[clang-tidy] Fix false positives with template in misc-unconventional-assign-operator check (#143292)

Fix false positives when copy assignment operator function in a template
class returns the result of another assignment to `*this`, this check
doesn't consider this situation that there will be a `BinaryOperator`
for assignment rather than `CXXOperatorCallExpr` since `this`'s type is
dependent.

Closes #143237.

Author: flovent

clang-tools-extra/clang-tidy/misc/UnconventionalAssignOperatorCheck.cpp

@@ -66,8 +66,11 @@ void UnconventionalAssignOperatorCheck::registerMatchers(
                                 hasArgument(0, cxxThisExpr())),
             cxxOperatorCallExpr(
                 hasOverloadedOperatorName("="),
-                hasArgument(
-                    0, unaryOperator(hasOperatorName("*"),
+                hasArgument(0, unaryOperator(hasOperatorName("*"),
+                                             hasUnaryOperand(cxxThisExpr())))),
+            binaryOperator(
+                hasOperatorName("="),
+                hasLHS(unaryOperator(hasOperatorName("*"),
                                      hasUnaryOperand(cxxThisExpr())))))))));
   const auto IsGoodAssign = cxxMethodDecl(IsAssign, HasGoodReturnType);
 

clang-tools-extra/docs/ReleaseNotes.rst

@@ -266,6 +266,11 @@ Changes in existing checks
   <clang-tidy/checks/misc/redundant-expression>` check by providing additional
   examples and fixing some macro related false positives.
 
+- Improved :doc:`misc-unconventional-assign-operator
+  <clang-tidy/checks/misc/unconventional-assign-operator>` check by fixing
+  false positives when copy assignment operator function in a template class
+  returns the result of another assignment to ``*this`` (``return *this=...``).
+
 - Improved :doc:`misc-unused-using-decls
   <clang-tidy/checks/misc/unused-using-decls>` check by fixing false positives
   on ``operator""`` with template parameters.

clang-tools-extra/test/clang-tidy/checkers/misc/unconventional-assign-operator.cpp

@@ -163,3 +163,16 @@ struct TemplateTypeAlias {
   Alias3<TypeAlias::Alias> &operator=(double) { return *this; }
   // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: operator=() should return 'TemplateTypeAlias&' [misc-unconventional-assign-operator]
 };
+
+namespace gh143237 {
+template<typename T>
+struct TemplateAssignment {
+  explicit TemplateAssignment(int) {
+  }
+
+  TemplateAssignment& operator=(int n) {
+    // No warning
+    return *this = TemplateAssignment(n);
+  }
+};
+}