================ @@ -6454,3 +6454,51 @@ 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 +======================================= + +In C23 (N3006), when an object is declared inside of another object and that +only exists in the scope of the declaration of the outer object. Clang allows +underspecified object declarations for structs, unions and enums when valid. ---------------- AaronBallman wrote:
I have a different formulation in mind. WDYT about this? ``` 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 different 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 different 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 ``` https://github.com/llvm/llvm-project/pull/140911 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
