Add a diagnostic group for tentative array definitions

This diagnostic is one of the ones that GCC also does not have a
warning group for, but a user requested adding a group to control
selectively turning off this diagnostic. So this adds the diagnostic
to a new group, -Wtentative-definition-array

Fixes #87766

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -352,6 +352,11 @@ Improvements to Clang's diagnostics
   (with initializer) entirely consist the condition expression of a if/while/for construct
   but are not actually used in the body of the if/while/for construct. Fixes #GH41447
 
+- Clang emits a diagnostic when a tentative array definition is assumed to have
+  a single element, but that diagnostic was never given a diagnostic group.
+  Added the ``-Wtentative-definition-array`` warning group to cover this.
+  Fixes #GH87766
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -7142,7 +7142,8 @@ def ext_typecheck_decl_incomplete_type : ExtWarn<
 def err_tentative_def_incomplete_type : Error<
   "tentative definition has type %0 that is never completed">;
 def warn_tentative_incomplete_array : Warning<
-  "tentative array definition assumed to have one element">;
+  "tentative array definition assumed to have one element">,
+  InGroup<DiagGroup<"tentative-definition-array">>;
 def err_typecheck_incomplete_array_needs_initializer : Error<
   "definition of variable with array type needs an explicit size "
   "or an initializer">;

clang/test/Misc/warning-flags.c

@@ -18,7 +18,7 @@ This test serves two purposes:
 
 The list of warnings below should NEVER grow.  It should gradually shrink to 0.
 
-CHECK: Warnings without flags (67):
+CHECK: Warnings without flags (66):
 
 CHECK-NEXT:   ext_expected_semi_decl_list
 CHECK-NEXT:   ext_explicit_specialization_storage_class
@@ -80,7 +80,6 @@ CHECK-NEXT:   warn_register_objc_catch_parm
 CHECK-NEXT:   warn_related_result_type_compatibility_class
 CHECK-NEXT:   warn_related_result_type_compatibility_protocol
 CHECK-NEXT:   warn_template_export_unsupported
-CHECK-NEXT:   warn_tentative_incomplete_array
 CHECK-NEXT:   warn_typecheck_function_qualifiers
 CHECK-NEXT:   warn_undef_interface
 CHECK-NEXT:   warn_undef_interface_suggest

clang/test/Sema/tentative-array-decl.c

@@ -0,0 +1,5 @@
+// RUN: %clang_cc1 -verify %s
+// RUN: %clang_cc1 -verify=good -Wno-tentative-definition-array %s
+// good-no-diagnostics
+
+int foo[]; // expected-warning {{tentative array definition assumed to have one element}}