Index: clang/lib/Sema/SemaOpenMP.cpp
===================================================================
--- clang/lib/Sema/SemaOpenMP.cpp
+++ clang/lib/Sema/SemaOpenMP.cpp
@@ -7052,7 +7052,8 @@
 
   QualType AdjustedFnType = FD->getType();
   if (NumAppendArgs) {
-    if (isa<FunctionNoProtoType>(FD->getType())) {
+    const auto *PTy = AdjustedFnType->getAsAdjusted<FunctionProtoType>();
+    if (!PTy) {
       Diag(FD->getLocation(), diag::err_omp_declare_variant_prototype_required)
           << SR;
       return None;
@@ -7070,8 +7071,7 @@
       Diag(SR.getBegin(), diag::err_omp_interop_type_not_found) << SR;
       return None;
     }
-    QualType InteropType = QualType(TD->getTypeForDecl(), 0);
-    auto *PTy = cast<FunctionProtoType>(FD->getType());
+    QualType InteropType = Context.getTypeDeclType(TD);
     if (PTy->isVariadic()) {
       Diag(FD->getLocation(), diag::err_omp_append_args_with_varargs) << SR;
       return None;
Index: clang/test/OpenMP/declare_variant_clauses_ast_print.c
===================================================================
--- /dev/null
+++ clang/test/OpenMP/declare_variant_clauses_ast_print.c
@@ -0,0 +1,48 @@
+//RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -fopenmp -fopenmp-version=51 \
+//RUN:   -Wno-source-uses-openmp -Wno-openmp-clauses                       \
+//RUN:   -ast-print -o - %s | FileCheck %s --check-prefix=PRINT
+
+//RUN: %clang_cc1 -triple x86_64-pc-windows-msvc -fopenmp -fopenmp-version=51 \
+//RUN:   -Wno-source-uses-openmp -Wno-openmp-clauses -DWIN -fms-compatibility \
+//RUN:   -ast-print -o - %s | FileCheck %s --check-prefixes=PRINT,PRINTW
+
+//RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -fopenmp -fopenmp-version=51 \
+//RUN:   -Wno-source-uses-openmp -Wno-openmp-clauses                       \
+//RUN:   -ast-dump -o - %s | FileCheck %s --check-prefix=DUMP
+
+//RUN: %clang_cc1 -triple x86_64-pc-windows-msvc -fopenmp -fopenmp-version=51 \
+//RUN:   -Wno-source-uses-openmp -Wno-openmp-clauses -DWIN -fms-compatibility \
+//RUN:   -ast-dump -o - %s | FileCheck %s --check-prefixes=DUMP,DUMPW
+
+typedef void *omp_interop_t;
+
+#ifdef WIN
+//DUMPW: FunctionDecl{{.*}}win_foov
+//PRINTW: void win_foov(int n, double *y, void *interop_obj);
+void win_foov(int n, double *y, void *interop_obj);
+
+//DUMPW: FunctionDecl{{.*}}win_foo
+//DUMPW: OMPDeclareVariantAttr
+//DUMPW-NEXT: DeclRefExpr{{.*}}win_foov
+//PRINTW: #pragma omp declare variant(win_foov) match(construct={dispatch}, device={arch(x86_64)}) append_args(interop(targetsync))
+//PRINTW: void win_foo(int n, double *y);
+#pragma omp declare variant (win_foov) \
+  match(construct={dispatch}, device={arch(x86_64)}) \
+  append_args(interop(targetsync))
+void _cdecl win_foo(int n, double *y);
+#endif // WIN
+
+//DUMP: FunctionDecl{{.*}}c_foov
+//PRINT: void c_foov(int n, double *y, void *interop_obj);
+void c_foov(int n, double *y, void *interop_obj);
+
+//DUMP: FunctionDecl{{.*}}c_foo
+//DUMP: OMPDeclareVariantAttr
+//DUMP-NEXT: DeclRefExpr{{.*}}c_foov
+//PRINT: #pragma omp declare variant(c_foov) match(construct={dispatch}, device={arch(x86_64)}) append_args(interop(targetsync))
+//PRINT: void c_foo(int n, double *y);
+
+#pragma omp declare variant (c_foov) \
+  match(construct={dispatch}, device={arch(x86_64)}) \
+  append_args(interop(targetsync))
+void c_foo(int n, double *y);
Index: clang/test/OpenMP/declare_variant_clauses_messages.cpp
===================================================================
--- clang/test/OpenMP/declare_variant_clauses_messages.cpp
+++ clang/test/OpenMP/declare_variant_clauses_messages.cpp
@@ -3,6 +3,8 @@
 // RUN:  -DNO_INTEROP_T_DEF -o - %s
 // RUN: %clang_cc1 -verify -triple x86_64-unknown-linux -fopenmp -fopenmp-version=50 -std=c++11 -o - %s
 // RUN: %clang_cc1 -verify -triple x86_64-unknown-linux -fopenmp -fopenmp-version=51 -DC -x c -o - %s
+// RUN: %clang_cc1 -verify -triple x86_64-pc-windows-msvc -fms-compatibility \
+// RUN:  -fopenmp -fopenmp-version=51 -DC -DWIN -x c -o - %s
 
 #ifdef NO_INTEROP_T_DEF
 void foo_v1(float *, void *);
@@ -201,7 +203,14 @@
 #ifdef C
 void c_variant(omp_interop_t);
 // expected-error@+3 {{function with '#pragma omp declare variant' must have a prototype when 'append_args' is used}}
-#pragma omp declare variant(foo_v1)                            \
+#pragma omp declare variant(c_variant)                         \
    append_args(interop(target)) match(construct={dispatch})
 void c_base() {}
+#ifdef WIN
+void _cdecl win_c_variant(omp_interop_t);
+// expected-error@+3 {{function with '#pragma omp declare variant' must have a prototype when 'append_args' is used}}
+#pragma omp declare variant(win_c_variant)                     \
+   append_args(interop(target)) match(construct={dispatch})
+void _cdecl win_c_base() {}
+#endif // WIN
 #endif