diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2985,6 +2985,9 @@
 def warn_declspec_allocator_nonpointer : Warning<
   "ignoring __declspec(allocator) because the function return type %0 is not "
   "a pointer or reference type">, InGroup<IgnoredAttributes>;
+def err_cconv_incomplete_param_type : Error<
+  "parameter %0 must have a complete type to use function %1 with the %2 "
+  "calling convention">;
 
 def ext_cannot_use_trivial_abi : ExtWarn<
   "'trivial_abi' cannot be applied to %0">, InGroup<IgnoredAttributes>;
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -14793,6 +14793,74 @@
   llvm_unreachable("Invalid context");
 }
 
+/// Return true if this function has a calling convention that requires mangling
+/// in the size of the parameter pack.
+static bool funcHasParameterSizeMangling(Sema &S, FunctionDecl *FD) {
+  // Do nothing on non-Windows, non-x86/x64 platforms.
+  const llvm::Triple &TT = S.Context.getTargetInfo().getTriple();
+  if (!TT.isOSWindows() || (TT.getArch() != llvm::Triple::x86 &&
+                            TT.getArch() != llvm::Triple::x86_64))
+    return false;
+
+  // Stdcall, fastcall, and vectorcall need this special treatment.
+  CallingConv CC = FD->getType()->castAs<FunctionType>()->getCallConv();
+  switch (CC) {
+  case CC_X86StdCall:
+  case CC_X86FastCall:
+  case CC_X86VectorCall:
+    return true;
+  default:
+    break;
+  }
+  return false;
+}
+
+/// Require that all of the parameter types of function be complete. Normally,
+/// parameter types are only required to be complete when a function is called
+/// or defined, but to mangle functions with certain calling conventions, the
+/// mangler needs to know the size of the parameter list. In this situation,
+/// MSVC doesn't emit an error or instantiate templates. Instead, MSVC mangles
+/// the function as _foo@0, i.e. zero bytes of parameters, which will usually
+/// result in a linker error. Clang doesn't implement this behavior, and instead
+/// attempts to error at compile time.
+static void CheckCompleteParameterTypesForMangler(Sema &S, FunctionDecl *FD,
+                                                  SourceLocation Loc) {
+  class ParamIncompleteTypeDiagnoser : public Sema::TypeDiagnoser {
+    FunctionDecl *FD;
+    ParmVarDecl *Param;
+
+  public:
+    ParamIncompleteTypeDiagnoser(FunctionDecl *FD, ParmVarDecl *Param)
+        : FD(FD), Param(Param) {}
+
+    void diagnose(Sema &S, SourceLocation Loc, QualType T) override {
+      CallingConv CC = FD->getType()->castAs<FunctionType>()->getCallConv();
+      StringRef CCName;
+      switch (CC) {
+      case CC_X86StdCall:
+        CCName = "stdcall";
+        break;
+      case CC_X86FastCall:
+        CCName = "fastcall";
+        break;
+      case CC_X86VectorCall:
+        CCName = "vectorcall";
+        break;
+      default:
+        llvm_unreachable("CC does not need mangling");
+      }
+
+      S.Diag(Loc, diag::err_cconv_incomplete_param_type)
+          << Param->getDeclName() << FD->getDeclName() << CCName;
+    }
+  };
+
+  for (ParmVarDecl *Param : FD->parameters()) {
+    ParamIncompleteTypeDiagnoser Diagnoser(FD, Param);
+    S.RequireCompleteType(Loc, Param->getType(), Diagnoser);
+  }
+}
+
 namespace {
 enum class OdrUseContext {
   /// Declarations in this context are not odr-used.
@@ -15038,6 +15106,12 @@
         UndefinedButUsed.insert(std::make_pair(Func->getCanonicalDecl(), Loc));
     }
 
+    // Some x86 Windows calling conventions mangle the size of the parameter
+    // pack into the name. Computing the size of the parameters requires the
+    // parameter types to be complete. Check that now.
+    if (funcHasParameterSizeMangling(*this, Func))
+      CheckCompleteParameterTypesForMangler(*this, Func, Loc);
+
     Func->markUsed(Context);
 
     if (LangOpts.OpenMP && LangOpts.OpenMPIsDevice)
diff --git a/clang/test/SemaCXX/calling-conv-complete-params.cpp b/clang/test/SemaCXX/calling-conv-complete-params.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/calling-conv-complete-params.cpp
@@ -0,0 +1,30 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++11 -fms-extensions -verify -triple i686-pc-win32 %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++11 -fms-extensions -verify -triple x86_64-pc-win32 %s
+
+struct Foo; // expected-note 1+ {{forward declaration of 'Foo'}}
+
+void __stdcall fwd_std(struct Foo p);
+#ifdef _M_IX86
+// expected-error@+2 {{parameter 'p' must have a complete type to use function 'fwd_std' with the stdcall calling convention}}
+#endif
+void (__stdcall *fp_fwd_std)(struct Foo) = &fwd_std;
+
+void __fastcall fwd_fast(struct Foo p);
+#ifdef _M_IX86
+// expected-error@+2 {{parameter 'p' must have a complete type to use function 'fwd_fast' with the fastcall calling convention}}
+#endif
+void (__fastcall *fp_fwd_fast)(struct Foo) = &fwd_fast;
+
+void __vectorcall fwd_vector(struct Foo p);
+// expected-error@+1 {{parameter 'p' must have a complete type to use function 'fwd_vector' with the vectorcall calling convention}}
+void (__vectorcall *fp_fwd_vector)(struct Foo) = &fwd_vector;
+
+template <typename T> struct TemplateWrapper {
+  T field; // expected-error {{field has incomplete type 'Foo'}}
+};
+
+void __vectorcall tpl_ok(TemplateWrapper<int> p);
+void(__vectorcall *fp_tpl_ok)(TemplateWrapper<int>) = &tpl_ok;
+
+void __vectorcall tpl_fast(TemplateWrapper<Foo> p);
+void(__vectorcall *fp_tpl_fast)(TemplateWrapper<Foo>) = &tpl_fast; // expected-note {{requested here}}