[Clang][Sema] Fix type of enumerators in incomplete enumerations

clang/lib/AST/Expr.cpp

@@ -264,11 +264,14 @@ namespace {
 }
 
 QualType Expr::getEnumCoercedType(const ASTContext &Ctx) const {
-  if (isa<EnumType>(this->getType()))
-    return this->getType();
-  else if (const auto *ECD = this->getEnumConstantDecl())
-    return Ctx.getTypeDeclType(cast<EnumDecl>(ECD->getDeclContext()));
-  return this->getType();
+  if (isa<EnumType>(getType()))
+    return getType();
+  if (const auto *ECD = getEnumConstantDecl()) {
+    const auto *ED = cast<EnumDecl>(ECD->getDeclContext());
+    if (ED->isCompleteDefinition())
+      return Ctx.getTypeDeclType(ED);
+  }
+  return getType();
 }
 
 SourceLocation Expr::getExprLoc() const {

clang/test/Sema/enum-constant-type.cpp

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -x c++ -fsyntax-only -verify %s -Wenum-compare
+// expected-no-diagnostics
+
+enum E1 {
+  E11 = 0
+};
+
+enum E2 {
+  E21 = 0,
+  E22 = E11,
+  E23 = E21 + E22
+};

clang/test/Sema/warn-compare-enum-types-mismatch.c

@@ -1,12 +1,21 @@
 // RUN: %clang_cc1 -x c -fsyntax-only -verify -Wenum-compare -Wno-unused-comparison %s
 // RUN: %clang_cc1 -x c++ -fsyntax-only -verify -Wenum-compare -Wno-unused-comparison %s
 
+// In C enumerators (i.e enumeration constants) have type int (until C23). In
+// order to support diagnostics such as -Wenum-compare we pretend they have the
+// type of their enumeration.
+
 typedef enum EnumA {
   A
 } EnumA;
 
 enum EnumB {
-  B
+  B,
+  B1 = 1,
+  // In C++ this comparison doesnt warn as enumerators dont have the type of
+  // their enumeration before the closing brace. We mantain the same behavior
+  // in C.
+  B2 = A == B1
 };
 
 enum {