Reenable POSIX builtin library functions in gnu2x mode

gnu17 and earlier modes automatically expose several POSIX C APIs, and
this was accidentally disabled for gnu2x in
7d644e1215.

This restores the behavior for gnu2x mode (without changing the
behavior in C standards modes instead of GNU modes).

Fixes #56607

clang/docs/ReleaseNotes.rst

@@ -227,6 +227,10 @@ Bug Fixes
   `Issue 57377 <https://github.com/llvm/llvm-project/issues/57377>`_.
 - Fix a crash when a ``btf_type_tag`` attribute is applied to the pointee of
   a function pointer.
+- Clang 14 predeclared some builtin POSIX library functions in ``gnu2x`` mode,
+  and Clang 15 accidentally stopped predeclaring those functions in that
+  language mode. Clang 16 now predeclares those functions again. This fixes
+  `Issue 56607 <https://github.com/llvm/llvm-project/issues/56607>`_.
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaLookup.cpp

@@ -939,11 +939,9 @@ bool Sema::LookupBuiltin(LookupResult &R) {
 
       // If this is a builtin on this (or all) targets, create the decl.
       if (unsigned BuiltinID = II->getBuiltinID()) {
-        // In C++, C2x, and OpenCL (spec v1.2 s6.9.f), we don't have any
-        // predefined library functions like 'malloc'. Instead, we'll just
-        // error.
-        if ((getLangOpts().CPlusPlus || getLangOpts().OpenCL ||
-             getLangOpts().C2x) &&
+        // In C++ and OpenCL (spec v1.2 s6.9.f), we don't have any predefined
+        // library functions like 'malloc'. Instead, we'll just error.
+        if ((getLangOpts().CPlusPlus || getLangOpts().OpenCL) &&
             Context.BuiltinInfo.isPredefinedLibFunction(BuiltinID))
           return false;
 

clang/test/Sema/gnu-builtins.c

@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -verify=gnu -std=gnu17 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=gnu -std=gnu2x %s
+// RUN: %clang_cc1 -fsyntax-only -verify=std -std=c17 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=std -std=c2x %s
+
+// std-no-diagnostics
+
+// 'index' is a builtin library function, but only in GNU mode. So this should
+// give an error in GNU modes but be okay in non-GNU mode.
+// FIXME: the error is correct, but these notes are pretty awful.
+int index; // gnu-error {{redefinition of 'index' as different kind of symbol}} \
+              gnu-note {{unguarded header; consider using #ifdef guards or #pragma once}} \
+              gnu-note {{previous definition is here}}