[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.

Author: to268

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 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
+
+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;

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
 ^^^^^^^^^^^^^^^^^^^

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;
+  });
+}

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>