[clang] Fix implicit integer conversion for opaque enums declared in class templates

clang/docs/ReleaseNotes.rst

@@ -938,6 +938,9 @@ Miscellaneous Clang Crashes Fixed
 - Fixed internal assertion firing when a declaration in the implicit global
   module is found through ADL. (GH#109879)
 
+- Fixed a crash when an unscoped enumeration declared by an opaque-enum-declaration within a class template
+  with a dependent underlying type is subject to integral promotion. (#GH117960)
+
 OpenACC Specific Changes
 ------------------------
 

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -1571,6 +1571,19 @@ Decl *TemplateDeclInstantiator::VisitEnumDecl(EnumDecl *D) {
         Enum->setIntegerType(SemaRef.Context.IntTy);
       else
         Enum->setIntegerTypeSourceInfo(NewTI);
+
+      // C++23 [conv.prom]p4
+      // if integral promotion can be applied to its underlying type, a prvalue
+      // of an unscoped enumeration type whose underlying type is fixed can also
+      // be converted to a prvalue of the promoted underlying type.
+      //
+      // FIXME: that logic is already implemented in ActOnEnumBody, factor out
+      // into (Re)BuildEnumBody.
+      QualType UnderlyingType = Enum->getIntegerType();
+      Enum->setPromotionType(
+          SemaRef.Context.isPromotableIntegerType(UnderlyingType)
+              ? SemaRef.Context.getPromotedIntegerType(UnderlyingType)
+              : UnderlyingType);
     } else {
       assert(!D->getIntegerType()->isDependentType()
              && "Dependent type without type source info");

