Correct the behavior of va_arg checking in C++

Clang checks whether the type given to va_arg will automatically cause
undefined behavior, but this check was issuing false positives for
enumerations in C++. The issue turned out to be because
typesAreCompatible() in C++ checks whether the types are *the same*, so
this uses custom logic if the type compatibility check fails.

This issue was found by a user on code like:

typedef enum {
  CURLINFO_NONE,
  CURLINFO_EFFECTIVE_URL,
  CURLINFO_LASTONE = 60
} CURLINFO;

...

__builtin_va_arg(list, CURLINFO); // false positive warning

Given that C++ defers to C for the rules around va_arg, the behavior
should be the same in both C and C++ and not diagnose because int and
CURLINFO are "compatible enough" types for va_arg.

clang/lib/Sema/SemaExpr.cpp

@@ -15752,8 +15752,46 @@ ExprResult Sema::BuildVAArgExpr(SourceLocation BuiltinLoc,
     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()
+      // directly to test this. However, in C++, that checks for *same type*,
+      // which causes false positives when passing an enumeration type to
+      // va_arg. Instead, get the underlying type of the enumeration and pass
+      // that.
+      QualType UnderlyingType = TInfo->getType();
+      if (const auto *ET = UnderlyingType->getAs<EnumType>())
+        UnderlyingType = ET->getDecl()->getIntegerType();
+      if (Context.typesAreCompatible(PromoteType, UnderlyingType,
+                                     /*CompareUnqualified*/ true))
         PromoteType = QualType();
+
+      // If the types are still not compatible, we need to test whether the
+      // promoted type and the underlying type are the same except for
+      // signedness. Ask the AST for the correctly corresponding type and see
+      // if that's compatible.
+      if (!PromoteType.isNull() &&
+          PromoteType->isUnsignedIntegerType() !=
+              UnderlyingType->isUnsignedIntegerType()) {
+        UnderlyingType =
+            UnderlyingType->isUnsignedIntegerType()
+                ? Context.getCorrespondingSignedType(UnderlyingType)
+                : Context.getCorrespondingUnsignedType(UnderlyingType);
+        if (Context.typesAreCompatible(PromoteType, UnderlyingType,
+                                       /*CompareUnqualified*/ true))
+          PromoteType = QualType();
+      }
     }
     if (TInfo->getType()->isSpecificBuiltinType(BuiltinType::Float))
       PromoteType = Context.DoubleTy;

clang/test/SemaCXX/varargs.cpp

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -std=c++03 -verify %s
-// RUN: %clang_cc1 -std=c++11 -verify %s
+// RUN: %clang_cc1 -std=c++03 -Wno-c++11-extensions -triple i386-pc-unknown -verify %s
+// RUN: %clang_cc1 -std=c++11 -triple x86_64-apple-darwin9 -verify %s
 
 __builtin_va_list ap;
 
@@ -28,6 +28,33 @@ void record_context(int a, ...) {
   };
 }
 
+// Ensure the correct behavior for promotable type UB checking.
+void promotable(int a, ...) {
+  enum Unscoped1 { One = 0x7FFFFFFF };
+  (void)__builtin_va_arg(ap, Unscoped1); // ok
+
+  enum Unscoped2 { Two = 0xFFFFFFFF };
+  (void)__builtin_va_arg(ap, Unscoped2); // ok
+
+  enum class Scoped { Three };
+  (void)__builtin_va_arg(ap, Scoped); // ok
+
+  enum Fixed : int { Four };
+  (void)__builtin_va_arg(ap, Fixed); // ok
+
+  enum FixedSmall : char { Five };
+  (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 { Six };
+  (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.