diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -11095,6 +11095,11 @@
     if (FD->getName() != "DllMain")
       FD->setHasImplicitReturnZero(true);
 
+  if (FT->getCallConv() != CC_C) {
+    FT = Context.adjustFunctionType(FT, FT->getExtInfo().withCallingConv(CC_C));
+    FD->setType(QualType(FT, 0));
+  }
+
   if (!FD->isInvalidDecl() && FD->getDescribedFunctionTemplate()) {
     Diag(FD->getLocation(), diag::err_mainlike_template_decl) << FD;
     FD->setInvalidDecl();
diff --git a/clang/test/CodeGenCXX/default_calling_conv.cpp b/clang/test/CodeGenCXX/default_calling_conv.cpp
--- a/clang/test/CodeGenCXX/default_calling_conv.cpp
+++ b/clang/test/CodeGenCXX/default_calling_conv.cpp
@@ -1,10 +1,14 @@
-// RUN: %clang_cc1 -triple i386-unknown-linux-gnu -fdefault-calling-conv=cdecl -emit-llvm -o - %s | FileCheck %s --check-prefix=CDECL --check-prefix=ALL
-// RUN: %clang_cc1 -triple i786-unknown-linux-gnu -target-feature +sse4.2 -fdefault-calling-conv=fastcall -emit-llvm -o - %s | FileCheck %s --check-prefix=FASTCALL --check-prefix=ALL
-// RUN: %clang_cc1 -triple i486-unknown-linux-gnu -fdefault-calling-conv=stdcall -emit-llvm -o - %s | FileCheck %s --check-prefix=STDCALL --check-prefix=ALL
-// RUN: %clang_cc1 -triple i486-unknown-linux-gnu -mrtd -emit-llvm -o - %s | FileCheck %s --check-prefix=STDCALL --check-prefix=ALL
-// RUN: %clang_cc1 -triple i986-unknown-linux-gnu -fdefault-calling-conv=vectorcall -emit-llvm -o - %s | FileCheck %s --check-prefix=VECTORCALL --check-prefix=ALL
-// RUN: %clang_cc1 -triple i986-unknown-linux-gnu -fdefault-calling-conv=regcall -emit-llvm -o - %s | FileCheck %s --check-prefix=REGCALL --check-prefix=ALL
-
+// RUN: %clang_cc1 -triple i386-unknown-linux-gnu -fdefault-calling-conv=cdecl -emit-llvm -o - %s -DMAIN | FileCheck %s --check-prefix=CDECL --check-prefix=ALL
+// RUN: %clang_cc1 -triple i786-unknown-linux-gnu -target-feature +sse4.2 -fdefault-calling-conv=fastcall -emit-llvm -o - %s -DMAIN | FileCheck %s --check-prefix=FASTCALL --check-prefix=ALL
+// RUN: %clang_cc1 -triple i486-unknown-linux-gnu -fdefault-calling-conv=stdcall -emit-llvm -o - %s -DMAIN | FileCheck %s --check-prefix=STDCALL --check-prefix=ALL
+// RUN: %clang_cc1 -triple i486-unknown-linux-gnu -mrtd -emit-llvm -o - %s -DMAIN | FileCheck %s --check-prefix=STDCALL --check-prefix=ALL
+// RUN: %clang_cc1 -triple i986-unknown-linux-gnu -fdefault-calling-conv=vectorcall -emit-llvm -o - %s -DMAIN | FileCheck %s --check-prefix=VECTORCALL --check-prefix=ALL
+// RUN: %clang_cc1 -triple i986-unknown-linux-gnu -fdefault-calling-conv=regcall -emit-llvm -o - %s -DMAIN | FileCheck %s --check-prefix=REGCALL --check-prefix=ALL
+// RUN: %clang_cc1 -triple i386-pc-win32  -target-feature +sse4.2 -fdefault-calling-conv=fastcall -emit-llvm -o - %s -DWMAIN | FileCheck %s  --check-prefix=WMAIN
+// RUN: %clang_cc1 -triple i386-pc-win32  -target-feature +sse4.2 -fdefault-calling-conv=fastcall -emit-llvm -o - %s -DWINMAIN | FileCheck %s  --check-prefix=WINMAIN
+// RUN: %clang_cc1 -triple i386-pc-win32  -target-feature +sse4.2 -fdefault-calling-conv=fastcall -emit-llvm -o - %s -DWWINMAIN | FileCheck %s  --check-prefix=WWINMAIN
+// RUN: %clang_cc1 -triple i386-pc-win32  -target-feature +sse4.2 -fdefault-calling-conv=fastcall -emit-llvm -o - %s -DDLLMAIN | FileCheck %s  --check-prefix=DLLMAIN
+//
 // CDECL: define void @_Z5test1v
 // FASTCALL: define x86_fastcallcc void @_Z5test1v
 // STDCALL: define x86_stdcallcc void @_Z5test1v
@@ -46,7 +50,37 @@
   a.test_member();
 }
 
+#ifdef MAIN
 // ALL: define i32 @main
 int main() {
   return 1;
 }
+#endif // main
+
+#ifdef WMAIN
+// WMAIN: define dso_local i32 @wmain
+int wmain() {
+  return 1;
+}
+#endif // wmain
+
+#ifdef WINMAIN
+// WINMAIN: define dso_local i32 @WinMain
+int WinMain() {
+  return 1;
+}
+#endif // WinMain
+
+#ifdef WWINMAIN
+// WWINMAIN: define dso_local i32 @wWinMain
+int wWinMain() {
+  return 1;
+}
+#endif // wWinMain
+
+#ifdef DLLMAIN
+// DLLMAIN: define dso_local i32 @DllMain
+int DllMain() {
+  return 1;
+}
+#endif // DllMain