
+  // CHECK:STDERR: fail_require_impls_incomplete_self_in_period_self_impls.carbon:[[@LINE+4]]:17: error: facet type in `require` declaration refers to the named constraint `Z` from within its definition [RequireImplsReferenceCycle]
+  // CHECK:STDERR:   require impls type where .Self impls Z;
+  // CHECK:STDERR:                 ^~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  require impls type where .Self impls Z;
+}
+
+// --- fail_require_impls_incomplete_self_in_class_impls.carbon
+library "[[@TEST_NAME]]";
+
+class C;
+constraint Z {
+  // TODO: This should be a RequireImplsReferenceCycle error since `Z` is being
+  // used inside `Z`.
+  // CHECK:STDERR: fail_require_impls_incomplete_self_in_class_impls.carbon:[[@LINE+4]]:17: error: semantics TODO: `facet type has constraints that we don't handle yet` [SemanticsTodo]
+  // CHECK:STDERR:   require impls type where C impls Z;
+  // CHECK:STDERR:                 ^~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  require impls type where C impls Z;
+}
+
+// --- fail_require_impls_incomplete_indirect.carbon
+library "[[@TEST_NAME]]";
+
+constraint Z;
+
+constraint Y {
+  // CHECK:STDERR: fail_require_impls_incomplete_indirect.carbon:[[@LINE+7]]:17: error: facet type `Z` cannot be identified in `require` declaration [RequireImplsUnidentifiedFacetType]
+  // CHECK:STDERR:   require impls Z;
+  // CHECK:STDERR:                 ^
+  // CHECK:STDERR: fail_require_impls_incomplete_indirect.carbon:[[@LINE-6]]:1: note: constraint was forward declared here [NamedConstraintForwardDeclaredHere]
+  // CHECK:STDERR: constraint Z;
+  // CHECK:STDERR: ^~~~~~~~~~~~~
+  // CHECK:STDERR:
+  require impls Z;
+}
+
+constraint Z {
+  require impls Y;
+}
 
 // --- fail_require_impls_incomplete_self_specific.carbon
 library "[[@TEST_NAME]]";
 
-//@dump-sem-ir-begin
 constraint Z(T:! type) {
-  // CHECK:STDERR: fail_require_impls_incomplete_self_specific.carbon:[[@LINE+7]]:19: error: facet type `Z(Self)` cannot be identified in `require` declaration [RequireImplsUnidentifiedFacetType]
+  // CHECK:STDERR: fail_require_impls_incomplete_self_specific.carbon:[[@LINE+4]]:19: error: facet type in `require` declaration refers to the named constraint `Z` from within its definition [RequireImplsReferenceCycle]
   // CHECK:STDERR:   require T impls Z(Self);
   // CHECK:STDERR:                   ^~~~~~~
-  // CHECK:STDERR: fail_require_impls_incomplete_self_specific.carbon:[[@LINE-4]]:1: note: constraint is currently being defined [NamedConstraintIncompleteWithinDefinition]
-  // CHECK:STDERR: constraint Z(T:! type) {
-  // CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~
   // CHECK:STDERR:
   require T impls Z(Self);
 }
-//@dump-sem-ir-end
 
