[clang] Add -Wunused-but-set-global (#188291) Commit fd11cf430e5a extended `-Wunused-but-set-variable` to static globals. To make it easier for downstream projects to integrate this new functionality, this commit introduces `-Wunused-but-set-global` so it can be easily disabled as projects investigate and fix new findings.

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 0dbe667..03fffc4 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst

@@ -231,7 +231,10 @@
 Improvements to Clang's diagnostics
 -----------------------------------
 - ``-Wunused-but-set-variable`` now diagnoses file-scope variables with
-  internal linkage (``static`` storage class) that are assigned but never used. (#GH148361)
+  internal linkage (``static`` storage class) that are assigned but never used.
+  This new coverage is added under the subgroup ``-Wunused-but-set-global``,
+  allowing it to be disabled independently with ``-Wno-unused-but-set-global``.
+  (#GH148361)
 
 - Added ``-Wlifetime-safety`` to enable lifetime safety analysis,
   a CFG-based intra-procedural analysis that detects use-after-free and related

diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index e440c9d..a8d9745 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td

@@ -1158,7 +1158,9 @@
 def UnusedConstVariable : DiagGroup<"unused-const-variable">;
 def UnusedVariable : DiagGroup<"unused-variable",
                                [UnusedConstVariable]>;
-def UnusedButSetVariable : DiagGroup<"unused-but-set-variable">;
+def UnusedButSetGlobal : DiagGroup<"unused-but-set-global">;
+def UnusedButSetVariable
+    : DiagGroup<"unused-but-set-variable", [UnusedButSetGlobal]>;
 def UnusedLocalTypedef : DiagGroup<"unused-local-typedef">;
 def UnusedPropertyIvar :  DiagGroup<"unused-property-ivar">;
 def UnusedGetterReturnValue : DiagGroup<"unused-getter-return-value">;

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index e766d6e..63f2412 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -433,6 +433,9 @@
   InGroup<UnusedVariable>, DefaultIgnore;
 def warn_unused_but_set_variable : Warning<"variable %0 set but not used">,
   InGroup<UnusedButSetVariable>, DefaultIgnore;
+def warn_unused_but_set_global : Warning<"variable %0 set but not used">,
+                                 InGroup<UnusedButSetGlobal>,
+                                 DefaultIgnore;
 def warn_unused_local_typedef : Warning<
   "unused %select{typedef|type alias}0 %1">,
   InGroup<UnusedLocalTypedef>, DefaultIgnore;

diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index 620b135..98318fc 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp

@@ -1610,7 +1610,7 @@
     emitAndClearUnusedLocalTypedefWarnings();
   }
 
-  if (!Diags.isIgnored(diag::warn_unused_but_set_variable, SourceLocation())) {
+  if (!Diags.isIgnored(diag::warn_unused_but_set_global, SourceLocation())) {
     // Diagnose unused-but-set static globals in a deterministic order.
     // Not tracking shadowing info for static globals; there's nothing to
     // shadow.

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index c862c09..5d204c4 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp

@@ -2245,8 +2245,13 @@
       return;
   }
 
-  unsigned DiagID = isa<ParmVarDecl>(VD) ? diag::warn_unused_but_set_parameter
-                                         : diag::warn_unused_but_set_variable;
+  unsigned DiagID;
+  if (isa<ParmVarDecl>(VD))
+    DiagID = diag::warn_unused_but_set_parameter;
+  else if (VD->isFileVarDecl())
+    DiagID = diag::warn_unused_but_set_global;
+  else
+    DiagID = diag::warn_unused_but_set_variable;
   DiagReceiver(VD->getLocation(), PDiag(DiagID) << VD);
 }
 

diff --git a/clang/test/C/C2y/n3622.c b/clang/test/C/C2y/n3622.c
index d90b0c5..afcd831 100644
--- a/clang/test/C/C2y/n3622.c
+++ b/clang/test/C/C2y/n3622.c

@@ -1,7 +1,7 @@
-// RUN: %clang_cc1 -verify=good -pedantic -Wall -Wno-unused-but-set-variable -std=c2y %s
-// RUN: %clang_cc1 -verify=compat,expected -pedantic -Wall -Wno-unused-but-set-variable -Wpre-c2y-compat -std=c2y %s
-// RUN: %clang_cc1 -verify=ped,expected -pedantic -Wall -Wno-unused-but-set-variable -std=c23 %s
-// RUN: %clang_cc1 -verify=ped,expected -pedantic -Wall -Wno-unused-but-set-variable -std=c17 %s
+// RUN: %clang_cc1 -verify=good -pedantic -Wall -Wno-unused-but-set-global -std=c2y %s
+// RUN: %clang_cc1 -verify=compat,expected -pedantic -Wall -Wno-unused-but-set-global -Wpre-c2y-compat -std=c2y %s
+// RUN: %clang_cc1 -verify=ped,expected -pedantic -Wall -Wno-unused-but-set-global -std=c23 %s
+// RUN: %clang_cc1 -verify=ped,expected -pedantic -Wall -Wno-unused-but-set-global -std=c17 %s
 // good-no-diagnostics
 
 /* WG14 N3622: Clang 22

diff --git a/clang/test/Misc/warning-wall.c b/clang/test/Misc/warning-wall.c
index 83b8d4d..6d6c4e5 100644
--- a/clang/test/Misc/warning-wall.c
+++ b/clang/test/Misc/warning-wall.c

@@ -84,6 +84,7 @@
 CHECK-NEXT:      -Wunused-variable
 CHECK-NEXT:        -Wunused-const-variable
 CHECK-NEXT:      -Wunused-but-set-variable
+CHECK-NEXT:        -Wunused-but-set-global
 CHECK-NEXT:      -Wunused-property-ivar
 CHECK-NEXT:    -Wvolatile-register-var
 CHECK-NEXT:    -Wobjc-missing-super-calls

diff --git a/clang/test/Sema/warn-unused-but-set-static-global.c b/clang/test/Sema/warn-unused-but-set-static-global.c
index 7c4d20a..e6abcad 100644
--- a/clang/test/Sema/warn-unused-but-set-static-global.c
+++ b/clang/test/Sema/warn-unused-but-set-static-global.c

@@ -1,4 +1,6 @@
 // RUN: %clang_cc1 -fsyntax-only -Wunused-but-set-variable -I %S/Inputs -verify %s
+// RUN: %clang_cc1 -fsyntax-only -Wunused-but-set-variable -Wno-unused-but-set-global -I %S/Inputs -verify=no-global %s
+// no-global-no-diagnostics
 
 // Test that header-defined static globals don't warn.
 #include "warn-unused-but-set-static-global-header.h"

diff --git a/clang/test/Sema/warn-unused-but-set-static-global.cpp b/clang/test/Sema/warn-unused-but-set-static-global.cpp
index 1ddd7eb..e035484 100644
--- a/clang/test/Sema/warn-unused-but-set-static-global.cpp
+++ b/clang/test/Sema/warn-unused-but-set-static-global.cpp

@@ -1,4 +1,6 @@
 // RUN: %clang_cc1 -fsyntax-only -Wunused-but-set-variable -verify -std=c++17 %s
+// RUN: %clang_cc1 -fsyntax-only -Wunused-but-set-variable -Wno-unused-but-set-global -verify=no-global -std=c++17 %s
+// no-global-no-diagnostics
 
 static thread_local int tl_set_unused;  // expected-warning {{variable 'tl_set_unused' set but not used}}
 static thread_local int tl_set_and_used;