[Clang] -Wunused-but-set-variable - ignore static variables

[xbolva00]

Summary

Currently, Clang emits false positive for

void test(void) {
  static int counter = 0;
  counter += 1;
}

warning: variable 'counter' set but not used [-Wunused-but-set-variable]

We should check for correct local storage (a variable with function scope is a non-static local variable).

[xbolva00]

ignore me, this seems okay (interestigly, gcc does not warn)

clang/lib/Sema/SemaExpr.cpp

   auto *MSI = Var->getMemberSpecializationInfo();
   TemplateSpecializationKind TSK = MSI ? MSI->getTemplateSpecializationKind()
                                        : Var->getTemplateSpecializationKind();
 
   OdrUseContext OdrUse = isOdrUseContext(SemaRef);
   bool UsableInConstantExpr =
       Var->mightBeUsableInConstantExpressions(SemaRef.Context);
 
-  if (Var->isLocalVarDeclOrParm() && !Var->hasExternalStorage()) {
+  if (Var->isLocalVarDeclOrParm() && Var->hasLocalStorage()) {
     RefsMinusAssignments.insert({Var, 0}).first->getSecond()++;
   }
 
   // C++20 [expr.const]p12:
   //   A variable [...] is needed for constant evaluation if it is [...] a
   //   variable whose name appears as a potentially constant evaluated
   //   expression that is either a contexpr variable or is of non-volatile
   //   const-qualified integral type or of reference type

clang/test/Sema/warn-unused-but-set-variables.c

void for_cleanup(int *x) {
   *x = 0;
 }
 
 void f3(void) {
   // Don't warn if the __cleanup__ attribute is used.
   __attribute__((__cleanup__(for_cleanup))) int x;
   x = 5;
 }
+
+void f4(void) {
+  static int c1 = 0;
+  c1 += 1;
+}