Fix implicit integer conversion for enum declared in class template

Previously, the promotion type of the instantiated enum was set only in `Sema::ActOnEnumBody`,
which is not called if there are no curly braces are after the enum-base.
This fixes GitHub issue #117960.

Author: thebrandre

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -1620,6 +1620,9 @@ Decl *TemplateDeclInstantiator::VisitEnumDecl(EnumDecl *D) {
   if (isDeclWithinFunction(D) ? D == Def : Def && !Enum->isScoped()) {
     SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, Enum);
     InstantiateEnumDefinition(Enum, Def);
+  } else {
+    if (D->isFixed() && !Def)
+      Enum->setPromotionType(Enum->getIntegerType());
   }
 
   return Enum;

clang/test/SemaCXX/enum-base-in-class-template.cpp

@@ -0,0 +1,66 @@
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only %s -verify
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only %s -verify
+
+// 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(), "");
+
+
+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(), "");
+