[clang] [Clang] Implement CWG2351 `void{}` (PR #78060)

Mital Ashok via cfe-commits cfe-commits at lists.llvm.org
Tue Jun 4 07:17:45 PDT 2024


https://github.com/MitalAshok updated https://github.com/llvm/llvm-project/pull/78060

>From 6ed7cad5d4993603221c3d9a777463675d69643b Mon Sep 17 00:00:00 2001
From: Mital Ashok <mital at mitalashok.co.uk>
Date: Sat, 13 Jan 2024 18:03:15 +0000
Subject: [PATCH 1/2] [SemaCXX] Implement CWG2351 `void{}`

---
 clang/docs/ReleaseNotes.rst                |  3 ++
 clang/include/clang/AST/ExprCXX.h          |  5 +--
 clang/lib/Sema/SemaExprCXX.cpp             | 23 ++++++++----
 clang/lib/Sema/SemaInit.cpp                |  1 +
 clang/test/CXX/drs/dr23xx.cpp              | 41 +++++++++++++++++++++-
 clang/test/SemaCXX/attr-annotate.cpp       |  4 +--
 clang/test/SemaCXX/cxx2a-explicit-bool.cpp |  4 +--
 clang/test/SemaCXX/sugared-auto.cpp        |  6 ++++
 clang/www/cxx_dr_status.html               |  2 +-
 9 files changed, 75 insertions(+), 14 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 92563262cc673..28cd1a8abd4c6 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -157,6 +157,9 @@ Resolutions to C++ Defect Reports
 - Clang now diagnoses declarative nested-name-specifiers with pack-index-specifiers.
   (`CWG2858: Declarative nested-name-specifiers and pack-index-specifiers <https://cplusplus.github.io/CWG/issues/2858.html>`_).
 
+- Implemented `CWG2351 <https://wg21.link/CWG2351>`_ which allows ``void{}``
+  as a prvalue of type ``void``.
+
 C Language Changes
 ------------------
 
diff --git a/clang/include/clang/AST/ExprCXX.h b/clang/include/clang/AST/ExprCXX.h
index a915745d2d732..8fa5c0c79f6a0 100644
--- a/clang/include/clang/AST/ExprCXX.h
+++ b/clang/include/clang/AST/ExprCXX.h
@@ -2170,8 +2170,9 @@ class LambdaExpr final : public Expr,
   const_child_range children() const;
 };
 
-/// An expression "T()" which creates a value-initialized rvalue of type
-/// T, which is a non-class type.  See (C++98 [5.2.3p2]).
+/// An expression "T()" which creates an rvalue of a non-class type T.
+/// For non-void T, the rvalue is value-initialized.
+/// See (C++98 [5.2.3p2]).
 class CXXScalarValueInitExpr : public Expr {
   friend class ASTStmtReader;
 
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index 779a41620033d..269aedb59ca8f 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -1660,12 +1660,23 @@ Sema::BuildCXXTypeConstructExpr(TypeSourceInfo *TInfo,
     return ExprError(Diag(TyBeginLoc, diag::err_init_for_function_type)
                        << Ty << FullRange);
 
-  // C++17 [expr.type.conv]p2:
-  //   If the type is cv void and the initializer is (), the expression is a
-  //   prvalue of the specified type that performs no initialization.
-  if (!Ty->isVoidType() &&
-      RequireCompleteType(TyBeginLoc, ElemTy,
-                          diag::err_invalid_incomplete_type_use, FullRange))
+  // C++17 [expr.type.conv]p2, per DR2351:
+  //   If the type is cv void and the initializer is () or {}, the expression is
+  //   a prvalue of the specified type that performs no initialization.
+  if (Ty->isVoidType()) {
+    if (Exprs.empty())
+      return new (Context) CXXScalarValueInitExpr(
+          Ty.getUnqualifiedType(), TInfo, Kind.getRange().getEnd());
+    if (ListInitialization &&
+        cast<InitListExpr>(Exprs[0])->getNumInits() == 0) {
+      return CXXFunctionalCastExpr::Create(
+          Context, Ty.getUnqualifiedType(), VK_PRValue, TInfo, CK_NoOp,
+          Exprs[0], /*Path=*/nullptr, CurFPFeatureOverrides(),
+          Exprs[0]->getBeginLoc(), Exprs[0]->getEndLoc());
+    }
+  } else if (RequireCompleteType(TyBeginLoc, ElemTy,
+                                 diag::err_invalid_incomplete_type_use,
+                                 FullRange))
     return ExprError();
 
