llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang
Author: Aaron Ballman (AaronBallman)
<details>
<summary>Changes</summary>
This paper removes UB around use of void expressions. Previously, code like
this had undefined behavior:
```
void foo(void) {
(void)(void)1;
extern void x;
x;
}
```
and this is now well-defined in C2y. Functionally, this now means that it is
valid to use `void` as a `_Generic` association.
---
Full diff: https://github.com/llvm/llvm-project/pull/130299.diff
4 Files Affected:
- (modified) clang/docs/ReleaseNotes.rst (+4)
- (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+7-2)
- (modified) clang/lib/Sema/SemaExpr.cpp (+6-1)
- (added) clang/test/C/C2y/n3409.c (+32)
``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 577b3f2130df7..df8d2eed1ec0c 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -114,6 +114,10 @@ C2y Feature Support
- Implemented N3411 which allows a source file to not end with a newline
character. This is still reported as a conforming extension in earlier
language modes.
+- Implement `WG14 N3409
<https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3409.pdf>`_
+ which removes UB around use of ``void`` expressions. In practice, this means
+ that ``_Generic`` selection associations may now have ``void`` type, but it
+ also removes UB with code like ``(void)(void)1;``.
C23 Feature Support
^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td
b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 1b46920e09619..d6e5005003322 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -10425,8 +10425,13 @@ def warn_type_safety_null_pointer_required : Warning<
"specified %0 type tag requires a null pointer">, InGroup<TypeSafety>;
// Generic selections.
-def err_assoc_type_incomplete : Error<
- "type %0 in generic association incomplete">;
+def ext_assoc_type_incomplete : Extension<
+ "ISO C requires a complete type in a '_Generic' association; %0 is an "
+ "incomplete type">;
+def warn_c2y_compat_assoc_type_incomplete : Warning<
+ "use of an incomplete type in a '_Generic' association is incompatible with "
+ "C standards before C2y; %0 is an incomplete type">,
+ InGroup<CPre2yCompat>, DefaultIgnore;
def err_assoc_type_nonobject : Error<
"type %0 in generic association not an object type">;
def err_assoc_type_variably_modified : Error<
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index f896ccab53a54..de7be6b2805af 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -1748,9 +1748,14 @@ ExprResult Sema::CreateGenericSelectionExpr(
//
// C11 6.5.1.1p2 "The type name in a generic association shall specify
a
// complete object type other than a variably modified type."
+ // C2y removed the requirement that an expression form must
+ // use a complete type, though it's still as-if the type has undergone
+ // lvalue conversion. We support this as an extension in C23 and
+ // earlier because GCC does so.
unsigned D = 0;
if (ControllingExpr && Types[i]->getType()->isIncompleteType())
- D = diag::err_assoc_type_incomplete;
+ D = LangOpts.C2y ? diag::warn_c2y_compat_assoc_type_incomplete
+ : diag::ext_assoc_type_incomplete;
else if (ControllingExpr && !Types[i]->getType()->isObjectType())
D = diag::err_assoc_type_nonobject;
else if (Types[i]->getType()->isVariablyModifiedType())
diff --git a/clang/test/C/C2y/n3409.c b/clang/test/C/C2y/n3409.c
new file mode 100644
index 0000000000000..2fc789891c71d
--- /dev/null
+++ b/clang/test/C/C2y/n3409.c
@@ -0,0 +1,32 @@
+// RUN: %clang_cc1 -verify -std=c2y -pedantic %s
+// RUN: %clang_cc1 -verify=pre-c2y -std=c2y -Wpre-c2y-compat %s
+// RUN: %clang_cc1 -verify=ext -std=c23 -pedantic %s
+// expected-no-diagnostics
+
+/* WG14 N3409: Clang 21
+ * Slay Some Earthly Demons X
+ *
+ * Removes the requirement that an expression with type void cannot be used in
+ * any way. This was making it UB to use a void expression in a _Generic
+ * selection expression for no good reason, as well as making it UB to cast a
+ * void expression to void, etc.
+ */
+
+extern void x;
+void foo() {
+ // FIXME: this is technically an extension before C2y and should be diagnosed
+ // under -pedantic.
+ (void)(void)1;
+ // FIXME: same with this.
+ x;
+ _Generic(x, void: 1); /* pre-c2y-warning {{use of an incomplete type in
a '_Generic' association is incompatible with C standards before C2y; 'void' is
an incomplete type}}
+ ext-warning {{ISO C requires a complete type
in a '_Generic' association; 'void' is an incomplete type}}
+ */
+ _Generic(x, typeof(x): 1); /* pre-c2y-warning {{use of an incomplete type in
a '_Generic' association is incompatible with C standards before C2y; 'typeof
(x)' (aka 'void') is an incomplete type}}
+ ext-warning {{ISO C requires a complete type
in a '_Generic' association; 'typeof (x)' (aka 'void') is an incomplete type}}
+ */
+ (void)_Generic(void, default : 1); /* pre-c2y-warning {{passing a type
argument as the first operand to '_Generic' is incompatible with C standards
before C2y}}
+ ext-warning {{passing a type argument
as the first operand to '_Generic' is a C2y extension}}
+ */
+}
+
``````````
</details>
https://github.com/llvm/llvm-project/pull/130299
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