3 files changed, 21 insertions, 13 deletions
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 2f8295aa977..4aa9dc25ac9 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -3136,6 +3136,15 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD,
       // there but not here.
       NewTypeInfo = NewTypeInfo.withCallingConv(OldTypeInfo.getCC());
       RequiresAdjustment = true;
+    } else if (New->getBuiltinID()) {
+      // Calling Conventions on a Builtin aren't really useful and setting a
+      // default calling convention and cdecl'ing some builtin redeclarations is
+      // common, so warn and ignore the calling convention on the redeclaration.
+      Diag(New->getLocation(), diag::warn_cconv_ignored)
+          << FunctionType::getNameForCallConv(NewTypeInfo.getCC())
+          << (int)CallingConventionIgnoredReason::BuiltinFunction;
+      NewTypeInfo = NewTypeInfo.withCallingConv(OldTypeInfo.getCC());
+      RequiresAdjustment = true;
     } else {
       // Calling conventions aren't compatible, so complain.
       bool FirstCCExplicit = getCallingConvAttributedType(First->getType());
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index e8a77b3dc28..dcccb4b468a 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -4652,7 +4652,8 @@ bool Sema::CheckCallingConvAttr(const ParsedAttr &Attrs, CallingConv &CC,
   }
   if (A != TargetInfo::CCCR_OK) {
     if (A == TargetInfo::CCCR_Warning)
-      Diag(Attrs.getLoc(), diag::warn_cconv_ignored) << Attrs;
+      Diag(Attrs.getLoc(), diag::warn_cconv_ignored)
+          << Attrs << (int)CallingConventionIgnoredReason::ForThisTarget;
 
     // This convention is not valid for the target. Use the default function or
     // method calling convention.
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index 7c36abfa5b8..d4a6fc8ff64 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -6930,19 +6930,16 @@ static bool handleFunctionTypeAttr(TypeProcessingState &state, ParsedAttr &attr,
   if (!supportsVariadicCall(CC)) {
     const FunctionProtoType *FnP = dyn_cast<FunctionProtoType>(fn);
     if (FnP && FnP->isVariadic()) {
-      unsigned DiagID = diag::err_cconv_varargs;
-
       // stdcall and fastcall are ignored with a warning for GCC and MS
       // compatibility.
-      bool IsInvalid = true;
-      if (CC == CC_X86StdCall || CC == CC_X86FastCall) {
-        DiagID = diag::warn_cconv_varargs;
-        IsInvalid = false;
-      }
+      if (CC == CC_X86StdCall || CC == CC_X86FastCall)
+        return S.Diag(attr.getLoc(), diag::warn_cconv_ignored)
+               << FunctionType::getNameForCallConv(CC)
+               << (int)Sema::CallingConventionIgnoredReason::VariadicFunction;
 
-      S.Diag(attr.getLoc(), DiagID) << FunctionType::getNameForCallConv(CC);
-      if (IsInvalid) attr.setInvalid();
-      return true;
+      attr.setInvalid();
+      return S.Diag(attr.getLoc(), diag::err_cconv_varargs)
+             << FunctionType::getNameForCallConv(CC);
     }
   }
 
@@ -6997,8 +6994,9 @@ void Sema::adjustMemberFunctionCC(QualType &T, bool IsStatic, bool IsCtorOrDtor,
     // Issue a warning on ignored calling convention -- except of __stdcall.
     // Again, this is what MS compiler does.
     if (CurCC != CC_X86StdCall)
-      Diag(Loc, diag::warn_cconv_structors)
-          << FunctionType::getNameForCallConv(CurCC);
+      Diag(Loc, diag::warn_cconv_ignored)
+          << FunctionType::getNameForCallConv(CurCC)
+          << (int)Sema::CallingConventionIgnoredReason::ConstructorDestructor;
   // Default adjustment.
   } else {
     // Only adjust types with the default convention.  For example, on Windows