[clang] [clang-tools-extra] [clang] Implement CWG2413 (implicit `typename` in conversion-function-ids) (PR #195207)

[Victor Chernyakin via cfe-commits]

https://github.com/localspook created https://github.com/llvm/llvm-project/pull/195207

Towards #54150.

>From d0a522553ca6dcefe6d87dc425b3fcba07aebc58 Mon Sep 17 00:00:00 2001
From: Victor Chernyakin <chernyakin.victor.j at outlook.com>
Date: Thu, 30 Apr 2026 18:47:05 -0700
Subject: [PATCH] [clang] Implement CWG2413 (implicit `typename` in
 conversion-function-ids)

---
 .../clang-tidy/readability/RedundantTypenameCheck.cpp  |  3 ++-
 clang/docs/ReleaseNotes.rst                            |  3 +++
 clang/include/clang/Parse/Parser.h                     |  2 +-
 clang/lib/Parse/ParseDecl.cpp                          |  5 +----
 clang/test/CXX/drs/cwg24xx.cpp                         | 10 ++++++++++
 clang/test/CXX/temp/temp.res/p4.cpp                    |  4 ++--
 6 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/readability/RedundantTypenameCheck.cpp b/clang-tools-extra/clang-tidy/readability/RedundantTypenameCheck.cpp
index 77ef2b8622c93..33359fdc3f4bc 100644
--- a/clang-tools-extra/clang-tidy/readability/RedundantTypenameCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/RedundantTypenameCheck.cpp
@@ -34,7 +34,8 @@ void RedundantTypenameCheck::registerMatchers(MatchFinder *Finder) {
                     cxxMethodDecl(), hasParent(friendDecl()),
                     functionDecl(has(nestedNameSpecifier())),
                     cxxDeductionGuideDecl(hasDeclContext(recordDecl())))))))),
-                // Match return types.
+                // Match return types. FIXME: CWG2413 made conversion operators
+                // an implicit typename context.
                 functionDecl(unless(cxxConversionDecl()))))),
             hasParent(expr(anyOf(cxxNamedCastExpr(), cxxNewExpr()))));
   Finder->addMatcher(
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index c5c8c1fa12e7a..44750a1485a49 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -155,6 +155,9 @@ C++17 Feature Support
 Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+- Clang now allows omitting ``typename`` before a template name in a
+  conversion operator, implementing `CWG2413 <https://wg21.link/cwg2413>`_.
+
 C Language Changes
 ------------------
 
diff --git a/clang/include/clang/Parse/Parser.h b/clang/include/clang/Parse/Parser.h
index 1d07d8dbcfa01..7c89a88dd37ff 100644
--- a/clang/include/clang/Parse/Parser.h
+++ b/clang/include/clang/Parse/Parser.h
@@ -1685,13 +1685,13 @@ class Parser : public CodeCompletionHandler {
     case DeclSpecContext::DSC_alias_declaration:
     case DeclSpecContext::DSC_template_param:
     case DeclSpecContext::DSC_new:
+    case DeclSpecContext::DSC_conv_operator:
       return ImplicitTypenameContext::Yes;
 
     case DeclSpecContext::DSC_normal:
     case DeclSpecContext::DSC_objc_method_result:
     case DeclSpecContext::DSC_condition:
     case DeclSpecContext::DSC_template_arg:
-    case DeclSpecContext::DSC_conv_operator:
     case DeclSpecContext::DSC_association:
       return ImplicitTypenameContext::No;
     }
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 4f37e1471c29e..03b68cc88369b 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -3364,11 +3364,8 @@ void Parser::ParseDeclarationSpecifiers(
 
   // If we are in a operator context, convert it back into a type specifier
   // context for better error handling later on.
-  if (DSContext == DeclSpecContext::DSC_conv_operator) {
-    // No implicit typename here.
-    AllowImplicitTypename = ImplicitTypenameContext::No;
+  if (DSContext == DeclSpecContext::DSC_conv_operator)
     DSContext = DeclSpecContext::DSC_type_specifier;
-  }
 
   bool EnteringContext = (DSContext == DeclSpecContext::DSC_class ||
                           DSContext == DeclSpecContext::DSC_top_level);
diff --git a/clang/test/CXX/drs/cwg24xx.cpp b/clang/test/CXX/drs/cwg24xx.cpp
index 0a6a05c125451..428ba97e77356 100644
--- a/clang/test/CXX/drs/cwg24xx.cpp
+++ b/clang/test/CXX/drs/cwg24xx.cpp
@@ -8,6 +8,16 @@
 
 // cwg2406 is in cwg2406.cpp
 
+namespace cwg2413 { // cwg2413
+#if __cplusplus >= 202002L
+template <typename T>
+struct S {
+  operator T::R();
+  void f() { operator T::R(); }
+};
+#endif
+} // namespace cwg2413
+
 namespace cwg2428 { // cwg2428: 19
 #if __cplusplus >= 202002L
 template <typename>
diff --git a/clang/test/CXX/temp/temp.res/p4.cpp b/clang/test/CXX/temp/temp.res/p4.cpp
index 9dbdd235e925d..6ca609b58cf2c 100644
--- a/clang/test/CXX/temp/temp.res/p4.cpp
+++ b/clang/test/CXX/temp/temp.res/p4.cpp
@@ -157,8 +157,8 @@ template int Test<X>;
 
 template<typename T> struct A {
   enum E : T::type {}; // expected-error{{missing 'typename'}}
-  operator T::type() {} // expected-error{{missing 'typename'}}
-  void f() { this->operator T::type(); } // expected-error{{missing 'typename'}}
+  operator T::type() {}
+  void f() { this->operator T::type(); }
 };
 
 template<typename T>