[clang-tidy]performance-unnecessary-copy-initialization: fix false ne…

…gative

We're missing all cases where the return value is a type alias.

Unfortunately, this includes things we care about, such as
`std::vector<T>::operator[]` (return value is `const_reference`,
not `const T&`).

Match the canonical type instead.

Differential Revision: https://reviews.llvm.org/D112722

clang-tools-extra/clang-tidy/performance/UnnecessaryCopyInitialization.cpp

@@ -84,7 +84,8 @@ AST_MATCHER_FUNCTION_P(StatementMatcher, isConstRefReturningMethodCall,
  // returned either points to a global static variable or to a member of the
  // called object.
  return cxxMemberCallExpr(
- callee(cxxMethodDecl(returns(matchers::isReferenceToConst()))
+ callee(cxxMethodDecl(
+ returns(hasCanonicalType(matchers::isReferenceToConst())))
  .bind(MethodDeclId)),
  on(declRefExpr(to(
  varDecl(
@@ -97,7 +98,8 @@ AST_MATCHER_FUNCTION(StatementMatcher, isConstRefReturningFunctionCall) {
  // Only allow initialization of a const reference from a free function if it
  // has no arguments. Otherwise it could return an alias to one of its
  // arguments and the arguments need to be checked for const use as well.
- return callExpr(callee(functionDecl(returns(matchers::isReferenceToConst()))
+ return callExpr(callee(functionDecl(returns(hasCanonicalType(
+ matchers::isReferenceToConst())))
  .bind(FunctionDeclId)),
  argumentCountIs(0), unless(callee(cxxMethodDecl())))
  .bind(InitFunctionCallId);

clang-tools-extra/test/clang-tidy/checkers/performance-unnecessary-copy-initialization.cpp

@@ -12,6 +12,8 @@ struct ExpensiveToCopyType {
  ExpensiveToCopyType();
  virtual ~ExpensiveToCopyType();
  const ExpensiveToCopyType &reference() const;
+ using ConstRef = const ExpensiveToCopyType &;
+ ConstRef referenceWithAlias() const;
  const ExpensiveToCopyType *pointer() const;
  Iterator<ExpensiveToCopyType> begin() const;
  Iterator<ExpensiveToCopyType> end() const;
@@ -206,6 +208,15 @@ void positiveNonConstVarInCodeBlock(const ExpensiveToCopyType &Obj) {
  }
 }
 
+void positiveNonConstVarInCodeBlockWithAlias(const ExpensiveToCopyType &Obj) {
+ {
+ const ExpensiveToCopyType Assigned = Obj.referenceWithAlias();
+ // CHECK-MESSAGES: [[@LINE-1]]:31: warning: the const qualified variable
+ // CHECK-FIXES: const ExpensiveToCopyType& Assigned = Obj.referenceWithAlias();
+ useAsConstReference(Assigned);
+ }
+}
+
 void negativeNonConstVarWithNonConstUse(const ExpensiveToCopyType &Obj) {
  {
  auto NonConstInvoked = Obj.reference();