1 files changed, 12 insertions, 0 deletions
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 9ce09288de1..4abbbebf808 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -7821,6 +7821,18 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
   }
 
   // Semantic checking for this function declaration (in isolation).
+
+  // Diagnose the use of X86 fastcall on unprototyped functions.
+  QualType NewQType = Context.getCanonicalType(NewFD->getType());
+  const FunctionType *NewType = cast<FunctionType>(NewQType);
+  if (isa<FunctionNoProtoType>(NewType)) {
+    FunctionType::ExtInfo NewTypeInfo = NewType->getExtInfo();
+    if (NewTypeInfo.getCC() == CC_X86FastCall)
+      Diag(NewFD->getLocation(), diag::err_cconv_knr)
+          << FunctionType::getNameForCallConv(CC_X86FastCall);
+    // TODO: Also diagnose unprototyped stdcall functions?
+  }
+
   if (getLangOpts().CPlusPlus) {
     // C++-specific checks.
     if (CXXConstructorDecl *Constructor = dyn_cast<CXXConstructorDecl>(NewFD)) {