[clang-tidy] Added a case to UnconventionalAssignOperatorCheck.

[balazske]

Summary

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

[aaron.ballman]

LGTM!

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

void UnconventionalAssignOperatorCheck::registerMatchers(
   Finder->addMatcher(
       cxxMethodDecl(IsSelfAssign, anyOf(isConst(), isVirtual())).bind("cv"),
       this);
 
   const auto IsBadReturnStatement = returnStmt(unless(has(ignoringParenImpCasts(
       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))
                          .bind("returnStmt"),
                      this);
 }
 
 void UnconventionalAssignOperatorCheck::check(

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

 template <typename>
 struct Template {
   Template &operator=(const Template &) { return this; }
   // CHECK-MESSAGES: :[[@LINE-1]]:43: warning: operator=() should always return '*this'
 };
 
 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'
+  }
+};