[clang] Allow parentheses around CTAD declarators #132829

offsetof · 2025-03-24T21:07:06Z

Fixes #39811

llvmbot · 2025-03-24T21:07:47Z

@llvm/pr-subscribers-clang

Author: None (offsetof)

Changes

Fixes #39811

Full diff: https://github.com/llvm/llvm-project/pull/132829.diff

5 Files Affected:

(modified) clang/docs/ReleaseNotes.rst (+2)
(modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+3-4)
(modified) clang/lib/Sema/SemaType.cpp (+2-7)
(modified) clang/test/Parser/cxx1z-class-template-argument-deduction.cpp (+4-4)
(modified) clang/test/SemaCXX/ctad.cpp (+26)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index f919b66dd0e41..24c65dbef5869 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -358,6 +358,8 @@ Bug Fixes to C++ Support
 - Fixed a Clang regression in C++20 mode where unresolved dependent call expressions were created inside non-dependent contexts (#GH122892)
 - Clang now emits the ``-Wunused-variable`` warning when some structured bindings are unused
   and the ``[[maybe_unused]]`` attribute is not applied. (#GH125810)
+- Declarations using class template argument deduction with redundant
+  parentheses around the declarator are no longer rejected. (#GH39811)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index c77cde297dc32..a19975f6d6bdf 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2612,10 +2612,9 @@ def err_decltype_auto_initializer_list : Error<
   "cannot deduce 'decltype(auto)' from initializer list">;
 
 // C++17 deduced class template specialization types
-def err_deduced_class_template_compound_type : Error<
-  "cannot %select{form pointer to|form reference to|form array of|"
-  "form function returning|use parentheses when declaring variable with}0 "
-  "deduced class template specialization type">;
+def err_deduced_class_template_compound_type
+    : Error<"cannot form %select{pointer to|reference to|array of|function "
+            "returning}0 deduced class template specialization type">;
 def err_deduced_non_class_or_alias_template_specialization_type : Error<
   "%select{<error>|function template|variable template|alias template|"
   "template template parameter|concept|template}0 %1 requires template "
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index aec33303780a0..16315d93ef8ce 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -4282,8 +4282,8 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
 
   // If T is 'decltype(auto)', the only declarators we can have are parens
   // and at most one function declarator if this is a function declaration.
-  // If T is a deduced class template specialization type, we can have no
-  // declarator chunks at all.
+  // If T is a deduced class template specialization type, only parentheses
+  // are allowed.
   if (auto *DT = T->getAs<DeducedType>()) {
     const AutoType *AT = T->getAs<AutoType>();
     bool IsClassTemplateDeduction = isa<DeducedTemplateSpecializationType>(DT);
@@ -4297,11 +4297,6 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
         unsigned DiagKind = 0;
         switch (DeclChunk.Kind) {
         case DeclaratorChunk::Paren:
-          // FIXME: Rejecting this is a little silly.
-          if (IsClassTemplateDeduction) {
-            DiagKind = 4;
-            break;
-          }
           continue;
         case DeclaratorChunk::Function: {
           if (IsClassTemplateDeduction) {
diff --git a/clang/test/Parser/cxx1z-class-template-argument-deduction.cpp b/clang/test/Parser/cxx1z-class-template-argument-deduction.cpp
index a1594333abae7..d29eed40b1864 100644
--- a/clang/test/Parser/cxx1z-class-template-argument-deduction.cpp
+++ b/clang/test/Parser/cxx1z-class-template-argument-deduction.cpp
@@ -39,7 +39,7 @@ namespace template_template_arg_pack {
   template<typename...> struct YP {};
 
   struct Z { template<typename T> struct Q {}; }; // expected-note 2{{here}}
-  
+
   template<typename T> using ZId = Z;
 
   template<typename ...Ts> struct A {
@@ -152,7 +152,7 @@ namespace decl {
   A a;
   A b = 0;
   const A c = 0;
-  A (parens) = 0; // expected-error {{cannot use parentheses when declaring variable with deduced class template specialization type}}
+  A (parens) = 0;
   A *p = 0; // expected-error {{cannot form pointer to deduced class template specialization type}}
   A &r = *p; // expected-error {{cannot form reference to deduced class template specialization type}}
   A arr[3] = 0; // expected-error {{cannot form array of deduced class template specialization type}}
@@ -179,7 +179,7 @@ namespace typename_specifier {
   }
   typename ::A a = 0;
   const typename ::A b = 0;
-  typename ::A (parens) = 0; // expected-error {{cannot use parentheses when declaring variable with deduced class template specialization type}}
+  typename ::A (parens) = 0;
   typename ::A *p = 0; // expected-error {{cannot form pointer to deduced class template specialization type}}
   typename ::A &r = *p; // expected-error {{cannot form reference to deduced class template specialization type}}
   typename ::A arr[3] = 0; // expected-error {{cannot form array of deduced class template specialization type}}
@@ -217,7 +217,7 @@ namespace typename_specifier {
 }
 
