[Clang] -Wunused-but-set-variable warning - handle also pre/post unary operators

Clang fails to diagnose:
```
void test() {
    int j = 0;
    for (int i = 0; i < 1000; i++)
            j++;
    return;
}
```

Reason: Missing support for UnaryOperator.

We should not warn with volatile variables... so add check for it.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D122271

clang/lib/Sema/SemaExprCXX.cpp

@@ -7921,6 +7921,7 @@ static void MaybeDecrementCount(
     Expr *E, llvm::DenseMap<const VarDecl *, int> &RefsMinusAssignments) {
   DeclRefExpr *LHS = nullptr;
   bool IsCompoundAssign = false;
+  bool isIncrementDecrementUnaryOp = false;
   if (BinaryOperator *BO = dyn_cast<BinaryOperator>(E)) {
     if (BO->getLHS()->getType()->isDependentType() ||
         BO->getRHS()->getType()->isDependentType()) {
@@ -7935,6 +7936,11 @@ static void MaybeDecrementCount(
     if (COCE->getOperator() != OO_Equal)
       return;
     LHS = dyn_cast<DeclRefExpr>(COCE->getArg(0));
+  } else if (UnaryOperator *UO = dyn_cast<UnaryOperator>(E)) {
+    if (!UO->isIncrementDecrementOp())
+      return;
+    isIncrementDecrementUnaryOp = true;
+    LHS = dyn_cast<DeclRefExpr>(UO->getSubExpr());
   }
   if (!LHS)
     return;
@@ -7942,8 +7948,10 @@ static void MaybeDecrementCount(
   if (!VD)
     return;
   // Don't decrement RefsMinusAssignments if volatile variable with compound
-  // assignment (+=, ...) to avoid potential unused-but-set-variable warning.
-  if (IsCompoundAssign && VD->getType().isVolatileQualified())
+  // assignment (+=, ...) or increment/decrement unary operator to avoid
+  // potential unused-but-set-variable warning.
+  if ((IsCompoundAssign || isIncrementDecrementUnaryOp) &&
+      VD->getType().isVolatileQualified())
     return;
   auto iter = RefsMinusAssignments.find(VD);
   if (iter == RefsMinusAssignments.end())

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

@@ -73,3 +73,20 @@ void f3(void) {
   __attribute__((__cleanup__(for_cleanup))) int x;
   x = 5;
 }
+
+void f4(void) {
+  int x1 = 0; // expected-warning{{variable 'x1' set but not used}}
+  x1++;
+  int x2 = 0; // expected-warning{{variable 'x2' set but not used}}
+  x2--;
+  int x3 = 0; // expected-warning{{variable 'x3' set but not used}}
+  ++x3;
+  int x4 = 0; // expected-warning{{variable 'x4' set but not used}}
+  --x4;
+
+  volatile int v1 = 0;
+  ++v1;
+  typedef volatile int volint;
+  volint v2 = 0;
+  v2++;
+}

clang/test/SemaCXX/warn-unused-but-set-variables-cpp.cpp

@@ -7,6 +7,7 @@ struct S {
 struct __attribute__((warn_unused)) SWarnUnused {
   int j;
   void operator +=(int);
+  void operator ++();
 };
 
 int f0() {
@@ -62,3 +63,9 @@ template<typename T> void f4(T n) {
   SWarnUnused swu;
   swu += n;
 }
+
+template <typename T> void f5() {
+  // Don't warn for overloaded pre/post operators in template code.
+  SWarnUnused swu;
+  ++swu;
+}