Silently accept -Wgnu-empty-initializer

5d8aaad
removed the warning group as the functionality is no longer a GNU
extension. However, users have asked for the warning group to be
supported so that code transitioning from Clang 16 to Clang 17 has an
easier migration path when compiling with -Werror. This patch restores
the warning group, but as an ignored warning group because the
functionality is now always considered to be a C extension rather than
a GNU extension. This allows users to do:

  -Werror -pedantic -Wno-gnu-empty-intializer -Wno-c2x-extensions

to silence the diagnostics in both Clang 16 and Clang 17.

Fixes #64357
Differential Revision: https://reviews.llvm.org/D157503

(cherry picked from commit 151214b)

Author: AaronBallman

clang/include/clang/Basic/DiagnosticGroups.td

@@ -20,6 +20,7 @@ def DeprecatedStaticAnalyzerFlag : DiagGroup<"deprecated-static-analyzer-flag">;
 // Empty DiagGroups are recognized by clang but ignored.
 def ODR : DiagGroup<"odr">;
 def : DiagGroup<"abi">;
+def : DiagGroup<"gnu-empty-initializer">; // Now a C extension, not GNU.
 def AbsoluteValue : DiagGroup<"absolute-value">;
 def MisspelledAssumption : DiagGroup<"misspelled-assumption">;
 def UnknownAssumption : DiagGroup<"unknown-assumption">;

clang/test/Sema/empty-init.c

@@ -0,0 +1,30 @@
+// RUN: %clang_cc1 %s -std=c2x -Wall -pedantic -fsyntax-only -verify=good
+// RUN: %clang_cc1 %s -std=c2x -Wpre-c2x-compat -fsyntax-only -verify=c2x
+// RUN: %clang_cc1 %s -std=c2x -Wpre-c2x-compat -Wno-gnu-empty-initializer -fsyntax-only -verify=c2x
+// RUN: %clang_cc1 %s -std=c2x -Wgnu-empty-initializer -fsyntax-only -verify=good
+// RUN: %clang_cc1 %s -std=c17 -Wall -pedantic -fsyntax-only -verify=c2x-ext
+// RUN: %clang_cc1 %s -std=c17 -Wgnu-empty-initializer -fsyntax-only -verify=good
+// RUN: %clang_cc1 %s -std=c17 -Wc2x-extensions -fsyntax-only -verify=c2x-ext
+// RUN: %clang_cc1 %s -std=c17 -Wpre-c2x-compat -fsyntax-only -verify=good
+
+// good-no-diagnostics
+
+// Empty brace initialization used to be a GNU extension, but the feature was
+// added to C2x. We now treat empty initialization as a C extension rather than
+// a GNU extension. Thus, -Wgnu-empty-initializer is always silently ignored.
+
+struct S {
+  int a;
+};
+
+struct S s = {};     /* c2x-warning {{use of an empty initializer is incompatible with C standards before C2x}}
+                        c2x-ext-warning {{use of an empty initializer is a C2x extension}}
+                      */
+
+void func(void) {
+  struct S s2 = {};  /* c2x-warning {{use of an empty initializer is incompatible with C standards before C2x}}
+                        c2x-ext-warning {{use of an empty initializer is a C2x extension}}
+                      */
+  (void)s2;
+}
+