[PATCH] D119470: [clang-tidy] Don't check decltype return types in `readability-const-return-type`

[Evgeny Shulgin via Phabricator via cfe-commits]

Izaron created this revision.
Izaron added reviewers: aaron.ballman, alexfh.
Herald added subscribers: carlosgalvezp, xazax.hun.
Izaron requested review of this revision.
Herald added a project: clang-tools-extra.
Herald added a subscriber: cfe-commits.

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


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D119470

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


Index: clang-tools-extra/test/clang-tidy/checkers/readability-const-return-type.cpp
===================================================================
--- clang-tools-extra/test/clang-tidy/checkers/readability-const-return-type.cpp
+++ 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
@@ -271,3 +271,25 @@
 
 int **const * n_multiple_ptr();
 int *const & n_pointer_ref();
+
+// 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 as well
+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;
+}
Index: clang-tools-extra/clang-tidy/readability/ConstReturnTypeCheck.cpp
===================================================================
--- clang-tools-extra/clang-tidy/readability/ConstReturnTypeCheck.cpp
+++ clang-tools-extra/clang-tidy/readability/ConstReturnTypeCheck.cpp
@@ -53,6 +53,10 @@
 
 namespace {
 
+AST_MATCHER(QualType, isLocalConstQualified) {
+  return Node.isLocalConstQualified();
+}
+
 struct CheckResult {
   // Source range of the relevant `const` token in the definition being checked.
   CharSourceRange ConstRange;
@@ -95,9 +99,14 @@
 
 void ConstReturnTypeCheck::registerMatchers(MatchFinder *Finder) {
   // Find all function definitions for which the return types are `const`
-  // qualified.
+  // qualified, ignoring decltype types.
+  const auto NonLocalConstDecltypeType = qualType(
+      unless(isLocalConstQualified()), anyOf(decltypeType(), autoType()));
   Finder->addMatcher(
-      functionDecl(returns(isConstQualified()), isDefinition()).bind("func"),
+      functionDecl(
+          returns(allOf(isConstQualified(), unless(NonLocalConstDecltypeType))),
+          isDefinition())
+          .bind("func"),
       this);
 }
 


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