Index: clang/lib/Sema/SemaExpr.cpp
===================================================================
--- clang/lib/Sema/SemaExpr.cpp
+++ clang/lib/Sema/SemaExpr.cpp
@@ -15750,7 +15750,27 @@
     QualType PromoteType;
     if (TInfo->getType()->isPromotableIntegerType()) {
       PromoteType = Context.getPromotedIntegerType(TInfo->getType());
-      if (Context.typesAreCompatible(PromoteType, TInfo->getType()))
+      // [cstdarg.syn]p1 defers the C++ behavior to what the C standard says,
+      // and C2x 7.16.1.1p2 says, in part:
+      //   If type is not compatible with the type of the actual next argument
+      //   (as promoted according to the default argument promotions), the
+      //   behavior is undefined, except for the following cases:
+      //     - both types are pointers to qualified or unqualified versions of
+      //       compatible types;
+      //     - one type is a signed integer type, the other type is the
+      //       corresponding unsigned integer type, and the value is
+      //       representable in both types;
+      //     - one type is pointer to qualified or unqualified void and the
+      //       other is a pointer to a qualified or unqualified character type.
+      // Given that type compatibility is the primary requirement (ignoring
+      // qualifications), you would think we could call typesAreCompatible() to
+      // test this. However, in C++, that checks for *same type*, which causes
+      // false positives when passing an enumeration type to va_arg. Instead,
+      // attempt to merge the types and see how well that works.
+      QualType Merged =
+          Context.mergeTypes(PromoteType, TInfo->getType(),
+                             /*OfBlockPointer*/ false, /*Unqualified*/ true);
+      if (!Merged.isNull())
         PromoteType = QualType();
     }
     if (TInfo->getType()->isSpecificBuiltinType(BuiltinType::Float))
Index: clang/test/SemaCXX/varargs.cpp
===================================================================
--- clang/test/SemaCXX/varargs.cpp
+++ clang/test/SemaCXX/varargs.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -std=c++03 -verify %s
+// RUN: %clang_cc1 -std=c++03 -Wno-c++11-extensions -verify %s
 // RUN: %clang_cc1 -std=c++11 -verify %s
 
 __builtin_va_list ap;
@@ -28,6 +28,30 @@
   };
 }
 
+// Ensure the correct behavior for promotable type UB checking.
+void promotable(int a, ...) {
+  enum Unscoped { One };
+  (void)__builtin_va_arg(ap, Unscoped); // ok
+
+  enum class Scoped { Two };
+  (void)__builtin_va_arg(ap, Scoped); // ok
+
+  enum Fixed : int { Three };
+  (void)__builtin_va_arg(ap, Fixed); // ok
+
+  enum FixedSmall : char { Four };
+  (void)__builtin_va_arg(ap, FixedSmall); // expected-warning {{second argument to 'va_arg' is of promotable type 'FixedSmall'; this va_arg has undefined behavior because arguments will be promoted to 'int'}}
+
+  enum FixedLarge : long long { Five };
+  (void)__builtin_va_arg(ap, FixedLarge); // ok
+
+  // Ensure that qualifiers are ignored.
+  (void)__builtin_va_arg(ap, const volatile int);  // ok
+
+  // Ensure that signed vs unsigned doesn't matter either.
+  (void)__builtin_va_arg(ap, unsigned int);
+}
+
 #if __cplusplus >= 201103L
 // We used to have bugs identifying the correct enclosing function scope in a
 // lambda.