[clang-tidy] Added a case to UnconventionalAssignOperatorCheck.

Summary:
The check accepts now a `return (*this = something);` as return
statement too (beneath of `*this`).

Reviewers: alexfh, hokein, aaron.ballman, JonasToth

Reviewed By: aaron.ballman

Subscribers: xazax.hun, dkrupp, Szelethus, gamesh411, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D74529

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

@@ -60,7 +60,12 @@ void UnconventionalAssignOperatorCheck::registerMatchers(
       anyOf(unaryOperator(hasOperatorName("*"), hasUnaryOperand(cxxThisExpr())),
             cxxOperatorCallExpr(argumentCountIs(1),
                                 callee(unresolvedLookupExpr()),
-                                hasArgument(0, cxxThisExpr())))))));
+                                hasArgument(0, cxxThisExpr())),
+            cxxOperatorCallExpr(
+                hasOverloadedOperatorName("="),
+                hasArgument(
+                    0, unaryOperator(hasOperatorName("*"),
+                                     hasUnaryOperand(cxxThisExpr())))))))));
   const auto IsGoodAssign = cxxMethodDecl(IsAssign, HasGoodReturnType);
 
   Finder->addMatcher(returnStmt(IsBadReturnStatement, forFunction(IsGoodAssign))

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

@@ -109,3 +109,21 @@ struct Template {
 
 Template<int> TemplateInt;
 }
+
+struct AssignmentCallAtReturn {
+  AssignmentCallAtReturn &returnThis() {
+    return *this;
+  }
+  AssignmentCallAtReturn &operator=(int rhs) {
+    return *this;
+  }
+  AssignmentCallAtReturn &operator=(char rhs) {
+    // Allow call to assignment from other type.
+    return (*this = static_cast<int>(rhs));
+  }
+  AssignmentCallAtReturn &operator=(float rhs) {
+    // Do not allow calls to other functions.
+    return returnThis();
+    // CHECK-MESSAGES: :[[@LINE-1]]:5: warning: operator=() should always return '*this'
+  }
+};