   //   Otherwise, the expression is a prvalue of the specified type whose
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index 003a157990d30..3bb8407c893d9 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -5344,6 +5344,7 @@ static void TryValueInitialization(Sema &S,
   //
   //   To value-initialize an object of type T means:
   QualType T = Entity.getType();
+  assert(!T->isVoidType() && "Cannot value-init void");
 
   //     -- if T is an array type, then each element is value-initialized;
   T = S.Context.getBaseElementType(T);
diff --git a/clang/test/CXX/drs/dr23xx.cpp b/clang/test/CXX/drs/dr23xx.cpp
index db5b7c3cd3c9a..cd21af0daf3d0 100644
--- a/clang/test/CXX/drs/dr23xx.cpp
+++ b/clang/test/CXX/drs/dr23xx.cpp
@@ -1,4 +1,5 @@
-// RUN: %clang_cc1 -std=c++98 %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -std=c++98 %s -verify=expected,cxx98-03 -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -std=c++03 %s -verify=expected,cxx98-03 -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
 // RUN: %clang_cc1 -std=c++11 %s -verify=expected,since-cxx11 -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
 // RUN: %clang_cc1 -std=c++14 %s -verify=expected,since-cxx11,since-cxx14 -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
 // RUN: %clang_cc1 -std=c++17 %s -verify=expected,since-cxx11,since-cxx14,since-cxx17 -fexceptions -fcxx-exceptions -pedantic-errors 2>&1 | FileCheck %s
@@ -70,6 +71,44 @@ namespace cwg2346 { // cwg2346: 11
   }
 }
 
+namespace cwg2351 { // cwg2351: 19
+#if __cplusplus >= 201103L
+  static_assert((void{}, true), "");
+  // since-cxx11-warning at -1 {{left operand of comma operator has no effect}}
+
+  void f() {
+    return void{};
+  }
+
+  template<typename T>
+  void g() {
+    return T{};
+  }
+  template void g<void>();
+  template void g<const void>();
+
+  void h() {
+    return {};
+    // since-cxx11-error at -1 {{void function 'h' must not return a value}}
+  }
+
+  template<typename T, int... I>
+  T i() {
+    return T{I...};
+  }
+  template void i<void>();
+  template const void i<const void>();
+
+  static_assert((void({}), true), "");
+  // since-cxx11-error at -1 {{cannot initialize non-class type 'void' with a parenthesized initializer list}}
+#else
+  int I = (void{}, 0);
+  // cxx98-03-error at -1 {{expected ')'}}
+  //   cxx98-03-note at -2 {{to match this '('}}
+  // cxx98-03-error at -3 {{expected expression}}
+#endif
+}
+
 namespace cwg2352 { // cwg2352: 10
   int **p;
   const int *const *const &f1() { return p; }
diff --git a/clang/test/SemaCXX/attr-annotate.cpp b/clang/test/SemaCXX/attr-annotate.cpp
index 3854f72bbcac1..846ef4119f1d7 100644
--- a/clang/test/SemaCXX/attr-annotate.cpp
+++ b/clang/test/SemaCXX/attr-annotate.cpp
@@ -43,10 +43,10 @@ namespace test0 {
   template<typename T>
   struct B {
     [[clang::annotate("test", ((void)T{}, 9))]] void t() {}
-    // expected-error at -1 {{illegal initializer type 'void'}}
+    // expected-error at -1 {{cannot create object of function type 'void ()'}}
   };
   B<int> b;
-  B<void> b1;
+  B<void ()> b1;
 // expected-note at -1 {{in instantiation of template class}}
 }
 
diff --git a/clang/test/SemaCXX/cxx2a-explicit-bool.cpp b/clang/test/SemaCXX/cxx2a-explicit-bool.cpp
index 9fdc059493aac..55f99b2fd355f 100644
--- a/clang/test/SemaCXX/cxx2a-explicit-bool.cpp
+++ b/clang/test/SemaCXX/cxx2a-explicit-bool.cpp
@@ -75,11 +75,11 @@ struct D {
 template <typename T> struct E {
   // expected-note at -1+ {{candidate constructor}}
   explicit((T{}, false))
-  // expected-error at -1 {{illegal initializer type 'void'}}
+  // expected-error at -1 {{cannot create object of function type 'void ()'}}
   E(int);
 };
 