clang/test/SemaCXX/opaque-enum-declaration-in-class-template.cpp

@@ -0,0 +1,214 @@
+// RUN: %clang_cc1 -std=c++11 -Wredeclared-class-member -Wconstant-conversion -Wdeprecated-declarations -Wc++11-narrowing -fsyntax-only %s -verify
+// RUN: %clang_cc1 -std=c++14 -Wredeclared-class-member -Wconstant-conversion -Wdeprecated-declarations -Wc++11-narrowing -fsyntax-only %s -verify
+// RUN: %clang_cc1 -std=c++20 -Wredeclared-class-member -Wconstant-conversion -Wdeprecated-declarations -Wc++11-narrowing -fsyntax-only %s -verify
+
+// Test that opaque-enum-declarations are handled correctly w.r.t integral promotions.
+// The key sections in the C++11 standard are:
+// C++11 [dcl.enum]p3: An enumeration declared by an opaque-enum-declaration
+// has a fixed underlying type and is a complete type.
+// C++11 [conv.prom]: A prvalue of an unscoped enumeration type whose underlying type
+// is fixed ([dcl.enum]) can be converted to a prvalue of its underlying type.
+
+// This program causes clang 19 and earlier to crash because
+// EnumDecl::PromotionType has not been set on the instantiated enum.
+// See GitHub Issue #117960.
+namespace Issue117960 {
+template <typename T>
+struct A {
+  enum E : T;
+};
+
+int b = A<int>::E{} + 0;
+}
+
+
+namespace test {
+template <typename T1, typename T2>
+struct IsSame {
+  static constexpr bool check() { return false; }
+};
+
+template <typename T>
+struct IsSame<T, T> {
+  static constexpr bool check() { return true; }
+};
+}  // namespace test
+
+
+template <typename T>
+struct S1 {
+  enum E : T;
+};
+// checks if EnumDecl::PromotionType is set
+int X1 = S1<int>::E{} + 0;
+int Y1 = S1<unsigned>::E{} + 0;
+static_assert(test::IsSame<decltype(S1<int>::E{}+0), int>::check(), "");
+static_assert(test::IsSame<decltype(S1<unsigned>::E{}+0), unsigned>::check(), "");
+char Z1 = S1<unsigned>::E(-1) + 0; // expected-warning{{implicit conversion from 'unsigned int' to 'char'}}
+
+template <typename Traits>
+struct S2 {
+  enum E : typename Traits::IntegerType;
+};
+
+template <typename T>
+struct Traits {
+  typedef T IntegerType;
+};
+
+int X2 = S2<Traits<int>>::E{} + 0;
+int Y2 = S2<Traits<unsigned>>::E{} + 0;
+static_assert(test::IsSame<decltype(S2<Traits<int>>::E{}+0), int>::check(), "");
+static_assert(test::IsSame<decltype(S2<Traits<unsigned>>::E{}+0), unsigned>::check(), "");
+// C++11 [conv.prom]p4:
+// A prvalue of an unscoped enumeration type whose underlying type is fixed can be converted to a
+// prvalue of its underlying type. Moreover, if integral promotion can be applied to its underlying type, a
+// prvalue of an unscoped enumeration type whose underlying type is fixed can also be converted to a prvalue
+// of the promoted underlying type.
+static_assert(test::IsSame<decltype(S2<Traits<char>>::E{}+char(0)), int>::check(), "");
+
+
+template <typename T>
+struct S3 {
+  enum E : unsigned;
+};
+
+int X3 = S3<float>::E{} + 0;
+
+// fails in clang 19 and earlier (see the discussion on GitHub Issue #117960):
+static_assert(test::IsSame<decltype(S3<float>::E{}+0), unsigned>::check(), "");
+
+template <typename T>
+struct S4 {
+  enum E1 : char;
+  enum E2 : T;
+};
+
+int X4 = S4<char>::E1{} + '\0';
+int Y4 = S4<char>::E2{} + '\0';
+
+template <typename T>
+struct S5 {
+  enum class E1 : char;
+  enum class E2 : T;
+};
+
+int X5 = S5<char>::E1{} + '\0'; // expected-error{{invalid operands to binary expression}}
+int Y5 = S5<char>::E2{} + '\0'; // expected-error{{invalid operands to binary expression}}
+
+
+template <typename T>
+struct S6 {
+  enum E1 : T;
+  enum E2 : E1; // expected-error{{invalid underlying type}}
+};
+
+template struct S6<int>; // expected-note{{in instantiation of template class 'S6<int>' requested here}}
+
+
+template <typename T>
+struct S7 {
+  enum E : T;
+  enum E : T { X, Y, Z }; // expected-note{{previous declaration is here}}
+  enum E : T; // expected-warning{{class member cannot be redeclared}}
+};
+
+template struct S7<int>;
+
+template <typename T>
+struct S8 {
+  enum E : char;
+  enum E : char { X, Y, Z }; // expected-note{{previous declaration is here}}
+  enum E : char; // expected-warning{{class member cannot be redeclared}}
+};
+
+template struct S8<float>;
+
+template <typename T>
+struct S9 {
+  enum class E1 : T;
+  enum class E1 : T { X, Y, Z }; // expected-note{{previous declaration is here}}
+  enum class E1 : T; // expected-warning{{class member cannot be redeclared}}
+  enum class E2 : char;
+  enum class E2 : char { X, Y, Z }; // expected-note{{previous declaration is here}}
+  enum class E2 : char; // expected-warning{{class member cannot be redeclared}}
+};
+
+template struct S9<int>;
+
+#if defined(__cplusplus) && __cplusplus >= 201402L
+template <typename T>
+struct S10 {
+  enum [[deprecated("for reasons")]] E : T; // expected-note{{explicitly marked deprecated here}}
+};
+
+int X10 = S10<int>::E{} + 0; // expected-warning{{deprecated: for reasons}}
+#endif
+
+template <typename T>
+struct S11 {};
+
+template <>
+struct S11<unsigned> {
+  enum E : unsigned;
+};
+
+unsigned X11 = S11<unsigned>::E{} + 0u;
+
+#if defined(__cplusplus) && __cplusplus >= 201402L
+template <typename T>
+struct S12 {
+  enum [[deprecated("for reasons")]] E1 : T; // expected-note{{explicitly marked deprecated here}}
+  enum [[deprecated("for reasons")]] E2 : T;
+};
+
+template <>
+struct S12<float> {
+  enum E1 : unsigned;
+  enum E2 : unsigned;
+};
+
+unsigned X12 = S12<float>::E1{} + 0u;
+unsigned Y12 = S12<float>::E2{} + 0u;
+int Z12 = S12<int>::E1{} + 0; // expected-warning{{deprecated: for reasons}}
+#endif
+
+template <typename T>
+struct S13 {
+  enum __attribute__((packed)) E { X, Y };
+};
+
+static_assert(sizeof(S13<int>::E) == 1, "");
+
+template<typename T>
+struct S14 {
+  enum E : float; // expected-error {{invalid underlying type}}
+};
+
+template<typename T>
+struct S15 {
+  enum E : T; // expected-error {{invalid underlying type}}
+};
+
+template struct S15<float>; // expected-note {{in instantiation of template class 'S15<float>' requested here}}
+
+
+
+
+template <typename T>
+int f1() {
+  enum E : T;
+  return E{} + 0;
+}
+
+int F1 = f1<int>();
+
+template <typename T>
+int f2() {
+  struct LocalClass {
+    enum E : T;
+  };
+  return typename LocalClass::E{} + 0;
+}
+
+int F2 = f2<int>();