Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -7458,11 +7458,16 @@
     // Determine whether the function was written with a
     // prototype. This true when:
     //   - there is a prototype in the declarator, or
-    //   - the type R of the function is some kind of typedef or other reference
-    //     to a type name (which eventually refers to a function type).
+    //   - the type R of the function is some kind of typedef or other non-
+    //     attributed reference to a type name (which eventually refers to a
+    //     function type).
+    const Type *NonAttributedFTy = R.getTypePtr();
+    while (const auto *AttrTy = NonAttributedFTy->getAs<AttributedType>()) {
+      NonAttributedFTy = AttrTy->getModifiedType().getTypePtr();
+    }
     bool HasPrototype =
       (D.isFunctionDeclarator() && D.getFunctionTypeInfo().hasPrototype) ||
-      (!isa<FunctionType>(R.getTypePtr()) && R->isFunctionProtoType());
+      (!isa<FunctionType>(NonAttributedFTy) && R->isFunctionProtoType());
 
     NewFD = FunctionDecl::Create(SemaRef.Context, DC,
                                  D.getLocStart(), NameInfo, R,
@@ -11950,6 +11955,11 @@
           !LangOpts.CPlusPlus) {
         TypeSourceInfo *TI = FD->getTypeSourceInfo();
         TypeLoc TL = TI->getTypeLoc();
+        // The type location may be attributed; strip the attributes to get to
+        // the function type location.
+        while (auto ATL = TL.getAs<AttributedTypeLoc>()) {
+          TL = ATL.getModifiedLoc();
+        }
         FunctionTypeLoc FTL = TL.castAs<FunctionTypeLoc>();
         Diag(FTL.getLParenLoc(), diag::warn_strict_prototypes) << 1;
       }
Index: test/Sema/knr-def-call.c
===================================================================
--- test/Sema/knr-def-call.c
+++ test/Sema/knr-def-call.c
@@ -39,3 +39,9 @@
   proto(42.1); // expected-warning{{implicit conversion from 'double' to 'int' changes value from 42.1 to 42}}
   (&proto)(42.1); // expected-warning{{implicit conversion from 'double' to 'int' changes value from 42.1 to 42}}
 }
+
+// PR31020
+void func(short d) __attribute__((cdecl)); // expected-note{{previous declaration is here}}
+void __attribute__((cdecl)) func(d)
+  short d; // expected-warning{{promoted type 'int' of K&R function parameter is not compatible with the parameter type 'short' declared in a previous prototype}}
+{}
Index: test/Sema/warn-strict-prototypes.c
===================================================================
--- test/Sema/warn-strict-prototypes.c
+++ test/Sema/warn-strict-prototypes.c
@@ -60,3 +60,8 @@
 // K&R function definition with previous prototype declared is not diagnosed.
 void foo11(int p, int p2);
 void foo11(p, p2) int p; int p2; {}
+
+// PR31020
+void __attribute__((cdecl)) foo12(d) // expected-warning {{this old-style function definition is not preceded by a prototype}}
+  short d;
+{}