diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -3911,6 +3911,7 @@
     // ASTContext::typesAreCompatible().
     if (Old->hasPrototype() && !New->hasWrittenPrototype() && NewDeclIsDefn &&
         Old->getNumParams() != New->getNumParams()) {
+      Old = Old->getCanonicalDecl();
       Diag(New->getLocation(), diag::err_conflicting_types) << New;
       Diag(Old->getLocation(), PrevDiag) << Old << Old->getType();
       return true;
diff --git a/clang/test/Sema/prototype-redecls.c b/clang/test/Sema/prototype-redecls.c
--- a/clang/test/Sema/prototype-redecls.c
+++ b/clang/test/Sema/prototype-redecls.c
@@ -12,6 +12,10 @@
 void blerp(short);      // expected-note {{previous}}
 void blerp(x) int x; {} // expected-error {{conflicting types for 'blerp'}}
 
+void foo(int); // expected-note {{previous}}
+void foo();
+void foo() {} // expected-error {{conflicting types for 'foo'}}
+
 void glerp(int);
 void glerp(x) short x; {} // Okay, promoted type is fine