Change prototype merging error into a warning for builtins

As was observed in https://reviews.llvm.org/D123627#3707635, it's
confusing that a user can write:
```
float rintf(void) {}
```
and get a warning, but writing:
```
float rintf() {}
```
gives an error. This patch changes the behavior so that both are
warnings, so that users who have functions which conflict with a
builtin identifier can still use that identifier as they wish.

Differential Revision: https://reviews.llvm.org/D131499

(cherry picked from commit 4c02ab8c9742f6c32b17f49a306b3b072486f5c5)

1 files changed, 1 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 150168d..75ffa8b 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;