[clang] b896d26 - [C23][N3006] Documented behavior of underspecified object declarations (#140911)
via cfe-commits
cfe-commits at lists.llvm.org
Mon Jun 9 08:02:23 PDT 2025
Author: Guillot Tony
Date: 2025-06-09T11:02:20-04:00
New Revision: b896d262ebc530a92327b912b2bd5ac3b0bb0e56
URL: https://github.com/llvm/llvm-project/commit/b896d262ebc530a92327b912b2bd5ac3b0bb0e56
DIFF: https://github.com/llvm/llvm-project/commit/b896d262ebc530a92327b912b2bd5ac3b0bb0e56.diff
LOG: [C23][N3006] Documented behavior of underspecified object declarations (#140911)
This PR is documenting the behavior of Clang towards underspecified
object declarations in C23 as advised by @AaronBallman.
Added:
clang/test/C/C23/n3006.c
Modified:
clang/docs/LanguageExtensions.rst
clang/docs/ReleaseNotes.rst
clang/www/c_status.html
Removed:
################################################################################
diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
index 083c909088361..0ba301f438d04 100644
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -6502,3 +6502,77 @@ qualifications.
Note, Clang does not allow an ``_Atomic`` function type because
of explicit constraints against atomically qualified (arrays and) function
types.
+
+
+Underspecified Object Declarations in C
+=======================================
+
+C23 introduced the notion of `underspecified object declarations <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3006.htm>`_
+(note, the final standards text is
diff erent from WG14 N3006 due to changes
+during national body comment review). When an object is declared with the
+``constexpr`` storage class specifier or has a deduced type (with the ``auto``
+specifier), it is said to be "underspecified". Underspecified declarations have
+
diff erent requirements than non-underspecified declarations. In particular, the
+identifier being declared cannot be used in its initialization. e.g.,
+
+.. code-block:: c
+
+ auto x = x; // Invalid
+ constexpr int y = y; // Invalid
+
+The standard leaves it implementation-defined whether an underspecified
+declaration may introduce additional identifiers as part of the declaration.
+
+Clang allows additional identifiers to be declared in the following cases:
+
+* A compound literal may introduce a new type. e.g.,
+
+.. code-block:: c
+
+ auto x = (struct S { int x, y; }){ 1, 2 }; // Accepted by Clang
+ constexpr int i = (struct T { int x; }){ 1 }.x; // Accepted by Clang
+
+* The type specifier for a ``constexpr`` declaration may define a new type.
+ e.g.,
+
+.. code-block:: c
+
+ constexpr struct S { int x; } s = { 1 }; // Accepted by Clang
+
+* A function declarator may be declared with parameters, including parameters
+ which introduce a new type. e.g.,
+
+.. code-block:: c
+
+ constexpr int (*fp)(int x) = nullptr; // Accepted by Clang
+ auto f = (void (*)(struct S { int x; } s))nullptr; // Accepted by Clang
+
+* The initializer may contain a GNU statement expression which defines new
+ types or objects. e.g.,
+
+.. code-block:: c
+
+ constexpr int i = ({ // Accepted by Clang
+ constexpr int x = 12;
+ constexpr struct S { int x; } s = { x };
+ s.x;
+ });
+ auto x = ({ struct S { int x; } s = { 0 }; s; }); // Accepted by Clang
+
+Clang intentionally does not implement the changed scoping rules from C23
+for underspecified declarations. Doing so would significantly complicate the
+implementation in order to get reasonable diagnostic behavior and also means
+Clang fails to reject some code that should be rejected. e.g.,
+
+.. code-block:: c
+
+ // This should be rejected because 'x' is not in scope within the initializer
+ // of an underspecified declaration. Clang accepts because it treats the scope
+ // of the identifier as beginning immediately after the declarator, same as with
+ // a non-underspecified declaration.
+ constexpr int x = sizeof(x);
+
+ // Clang rejects this code with a diagnostic about using the variable within its
+ // own initializer rather than rejecting the code with an undeclared identifier
+ // diagnostic.
+ auto x = x;
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 322686fce0b04..db427451322a8 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -289,6 +289,8 @@ C23 Feature Support
directive. Fixes #GH126940.
- Fixed a crash when a declaration of a ``constexpr`` variable with an invalid
type. Fixes #GH140887
+- Documented `WG14 N3006 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3006.htm>`_
+ which clarified how Clang is handling underspecified object declarations.
