Index: lib/IR/Verifier.cpp
===================================================================
--- lib/IR/Verifier.cpp
+++ lib/IR/Verifier.cpp
@@ -1054,24 +1054,6 @@
                               Attribute::Builtin),
           "Attribute 'builtin' can only be applied to a callsite.", &F);
 
-  // Check that this function meets the restrictions on this calling convention.
-  switch (F.getCallingConv()) {
-  default:
-    break;
-  case CallingConv::C:
-    break;
-  case CallingConv::Fast:
-  case CallingConv::Cold:
-  case CallingConv::X86_FastCall:
-  case CallingConv::X86_ThisCall:
-  case CallingConv::Intel_OCL_BI:
-  case CallingConv::PTX_Kernel:
-  case CallingConv::PTX_Device:
-    Assert1(!F.isVarArg(),
-            "Varargs functions must have C calling conventions!", &F);
-    break;
-  }
-
   bool isLLVMdotName = F.getName().size() >= 5 &&
                        F.getName().substr(0, 5) == "llvm.";
 
Index: test/Verifier/musttail-valid.ll
===================================================================
--- test/Verifier/musttail-valid.ll
+++ test/Verifier/musttail-valid.ll
@@ -14,3 +14,21 @@
   %w = bitcast i8* %v to i32*
   ret i32* %w
 }
+
+declare x86_thiscallcc void @varargs_thiscall(i8*, ...)
+define x86_thiscallcc void @varargs_thiscall_thunk(i8* %this, ...) {
+  musttail call x86_thiscallcc void (i8*, ...)* @varargs_thiscall(i8* %this, ...)
+  ret void
+}
+
+declare x86_fastcallcc void @varargs_fastcall(i8*, ...)
+define x86_fastcallcc void @varargs_fastcall_thunk(i8* %this, ...) {
+  musttail call x86_fastcallcc void (i8*, ...)* @varargs_fastcall(i8* %this, ...)
+  ret void
+}
+
+; The verifier checks shouldn't be looking for musttail in the function. The
+; function might just abort. Doing a normal return is just UB.
+define x86_thiscallcc void @varargs_thiscall_unreachable(i8* %this, ...) {
+  unreachable
+}