[PATCH] D103949: Only consider built-in compound assignment operators for -Wunused-but-set-*

[Stephan Bergmann via Phabricator via cfe-commits]

sberg created this revision.
sberg added a reviewer: mbenfield.
sberg added a project: clang.
sberg requested review of this revision.

At least LibreOffice has, for mainly historic reasons that would be hard to change now, a class `Any` with an overloaded `operator >>=` that semantically does not assign to the LHS but rather extracts into the (by-reference) RHS.  Which thus caused false positive `-Wunused-but-set-parameter` and `-Wunused-but-set-variable` after those have been introduced recently.
This change is more conservative about the assumed semantics of overloaded operators, excluding compound assignment operators but keeping plain `operator =` ones.  At least for LibreOffice, that strikes a good balance of not producing false positives but still finding lots of true ones.


https://reviews.llvm.org/D103949

Files:
  clang/lib/Sema/SemaExprCXX.cpp
  clang/test/SemaCXX/warn-unused-but-set-variables-cpp.cpp


Index: clang/test/SemaCXX/warn-unused-but-set-variables-cpp.cpp
===================================================================
--- clang/test/SemaCXX/warn-unused-but-set-variables-cpp.cpp
+++ clang/test/SemaCXX/warn-unused-but-set-variables-cpp.cpp
@@ -6,6 +6,7 @@
 
 struct __attribute__((warn_unused)) SWarnUnused {
   int j;
+  void operator +=(int);
 };
 
 int f0() {
@@ -48,3 +49,16 @@
   char a[x];
   char b[y];
 }
+
+void f3(int n) {
+  // Don't warn for overloaded compound assignment operators.
+  SWarnUnused swu;
+  swu += n;
+}
+
+template<typename T> void f4(T n) {
+  // Don't warn for (potentially) overloaded compound assignment operators in
+  // template code.
+  SWarnUnused swu;
+  swu += n;
+}
Index: clang/lib/Sema/SemaExprCXX.cpp
===================================================================
--- clang/lib/Sema/SemaExprCXX.cpp
+++ clang/lib/Sema/SemaExprCXX.cpp
@@ -7791,11 +7791,16 @@
     Expr *E, llvm::DenseMap<const VarDecl *, int> &RefsMinusAssignments) {
   DeclRefExpr *LHS = nullptr;
   if (BinaryOperator *BO = dyn_cast<BinaryOperator>(E)) {
-    if (!BO->isAssignmentOp())
+    if (BO->getLHS()->getType()->isDependentType() ||
+        BO->getRHS()->getType()->isDependentType())
+    {
+      if (BO->getOpcode() != BO_Assign)
+        return;
+    } else if (!BO->isAssignmentOp())
       return;
     LHS = dyn_cast<DeclRefExpr>(BO->getLHS());
   } else if (CXXOperatorCallExpr *COCE = dyn_cast<CXXOperatorCallExpr>(E)) {
-    if (!COCE->isAssignmentOp())
+    if (COCE->getOperator() != OO_Equal)
       return;
     LHS = dyn_cast<DeclRefExpr>(COCE->getArg(0));
   }


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