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

[Izaron]

Summary

The checker removes consts 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

[Izaron]

I think we don't need to update the docs
(https://clang.llvm.org/extra/clang-tidy/checks/readability-const-return-type.html)
Because a user would expect this behaviour.

If this review is eventually approved, kindly please merge the commit on my behalf =) As I don't have merge access. My name is Evgeny Shulgin and email is [e-mail]. Sorry for inconvenience!

[aaron.ballman]

In D119470#3312223, @Izaron wrote:

I think we don't need to update the docs
(https://clang.llvm.org/extra/clang-tidy/checks/readability-const-return-type.html)
Because a user would expect this behaviour.

If this review is eventually approved, kindly please merge the commit on my behalf =) As I don't have merge access. My name is Evgeny Shulgin and email is [e-mail]. Sorry for inconvenience!

You've submitted some quality patches already, would you be interested in obtaining commit privileges (https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access)?

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

56–58

I think we might as well hit all the local qualifiers instead of just const, WDYT? e.g., volatile et al via hasLocalQualifiers()

111

We don't usually use top-level const qualification for locals.

112

How about a TypeOfType? (The GCC typeof(foo) extension)

[Izaron]

Don't warn on non-toplevel-const TypeOfType

[Izaron]

You've submitted some quality patches already, would you be interested in obtaining commit privileges (https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access)?

Thanks! I obtained the commit access =) Now I don't have to ask other peoples commit on my behalf.

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

56–58

In this case the checker will complain on this code:

const int i = 1;
volatile decltype(i) n() {
  return 12345;
}

warning: return type 'volatile decltype(i)' (aka 'const volatile int') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type]

That wouldn't be the correct outcome.

I added the test for the snippet above.

112

Thanks! Didn't know about this extension. Now the checker filters this out.

[aaron.ballman]

LGTM!

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

56–58

Great, thank you!

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

if (FileRange.isInvalid())
     return None;
 
   return utils::lexer::getQualifyingToken(
       tok::kw_const, FileRange, *Result.Context, *Result.SourceManager);
 }
 
 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;
 
   // FixItHints associated with the definition being checked.
   llvm::SmallVector<clang::FixItHint, 4> Hints;
 
   // Locations of any declarations that could not be fixed.
else
       // `getInnerLocStart` gives the start of the return type.
       Result.DeclLocs.push_back(Decl->getInnerLocStart());
   }
   return Result;
 }
 
 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))),
           .bind("func"),
       this);
 }
 
 void ConstReturnTypeCheck::check(const MatchFinder::MatchResult &Result) {
   const auto *Def = Result.Nodes.getNodeAs<FunctionDecl>("func");
   CheckResult CR = checkDef(Def, Result);
   {

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

-// 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
 
 namespace std {
 template< class T >
 struct add_cv { typedef const volatile T type; };
 

 };
 
 class PVDerive : public PVBase {
 public:
   const int getC() { return 1; }
   // 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;
+}