[Sema] -Wunused-variable warning for variables with array types should behave similarly to variables with scalar types

[arphaman]

Summary

This patch makes the -Wunused-variable warning behaviour more consistent: Now clang won't warn on variables with constant array types where it doesn't warn on variables with scalar types.

[rjmccall]

There's no reason for this to only consider constant-sized arrays, and you should use getBaseElementTypeUnsafe() so you look through nested array types. That method is a no-op on non-array types, so you can just call it unconditionally here.

[arphaman]

The updated patch incorporates John's suggestions into code.

[rjmccall]

Please add a line testing the nested-array case to warn-unused-variables.cpp, but with that, LGTM.

lib/Sema/SemaDecl.cpp

static bool ShouldDiagnoseUnusedDecl(const NamedDecl *D) {
   // White-list anything that isn't a local variable.
   if (!isa<VarDecl>(D) || isa<ParmVarDecl>(D) || isa<ImplicitParamDecl>(D))
     return false;
 
   // Types of valid local variables should be complete, so this should succeed.
   if (const VarDecl *VD = dyn_cast<VarDecl>(D)) {
 
     // White-list anything with an __attribute__((unused)) type.
-    QualType Ty = VD->getType();
+    const auto *Ty = VD->getType().getTypePtr();
 
     // Only look at the outermost level of typedef.
     if (const TypedefType *TT = Ty->getAs<TypedefType>()) {
       if (TT->getDecl()->hasAttr<UnusedAttr>())
         return false;
     }
 
     // If we failed to complete the type for some reason, or if the type is
     // dependent, don't diagnose the variable.
     if (Ty->isIncompleteType() || Ty->isDependentType())
       return false;
 
+    // Look at the element type to ensure that the warning behaviour is
+    // consistent for both scalars and arrays.
+    Ty = Ty->getBaseElementTypeUnsafe();
+
     if (const TagType *TT = Ty->getAs<TagType>()) {
       const TagDecl *Tag = TT->getDecl();
       if (Tag->hasAttr<UnusedAttr>())
         return false;
 
       if (const CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(Tag)) {
         if (!RD->hasTrivialDestructor() && !RD->hasAttr<WarnUnusedAttr>())
           return false;

test/SemaCXX/warn-everthing.cpp

 // RUN: %clang_cc1 -Weverything -fsyntax-only %s -verify
 
 // This previously crashed due to a bug in the CFG.  Exercising all
 // warnings helps check CFG construction.
 class PR12271 {
 public:
   PR12271();
   ~PR12271();
 };
 
 void testPR12271() { // expected-warning {{no previous prototype for function 'testPR12271'}}
-  PR12271 a[1][1]; // expected-warning {{unused variable 'a'}}
+  PR12271 a[1][1];
 }

test/SemaCXX/warn-unused-variables.cpp

struct S2 {
     S2(const S1&);
   };
   void func() {
     S2 s((S1()));
   }
 }
 
 #include "Inputs/warn-unused-variables.h"
+
+namespace arrayRecords {
+
+int total = 0;
+
+class Adder {
+public:
+  Adder(int x); // out of line below
+  ~Adder() {}
+};
+
+Adder::Adder(int x) {
+  total += x;
+}
+
+struct Foo {
+  int x;
+  Foo(int x) : x(x) {}
+};
+
+struct S1 {
+  S1();
+};
+
+void foo(int size) {
+  S1 y; // no warning
+  S1 yarray[2]; // no warning
+  S1 dynArray[size]; // no warning
+
+  Adder scalerInFuncScope = 134; // no warning
+  Adder arrayInFuncScope[] = { 135, 136 };  // no warning
+
+  Foo fooScalar = 1; // expected-warning {{unused variable 'fooScalar'}}
+  Foo fooArray[] = {1,2}; // expected-warning {{unused variable 'fooArray'}}
+}
+
+template<int N>
+void bar() {
+  Adder scaler = 123; // no warning
+  Adder array[N] = {1,2}; // no warning
+}
+
+void test() {
+  foo(10);
+  bar<2>();
+}
+
+}