diff --git lib/Sema/SemaDecl.cpp lib/Sema/SemaDecl.cpp
index 4745376..951150b 100644
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -1946,6 +1946,9 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, Decl *OldD, Scope *S) {
   // later declared or defined without one, the second decl assumes the
   // calling convention of the first.
   //
+  // It's OK if a function is first declared without a calling convention,
+  // but is later declared or defined with the default calling convention.
+  //
   // For the new decl, we have to look at the NON-canonical type to tell the
   // difference between a function that really doesn't have a calling
   // convention and one that is declared cdecl. That's because in
@@ -1954,6 +1957,14 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, Decl *OldD, Scope *S) {
   //
   // Note also that we DO NOT return at this point, because we still have
   // other tests to run.
+  CallingConv ABIDefaultCC;
+  if (isa<CXXMethodDecl>(New) && dyn_cast<CXXMethodDecl>(New)->isInstance()) {
+    ABIDefaultCC = Context.getDefaultCXXMethodCallConv(New->isVariadic());
+  } else {
+    // Free C function or a static method.
+    ABIDefaultCC = (Context.getLangOpts().MRTD ? CC_X86StdCall : CC_C);
+  }
+
   const FunctionType *OldType = cast<FunctionType>(OldQType);
   const FunctionType *NewType = New->getType()->getAs<FunctionType>();
   FunctionType::ExtInfo OldTypeInfo = OldType->getExtInfo();
@@ -1963,6 +1974,10 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, Decl *OldD, Scope *S) {
       NewTypeInfo.getCC() == CC_Default) {
     NewTypeInfo = NewTypeInfo.withCallingConv(OldTypeInfo.getCC());
     RequiresAdjustment = true;
+  } else if (OldTypeInfo.getCC() == CC_Default &&
+             NewTypeInfo.getCC() == ABIDefaultCC) {
+    NewTypeInfo = NewTypeInfo.withCallingConv(OldTypeInfo.getCC());
+    RequiresAdjustment = true;
   } else if (!Context.isSameCallConv(OldTypeInfo.getCC(),
                                      NewTypeInfo.getCC())) {
     // Calling conventions really aren't compatible, so complain.
diff --git test/CodeGenCXX/microsoft-abi-default-cc.cpp test/CodeGenCXX/microsoft-abi-default-cc.cpp
new file mode 100644
index 0000000..d0d25ce
--- /dev/null
+++ test/CodeGenCXX/microsoft-abi-default-cc.cpp
@@ -0,0 +1,47 @@
+// RUN: %clang_cc1 -emit-llvm %s -o - | FileCheck -check-prefix GCABI %s
+// RUN: %clang_cc1 -emit-llvm %s -o - -DMS_ABI -cxx-abi microsoft -triple=i386-pc-win32 | FileCheck -check-prefix MSABI %s
+
+#ifdef MS_ABI
+# define METHOD_CC __thiscall
+#else
+# define METHOD_CC __attribute__ ((cdecl))
+#endif
+
+// Test that it's OK to have multiple function declarations with the default CC
+// both mentioned explicitly and implied.
+void foo();
+void __cdecl foo();
+void __cdecl foo() {}
+// GCABI: define void @_Z3foov()
+// MSABI: define void @"\01?foo@@YAXXZ"
+
+void __cdecl bar();
+void bar();
+void bar() {}
+// GCABI: define void @_Z3barv()
+// MSABI: define void @"\01?bar@@YAXXZ"
+
+// Test that it's OK to mark either the method declaration or method definition
+// with a default CC explicitly.
+class A {
+public:
+  void baz();
+  void METHOD_CC qux();
+
+  void static_baz();
+  void __cdecl static_qux();
+};
+
+void METHOD_CC A::baz() {}
+// GCABI: define void @_ZN1A3bazEv
+// MSABI: define x86_thiscallcc void @"\01?baz@A@@QAEXXZ"
+void A::qux() {}
+// GCABI: define void @_ZN1A3quxEv
+// MSABI: define x86_thiscallcc void @"\01?qux@A@@QAEXXZ"
+
+void __cdecl static_baz() {}
+// GCABI: define void @_Z10static_bazv
+// MSABI: define void @"\01?static_baz@@YAXXZ"
+void static_qux() {}
+// GCABI: define void @_Z10static_quxv
+// MSABI: define void @"\01?static_qux@@YAXXZ"