[clang] [SemaCXX] Implement CWG2351 `void{}` (PR #78060)
via cfe-commits
cfe-commits at lists.llvm.org
Sat Jan 13 10:40:49 PST 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang
Author: Mital Ashok (MitalAshok)
<details>
<summary>Changes</summary>
As per [CWG2351](https://wg21.link/CWG2351), allow `void{}`, treated the same as `void()`: a prvalue expression of type `void` that performs no initialization.
Note that the AST for the expression `T{}` looks like:
```
// using T = int;
CXXFunctionalCastExpr 'T':'int' functional cast to T <NoOp>
`-InitListExpr 'T':'int'
// using T = const int;
CXXFunctionalCastExpr 'int' functional cast to T <NoOp>
`-InitListExpr 'int'
// using T = void;
CXXFunctionalCastExpr 'void' functional cast to T <NoOp>
`-InitListExpr 'void'
// using T = const void;
CXXFunctionalCastExpr 'void' functional cast to T <NoOp>
`-InitListExpr 'void'
```
(Since the `InitListExpr` already has `void` before anything is done, the type doesn't need to be adjusted)
As for `void()`/`T() [T = const void]`, that looked like `CXXScalarValueInitExpr 'void'` and is unchanged after this.
For reference, C++98 [5.2.3p2] says:
> The expression `T()`, where `T` is a simple-type-specifier (7.1.5.2) for a non-array complete object type or the (possibly cv-qualified) void type, creates an rvalue of the specified type, whose value is determined by default-initialization (8.5; no initialization is done for the `void()` case). [*Note:* if `T` is a non-class type that is *cv-qualified*, the `cv-qualifiers` are ignored when determining the type of the resulting rvalue (3.10). ]
Though it is a bit of a misnomer that, for `T = void`, `CXXScalarValueInitExpr` does not perform value initialization, it would be a breaking change to change the AST node for `void()`, so I simply reworded the doc comment.
---
Full diff: https://github.com/llvm/llvm-project/pull/78060.diff
5 Files Affected:
- (modified) clang/docs/ReleaseNotes.rst (+2)
- (modified) clang/include/clang/AST/ExprCXX.h (+3-2)
- (modified) clang/lib/Sema/SemaExprCXX.cpp (+15-6)
- (modified) clang/test/CXX/drs/dr23xx.cpp (+31)
- (modified) clang/www/cxx_dr_status.html (+1-1)
``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 3cbce1be159437..00e200ea76e5bc 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -226,6 +226,8 @@ C++2c Feature Support
Resolutions to C++ Defect Reports
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+- 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 24278016431837..e7b732e4181127 100644
--- a/clang/include/clang/AST/ExprCXX.h
+++ b/clang/include/clang/AST/ExprCXX.h
@@ -2160,8 +2160,9 @@ 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 type T, which is a
+/// non-class type. 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 4ae04358d5df7c..1792ec6bb292d5 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -1600,12 +1600,21 @@ 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(Context.VoidTy, TInfo,
+ Kind.getRange().getEnd());
+ if (ListInitialization && cast<InitListExpr>(Exprs[0])->getNumInits() == 0)
+ return CXXFunctionalCastExpr::Create(
+ Context, Context.VoidTy, 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/test/CXX/drs/dr23xx.cpp b/clang/test/CXX/drs/dr23xx.cpp
index d2f4e7652ab568..1bf30a80f743e5 100644
--- a/clang/test/CXX/drs/dr23xx.cpp
+++ b/clang/test/CXX/drs/dr23xx.cpp
@@ -69,6 +69,37 @@ namespace dr2346 { // dr2346: 11
}
}
+namespace dr2351 { // dr2351: 18
+#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>();
+
+ 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>();
+
+ static_assert((void({}), true), "");
+ // since-cxx11-error at -1 {{cannot initialize non-class type 'void' with a parenthesized initializer list}}
+#endif
+}
+
namespace dr2352 { // dr2352: 10
int **p;
const int *const *const &f1() { return p; }
diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html
index 4a3ed19161f9a5..207f032a5d45e4 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 18</td>
</tr>
<tr id="2352">
<td><a href="https://cplusplus.github.io/CWG/issues/2352.html">2352</a></td>
``````````
</details>
https://github.com/llvm/llvm-project/pull/78060
More information about the cfe-commits
mailing list