[clang-tidy] Don't check decltype return types in `readability-const-return-type`

The checker removes `const`s that are superfluos and badly affect
readability. `decltype(auto)`/`decltype(expr)` are often const-qualified, but
have no effect on readability and usually can't stop being const-qualified
without significant code change.

Fixes https://github.com/llvm/llvm-project/issues/52890

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D119470

clang-tools-extra/clang-tidy/readability/ConstReturnTypeCheck.cpp

@@ -53,6 +53,18 @@ findConstToRemove(const FunctionDecl *Def,
 
 namespace {
 
+AST_MATCHER(QualType, isLocalConstQualified) {
+  return Node.isLocalConstQualified();
+}
+
+AST_MATCHER(QualType, isTypeOfType) {
+  return isa<TypeOfType>(Node.getTypePtr());
+}
+
+AST_MATCHER(QualType, isTypeOfExprType) {
+  return isa<TypeOfExprType>(Node.getTypePtr());
+}
+
 struct CheckResult {
   // Source range of the relevant `const` token in the definition being checked.
   CharSourceRange ConstRange;
@@ -95,10 +107,14 @@ static CheckResult checkDef(const clang::FunctionDecl *Def,
 
 void ConstReturnTypeCheck::registerMatchers(MatchFinder *Finder) {
   // Find all function definitions for which the return types are `const`
-  // qualified.
+  // qualified, ignoring decltype types.
+  auto NonLocalConstType = qualType(
+      unless(isLocalConstQualified()),
+      anyOf(decltypeType(), autoType(), isTypeOfType(), isTypeOfExprType()));
   Finder->addMatcher(
-      functionDecl(returns(isConstQualified()),
-                   anyOf(isDefinition(), cxxMethodDecl(isPure())))
+      functionDecl(
+          returns(allOf(isConstQualified(), unless(NonLocalConstType))),
+          anyOf(isDefinition(), cxxMethodDecl(isPure())))
           .bind("func"),
       this);
 }

clang-tools-extra/test/clang-tidy/checkers/readability-const-return-type.cpp

@@ -1,4 +1,4 @@
-// RUN: %check_clang_tidy %s readability-const-return-type %t
+// RUN: %check_clang_tidy -std=c++14 %s readability-const-return-type %t
 
 //  p# = positive test
 //  n# = negative test
@@ -285,3 +285,40 @@ public:
   // CHECK-MESSAGES: [[@LINE-1]]:3: warning: return type 'const int' is 'const'-qualified at the top level, which may reduce code readability without improving const correctness
   // CHECK-NOT-FIXES: int getC() { return 1; }
 };
+
+// Don't warn about const auto types, because it may be impossible to make them non-const
+// without a significant semantics change. Since `auto` drops cv-qualifiers,
+// tests check `decltype(auto)`.
+decltype(auto) n16() {
+  static const int i = 42;
+  return i;
+}
+
+// Don't warn about `decltype(<expr>)` types
+const int n17i = 1;
+decltype(n17i) n17() {
+  return 17;
+}
+
+// Do warn when on decltype types with the local const qualifier
+// `const decltype(auto)` won't compile, so check only `const decltype(<expr>)`
+const decltype(n17i) n18() {
+  // CHECK-MESSAGES: [[@LINE-1]]:1: warning: return type 'const decltype(n17i)
+  // CHECK-FIXES: decltype(n17i) n18() {
+  return 18;
+}
+
+// `volatile` modifier doesn't affect the checker
+volatile decltype(n17i) n19() {
+  return 19;
+}
+
+// Don't warn about `__typeof__(<expr>)` types
+__typeof__(n17i) n20() {
+  return 20;
+}
+
+// Don't warn about `__typeof__(type)` types
+__typeof__(const int) n21() {
+  return 21;
+}