-E<void> e = 1;
+E<void ()> e = 1;
 // expected-error at -1 {{no viable conversion}}
 // expected-note at -2 {{in instantiation of}}
 
diff --git a/clang/test/SemaCXX/sugared-auto.cpp b/clang/test/SemaCXX/sugared-auto.cpp
index 5fdfb09689b66..b5bb4f0f85a77 100644
--- a/clang/test/SemaCXX/sugared-auto.cpp
+++ b/clang/test/SemaCXX/sugared-auto.cpp
@@ -112,6 +112,12 @@ N t6 = [] { // expected-error {{rvalue of type 'void'}}
   return;
 }();
 
+N t7 = [] { // expected-error {{rvalue of type 'Virus' (aka 'void')}}
+  if (true)
+    return Ebola();
+  return SARS{};
+}();
+
 } // namespace function_multiple_basic
 
 #define TEST_AUTO(X, A, B) \
diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html
index ea8872c91be60..d696cc523e41d 100755
--- a/clang/www/cxx_dr_status.html
+++ b/clang/www/cxx_dr_status.html
@@ -13914,7 +13914,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2351.html">2351</a></td>
     <td>CD5</td>
     <td><TT>void{}</TT></td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 19</td>
   </tr>
   <tr id="2352">
     <td><a href="https://cplusplus.github.io/CWG/issues/2352.html">2352</a></td>

>From 70b62c2246be90ea71dd9b836be55674cc385c0a Mon Sep 17 00:00:00 2001
From: Mital Ashok <mital at mitalashok.co.uk>
Date: Tue, 21 May 2024 16:58:33 +0100
Subject: [PATCH 2/2] Change cast kind to CK_ToVoid instead of CK_NoOp

---
 clang/docs/ReleaseNotes.rst    | 4 ++--
 clang/lib/Sema/SemaExprCXX.cpp | 2 +-
 clang/test/CXX/drs/cwg23xx.cpp | 1 -
 3 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 0fca837204ba5..67a5cc48043fb 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -232,8 +232,8 @@ Resolutions to C++ Defect Reports
 - P0522 implementation is enabled by default in all language versions, and
   provisional wording for CWG2398 is implemented.
 
-- Implemented `CWG2351 <https://wg21.link/CWG2351>`_ which allows ``void{}``
-  as a prvalue of type ``void``.
+- Allow ``void{}`` as a prvalue of type ``void``.
+  (`CWG2351: void{} <https://cplusplus.github.io/CWG/issues/2351.html>`_).
 
 C Language Changes
 ------------------
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index cfb0091bde51e..00579eeb1204e 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -1671,7 +1671,7 @@ Sema::BuildCXXTypeConstructExpr(TypeSourceInfo *TInfo,
     if (ListInitialization &&
         cast<InitListExpr>(Exprs[0])->getNumInits() == 0) {
       return CXXFunctionalCastExpr::Create(
-          Context, Ty.getUnqualifiedType(), VK_PRValue, TInfo, CK_NoOp,
+          Context, Ty.getUnqualifiedType(), VK_PRValue, TInfo, CK_ToVoid,
           Exprs[0], /*Path=*/nullptr, CurFPFeatureOverrides(),
           Exprs[0]->getBeginLoc(), Exprs[0]->getEndLoc());
     }
diff --git a/clang/test/CXX/drs/cwg23xx.cpp b/clang/test/CXX/drs/cwg23xx.cpp
index f89865a360b72..3293e92a36f7d 100644
--- a/clang/test/CXX/drs/cwg23xx.cpp
+++ b/clang/test/CXX/drs/cwg23xx.cpp
@@ -118,7 +118,6 @@ namespace cwg2346 { // cwg2346: 11
 namespace cwg2351 { // cwg2351: 19
 #if __cplusplus >= 201103L
   static_assert((void{}, true), "");
-  // since-cxx11-warning at -1 {{left operand of comma operator has no effect}}
 
   void f() {
     return void{};



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