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> _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
