diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 150168d9cffa8..75ffa8b0a90ab 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -4034,7 +4034,7 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD, Scope *S,
     // default argument promotion rules were already checked by
     // ASTContext::typesAreCompatible().
     if (Old->hasPrototype() && !New->hasWrittenPrototype() && NewDeclIsDefn &&
-        Old->getNumParams() != New->getNumParams()) {
+        Old->getNumParams() != New->getNumParams() && !Old->isImplicit()) {
       if (Old->hasInheritedPrototype())
         Old = Old->getCanonicalDecl();
       Diag(New->getLocation(), diag::err_conflicting_types) << New;
diff --git a/clang/test/Sema/prototype-redecls.c b/clang/test/Sema/prototype-redecls.c
index ed569b5223ce2..49305db10d240 100644
--- a/clang/test/Sema/prototype-redecls.c
+++ b/clang/test/Sema/prototype-redecls.c
@@ -29,7 +29,7 @@ void garp(x) int x; {}
 
 // Ensure redeclarations that conflict with a builtin use a note which makes it
 // clear that the previous declaration was a builtin.
-float rintf() { // expected-error {{conflicting types for 'rintf'}} \
+float rintf() { // expected-warning {{incompatible redeclaration of library function 'rintf'}} \
                    expected-note {{'rintf' is a builtin with type 'float (float)'}}
   return 1.0f;
 }