 namespace parenthesized {
-  template<typename T> struct X { X(T); };                    
+  template<typename T> struct X { X(T); };
   auto n = (X([]{}));
 }
 
diff --git a/clang/test/SemaCXX/ctad.cpp b/clang/test/SemaCXX/ctad.cpp
index 00a861d0f567c..d727c7e66c34c 100644
--- a/clang/test/SemaCXX/ctad.cpp
+++ b/clang/test/SemaCXX/ctad.cpp
@@ -1,5 +1,31 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -Wno-unused-value -std=c++20 %s
 
+namespace GH39811 {
+
+template<int = 0> class C {};
+
+C (a);
+C (b) = C();
+C (c) {};
+C (((((d)))));
+
+template<C (e)> class X;
+template<C (...f)> class Y;
+
+void test() {
+    C (g);
+    C (h) = C();
+    C (i) {};
+    (void)g;
+    (void)h;
+    (void)i;
+}
+
+C* (bad1); // expected-error {{cannot form pointer to deduced class template specialization type}}
+C (*bad2); // expected-error {{cannot form pointer to deduced class template specialization type}}
+
+}
+
 namespace GH64347 {
 
 template<typename X, typename Y> struct A { X x; Y y;};

Sirraide

So the implementation of this seems fine, but looking at how other compilers handle this, GCC and MSVC accept this, whereas EDG rejects. I agree w/ the comment that that is a bit silly to reject this, but can you check what the standard has to say about this? If this is disallowed by the standard, we might want to issue an extension warning (disabled by default) here.

Sirraide

Actually, I just checked #39811, and apparently this was a CWG issue that has been resolved, so in that case this seems fine.

Sirraide · 2025-03-25T11:41:33Z

CC @Endilll Should this also add a test to cwg23xx.cpp?

cor3ntin · 2025-03-25T14:00:18Z

CC @Endilll Should this also add a test to cwg23xx.cpp?

Yes, please!

Add test for CWG2376

offsetof · 2025-03-25T16:39:36Z

Added a test for CWG2376.

zwuis · 2025-04-01T08:48:36Z

clang/test/CXX/drs/cwg23xx.cpp

@@ -379,6 +379,20 @@ class C {
 };
 } // namespace cwg2370

+namespace cwg2376 { // cwg2376: 21
+#if __cpp_deduction_guides >= 201703


Suggested change

#if __cpp_deduction_guides >= 201703

#if __cplusplus >= 201703L

Replace feature-test macro with `__cplusplus`

cor3ntin

LGTM, thanks. Will you need me to merge that for you?

offsetof · 2025-04-09T18:13:15Z

Yes please, thank you @cor3ntin

Fixes llvm#39811

[clang] Allow parentheses around CTAD declarators

66833f4

llvmbot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Mar 24, 2025

Sirraide reviewed Mar 25, 2025

View reviewed changes

Sirraide approved these changes Mar 25, 2025

View reviewed changes

fixup! [clang] Allow parentheses around CTAD declarators

9d717f0

Add test for CWG2376

offsetof requested a review from Endilll as a code owner March 25, 2025 16:06

zwuis reviewed Apr 1, 2025

View reviewed changes

fixup! [clang] Allow parentheses around CTAD declarators

de8abdb

Replace feature-test macro with `__cplusplus`

cor3ntin approved these changes Apr 2, 2025

View reviewed changes

Merge branch 'main' into issue39811

de1418f

cor3ntin merged commit 9604bdf into llvm:main Apr 11, 2025
12 checks passed

var-const pushed a commit to ldionne/llvm-project that referenced this pull request Apr 17, 2025

[clang] Allow parentheses around CTAD declarators (llvm#132829)

325014b

Fixes llvm#39811

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	#if __cpp_deduction_guides >= 201703
	#if __cplusplus >= 201703L

[clang] Allow parentheses around CTAD declarators #132829

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