[clang] Prevent duplicated instantiation of enumerators of unscoped member enumerations (#124407)

This commit addresses a bug occurring when an unscoped member enumeration
of a class template is introduced with an opaque-enum-declaration and later
redeclared with an enum-specifier (per C++23 [class.mem] p6).
Previously, the enumerators, or EnumConstantDecl, of the enum-specifier
were instantiated at both declarations, leading to different issues:
* erroneous ambiguities when referencing the enumerators,
* duplicated diagnostics in the enumerator-list.

The issue is resolved by ensuring that enumerators are instantiated only
at the first instantiated declaration, analogous to nested classes.

Fixes #124405

Author: thebrandre

clang/docs/ReleaseNotes.rst

@@ -1003,6 +1003,7 @@ Bug Fixes to C++ Support
 - Fixed assertions or false compiler diagnostics in the case of C++ modules for
   lambda functions or inline friend functions defined inside templates (#GH122493).
 - Clang now rejects declaring an alias template with the same name as its template parameter. (#GH123423)
+- Fixed the rejection of valid code when referencing an enumerator of an unscoped enum member with a prior declaration. (#GH124405)
 - Fixed immediate escalation of non-dependent expressions. (#GH123405)
 - Fix type of expression when calling a template which returns an ``__array_rank`` querying a type depending on a
   template parameter. Now, such expression can be used with ``static_assert`` and ``constexpr``. (#GH123498)

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -1627,12 +1627,17 @@ Decl *TemplateDeclInstantiator::VisitEnumDecl(EnumDecl *D) {
   // specialization causes the implicit instantiation of the declarations, but
   // not the definitions of scoped member enumerations.
   //
-  // DR1484 clarifies that enumeration definitions inside of a template
+  // DR1484 clarifies that enumeration definitions inside a template
   // declaration aren't considered entities that can be separately instantiated
-  // from the rest of the entity they are declared inside of.
+  // from the rest of the entity they are declared inside.
   if (isDeclWithinFunction(D) ? D == Def : Def && !Enum->isScoped()) {
-    SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, Enum);
-    InstantiateEnumDefinition(Enum, Def);
+    // Prevent redundant instantiation of the enumerator-definition if the
+    // definition has already been instantiated due to a prior
+    // opaque-enum-declaration.
+    if (PrevDecl == nullptr) {
+      SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, Enum);
+      InstantiateEnumDefinition(Enum, Def);
+    }
   }
 
   return Enum;

clang/test/SemaCXX/member-enum-declarations.cpp

@@ -0,0 +1,112 @@
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only %s -verify
+// RUN: %clang_cc1 -std=c++14 -fsyntax-only %s -verify
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only %s -verify
+
+
+namespace ScopedEnumerations {
+
+template <typename T>
+struct S1 {
+  enum class E : T;
+};
+
+template <typename T>
+enum class S1<T>::E : T {
+  S1_X = 0x123
+};
+
+static_assert(static_cast<int>(S1<int>::E::S1_X) == 0x123, "");
+
+template <typename T>
+struct S2 {
+  static constexpr T f(int) { return 0; };
+  enum class E : T;
+  static constexpr T f(char) { return 1; };
+  enum class E : T { X = f(T{}) };
+};
+
+static_assert(static_cast<int>(S2<char>::E::X) == 1, "");
+
+template <typename T>
+struct S3 {
+  enum class E : T;
+  enum class E : T { X = 0x7FFFFF00 }; // expected-error {{cannot be narrowed to type 'char'}} expected-warning {{implicit conversion from 'int' to 'char'}}
+};
+template struct S3<char>; // expected-note {{in instantiation}}
+
+template <typename T>
+struct S4 {
+  enum class E : T;
+  enum class E : T { S4_X = 5 };
+};
+
+auto x4 = S4<int>::E::S4_X;
+
+template <typename T>
+T f1() {
+  enum class E : T { X_F1, Y_F1, Z_F1 };
+  return X_F1;  // expected-error {{use of undeclared identifier 'X_F1'}}
+}
+
+const int resf1 = f1<int>();
+
+}
+
+
+namespace UnscopedEnumerations {
+
+template <typename T>
+struct S1 {
+  enum E : T;
+};
+
+template <typename T>
+enum S1<T>::E : T {
+  S1_X = 0x123
+};
+
+static_assert(static_cast<int>(S1<int>::S1_X) == 0x123, "");
+
+template <typename T>
+struct S2 {
+  static constexpr T f(int) { return 0; };
+  enum E : T;
+  static constexpr T f(char) { return 1; };
+  enum E : T { S2_X = f(T{}) };
+};
+
+static_assert(static_cast<int>(S2<char>::E::S2_X) == 1, "");
+
+template <typename T>
+struct S3 {
+  enum E : T;
+  enum E : T { S3_X = 0x7FFFFF00 }; // expected-error {{cannot be narrowed to type 'char'}} expected-warning {{implicit conversion from 'int' to 'char'}}
+};
+template struct S3<char>; // expected-note {{in instantiation of template class}}
+
+template <typename T>
+struct S4 {
+  enum E : T;
+  enum E : T { S4_X = 5 };
+};
+
+auto x4 = S4<int>::S4_X;
+
+template <typename T>
+struct S5 {
+  enum E : T;
+  T S5_X = 5; // expected-note {{previous definition is here}}
+  enum E : T { S5_X = 5 }; // expected-error {{redefinition of 'S5_X'}}
+};
+
+
+template <typename T>
+T f1() {
+  enum E : T { X_F2, Y_F2, Z_F2 };
+  return X_F2;
+}
+
+const int resf1 = f1<int>();
+
+}
+