C11 Feature Support
^^^^^^^^^^^^^^^^^^^
diff --git a/clang/test/C/C23/n3006.c b/clang/test/C/C23/n3006.c
new file mode 100644
index 0000000000000..e0713fa9969c4
--- /dev/null
+++ b/clang/test/C/C23/n3006.c
@@ -0,0 +1,113 @@
+// RUN: %clang_cc1 -std=c23 -verify %s
+
+/* WG14 N3006: Yes
+ * Underspecified object declarations
+ */
+
+void struct_test(void) {
+ struct S1 { int x, y; }; // expected-note {{field 'x' has type 'int' here}}
+
+ auto normal_struct = (struct S1){ 1, 2 };
+ auto normal_struct2 = (struct S1) { .x = 1, .y = 2 };
+ auto underspecified_struct = (struct S2 { int x, y; }){ 1, 2 };
+ auto underspecified_struct_redef = (struct S1 { char x, y; }){ 'A', 'B'}; // expected-error {{type 'struct S1' has incompatible definitions}} \
+ expected-error {{cannot use 'auto' with array in C}} \
+ expected-note {{field 'x' has type 'char' here}}
+ auto underspecified_empty_struct = (struct S3 { }){ };
+ auto zero_init_struct = (struct S4 { int x; }){ 0 };
+ int field_struct = (struct S5 { int y; }){ 0 }.y;
+}
+
+void union_test(void) {
+ union U1 { int a; double b; }; // expected-note {{field 'a' has type 'int' here}}
+
+ auto normal_union_int = (union U1){ .a = 12 };
+ auto normal_union_double = (union U1){ .b = 2.4 };
+ auto underspecified_union = (union U2 { int a; double b; }){ .a = 34 };
+ auto underspecified_union_redef = (union U1 { char a; double b; }){ .a = 'A' }; // expected-error {{type 'union U1' has incompatible definitions}} \
+ expected-error {{cannot use 'auto' with array in C}} \
+ expected-note {{field 'a' has type 'char' here}}
+ auto underspecified_empty_union = (union U3 { }){ };
+}
+
+void enum_test(void) {
+ enum E1 { FOO, BAR }; // expected-note {{enumerator 'BAR' with value 1 here}}
+
+ auto normal_enum_foo = (enum E1){ FOO };
+ auto normal_enum_bar = (enum E1){ BAR };
+ auto underspecified_enum = (enum E2 { BAZ, QUX }){ BAZ };
+ auto underspecified_enum_redef = (enum E1 { ONE, TWO }){ ONE }; // expected-error {{type 'enum E1' has incompatible definitions}} \
+ expected-error {{cannot use 'auto' with array in C}} \
+ expected-note {{enumerator 'ONE' with value 0 here}}
+ auto underspecified_empty_enum = (enum E3 { }){ }; // expected-error {{use of empty enum}}
+ auto underspecified_undeclared_enum = (enum E4){ FOO }; // expected-error {{variable has incomplete type 'enum E4'}} \
+ expected-note {{forward declaration of 'enum E4'}}
+}
+
+void constexpr_test(void) {
+ constexpr auto ce_struct = (struct S1){ 1, 2 }; // expected-error {{variable has incomplete type 'struct S1'}} \
+ expected-note {{forward declaration of 'struct S1'}}
+ constexpr auto ce_struct_zero_init = (struct S2 { int x; }){ 0 };
+ constexpr int ce_struct_field = (struct S3 { int y; }){ 0 }.y;
+ constexpr auto ce_union = (union U1){ .a = 12 }; // expected-error {{variable has incomplete type 'union U1'}} \
+ expected-note {{forward declaration of 'union U1'}}
+
+ constexpr auto ce_enum = (enum E1 { BAZ, QUX }){ BAZ };
+ constexpr auto ce_empty_enum = (enum E2){ FOO }; // expected-error {{use of undeclared identifier 'FOO'}}
+}
+
+void self_reference_test(void) {
+ constexpr int i = i; // expected-error {{constexpr variable 'i' must be initialized by a constant expression}} \
+ expected-note {{read of object outside its lifetime is not allowed in a constant expression}}
+ auto j = j; // expected-error {{variable 'j' declared with deduced type 'auto' cannot appear in its own initializer}}
+}
+
+void redefinition_test(void) {
+ const struct S { int x; } s; // expected-warning {{default initialization of an object of type 'const struct S' leaves the object uninitialized}} \
+ expected-note {{previous definition is here}}
+ constexpr struct S s = {0}; // expected-error {{redefinition of 's'}}
+}
+
+void declaring_an_underspecified_defied_object_test(void) {
+ struct S { int x, y; };
+ constexpr int i = (struct T { int a, b; }){0, 1}.a;
+
+ struct T t = { 1, 2 };
+}
+
+void constexpr_complience_test(void) {
+ int x = (struct Foo { int x; }){ 0 }.x;
+ constexpr int y = (struct Bar { int x; }){ 0 }.x;
+}
+
+void builtin_functions_test(void) {
+ constexpr typeof(struct s *) x = 0;
+ auto so = sizeof(struct S {});
+ auto to = (typeof(struct S {})){};
+}
+
+void misc_test(void) {
+ constexpr struct S { int a, b; } y = { 0 };
+ constexpr int a = 0, b = 0;
+ auto c = (struct T { int x, y; }){0, 0};
+ auto s2 = ({struct T { int x; } s = {}; s.x; });
+ auto s3 = ((struct {}){},0); // expected-warning {{left operand of comma operator has no effect}}
+ constexpr int (*fp)(struct X { int x; } val) = 0;
+ auto v = (void (*)(int y))0;
+}
+
+void misc_struct_test(void) {
+ constexpr struct {
+ int a;
+ } a = {};
+
+ constexpr struct {
+ int b;
+ } b = (struct S { int x; }){ 0 }; // expected-error-re {{initializing 'const struct (unnamed struct at {{.*}}n3006.c:104:13)' with an expression of incompatible type 'struct S'}}
+
+ auto z = ({
+ int a = 12;
+ struct {} s;
+ a;
+ });
+}
diff --git a/clang/www/c_status.html b/clang/www/c_status.html
index e47466e3273f2..dcff2fc2b1a3e 100644
--- a/clang/www/c_status.html
+++ b/clang/www/c_status.html
@@ -864,7 +864,7 @@ <h2 id="c2x">C23 implementation status</h2>
<tr>
<td>Underspecified object definitions</td>
<td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3006.htm">N3006</a></td>
- <td class="none" align="center">No</td>
+ <td class="unreleased" align="center">Yes</td>
</tr>
<tr>
<td>Type inference for object declarations</td>
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