 // --- fail_require_impls_without_self.carbon
 library "[[@TEST_NAME]]";
@@ -1064,87 +1112,6 @@ fn F() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: specific @Z.WithSelf(constants.%Self) {}
 // CHECK:STDOUT:
-// CHECK:STDOUT: --- fail_require_impls_incomplete_self.carbon
-// CHECK:STDOUT:
-// CHECK:STDOUT: constants {
-// CHECK:STDOUT:   %Z.type: type = facet_type <@Z> [concrete]
-// CHECK:STDOUT:   %Self: %Z.type = symbolic_binding Self, 0 [symbolic]
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: file {
-// CHECK:STDOUT:   %Z.decl: type = constraint_decl @Z [concrete = constants.%Z.type] {} {}
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: constraint @Z {
-// CHECK:STDOUT:   %Self: %Z.type = symbolic_binding Self, 0 [symbolic = constants.%Self]
-// CHECK:STDOUT:   %Z.WithSelf.decl = constraint_with_self_decl @Z [concrete]
-// CHECK:STDOUT:
-// CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .Self = %Self
-// CHECK:STDOUT:   .Z = <poisoned>
-// CHECK:STDOUT:   .Z = <poisoned>
-// CHECK:STDOUT:
-// CHECK:STDOUT: !requires:
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: specific @Z.WithSelf(constants.%Self) {}
-// CHECK:STDOUT:
-// CHECK:STDOUT: --- fail_require_impls_incomplete_self_specific.carbon
-// CHECK:STDOUT:
-// CHECK:STDOUT: constants {
-// CHECK:STDOUT:   %T: type = symbolic_binding T, 0 [symbolic]
-// CHECK:STDOUT:   %pattern_type: type = pattern_type type [concrete]
-// CHECK:STDOUT:   %Z.type.7a8: type = generic_named_constaint_type @Z [concrete]
-// CHECK:STDOUT:   %empty_struct: %Z.type.7a8 = struct_value () [concrete]
-// CHECK:STDOUT:   %Z.type.d682d6.1: type = facet_type <@Z, @Z(%T)> [symbolic]
-// CHECK:STDOUT:   %Self: %Z.type.d682d6.1 = symbolic_binding Self, 1 [symbolic]
-// CHECK:STDOUT:   %Self.binding.as_type: type = symbolic_binding_type Self, 1, %Self [symbolic]
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: file {
-// CHECK:STDOUT:   %Z.decl: %Z.type.7a8 = constraint_decl @Z [concrete = constants.%empty_struct] {
-// CHECK:STDOUT:     %T.patt: %pattern_type = symbolic_binding_pattern T, 0 [concrete]
-// CHECK:STDOUT:   } {
-// CHECK:STDOUT:     %.loc4_18.1: type = splice_block %.loc4_18.2 [concrete = type] {
-// CHECK:STDOUT:       <elided>
-// CHECK:STDOUT:       %.loc4_18.2: type = type_literal type [concrete = type]
-// CHECK:STDOUT:     }
-// CHECK:STDOUT:     %T.loc4_14.2: type = symbolic_binding T, 0 [symbolic = %T.loc4_14.1 (constants.%T)]
-// CHECK:STDOUT:   }
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: generic constraint @Z(%T.loc4_14.2: type) {
-// CHECK:STDOUT:   %T.loc4_14.1: type = symbolic_binding T, 0 [symbolic = %T.loc4_14.1 (constants.%T)]
-// CHECK:STDOUT:
-// CHECK:STDOUT: !definition:
-// CHECK:STDOUT:   %Z.type: type = facet_type <@Z, @Z(%T.loc4_14.1)> [symbolic = %Z.type (constants.%Z.type.d682d6.1)]
-// CHECK:STDOUT:   %Self.loc4_24.2: @Z.%Z.type (%Z.type.d682d6.1) = symbolic_binding Self, 1 [symbolic = %Self.loc4_24.2 (constants.%Self)]
-// CHECK:STDOUT:
-// CHECK:STDOUT:   constraint {
-// CHECK:STDOUT:     %Self.loc4_24.1: @Z.%Z.type (%Z.type.d682d6.1) = symbolic_binding Self, 1 [symbolic = %Self.loc4_24.2 (constants.%Self)]
-// CHECK:STDOUT:     %Z.WithSelf.decl = constraint_with_self_decl @Z [concrete]
-// CHECK:STDOUT:
-// CHECK:STDOUT:   !members:
-// CHECK:STDOUT:     .Self = %Self.loc4_24.1
-// CHECK:STDOUT:     .T = <poisoned>
-// CHECK:STDOUT:     .Z = <poisoned>
-// CHECK:STDOUT:     .T = <poisoned>
-// CHECK:STDOUT:     .Z = <poisoned>
-// CHECK:STDOUT:
-// CHECK:STDOUT:   !requires:
-// CHECK:STDOUT:   }
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: specific @Z(constants.%T) {
-// CHECK:STDOUT:   %T.loc4_14.1 => constants.%T
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: specific @Z.WithSelf(constants.%T, constants.%Self) {}
-// CHECK:STDOUT:
-// CHECK:STDOUT: specific @Z(constants.%Self.binding.as_type) {
-// CHECK:STDOUT:   %T.loc4_14.1 => constants.%Self.binding.as_type
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
 // CHECK:STDOUT: --- fail_require_impls_without_self.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {

toolchain/diagnostics/kind.def

@@ -381,6 +381,7 @@ CARBON_DIAGNOSTIC_KIND(RequireImplsMissingFacetType)
 CARBON_DIAGNOSTIC_KIND(RequireImplsMissingSelf)
 CARBON_DIAGNOSTIC_KIND(RequireImplsMissingSelfEmptyFacetType)
 CARBON_DIAGNOSTIC_KIND(RequireImplsIncompleteFacetType)
+CARBON_DIAGNOSTIC_KIND(RequireImplsReferenceCycle)
 CARBON_DIAGNOSTIC_KIND(RequireImplsUnidentifiedFacetType)
 CARBON_DIAGNOSTIC_KIND(RequireImplsNotImplemented)
 CARBON_DIAGNOSTIC_KIND(RequireInWrongScope)