[clang][C23] N3006 Underspecified object declarations

clang/docs/ReleaseNotes.rst

@@ -98,6 +98,8 @@ C2y Feature Support
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^
 
+- Clang now supports `N3006 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3006.htm>`_ Underspecified object declarations.
+
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -7937,6 +7937,8 @@ def err_attribute_arm_mve_polymorphism : Error<
   "'__clang_arm_mve_strict_polymorphism' attribute can only be applied to an MVE/NEON vector type">;
 def err_attribute_webassembly_funcref : Error<
   "'__funcref' attribute can only be applied to a function pointer type">;
+def err_c23_underspecified_object_declaration: Error<
+  "'%select{struct|<ERROR>|union|<ERROR>|enum}0 %1' is defined as an underspecified object initializer">;
 
 def warn_setter_getter_impl_required : Warning<
   "property %0 requires method %1 to be defined - "

clang/lib/Sema/SemaExpr.cpp

@@ -7101,10 +7101,38 @@ Sema::BuildCompoundLiteralExpr(SourceLocation LParenLoc, TypeSourceInfo *TInfo,
                                            diagID))
         return ExprError();
     }
+  } else if (LangOpts.C23 &&
+             (literalType->isRecordType() || literalType->isEnumeralType())) {
+    // C23 6.2.1p7: Structure, union, and enumeration tags have scope that
+    // begins just after the appearance of the tag in a type specifier that
+    // declares the tag.
+    // [...]
+    // An ordinary identifier that has an underspecified definition has scope
+    // that starts when the definition is completed; if the same ordinary
+    // identifier declares another entity with a scope that encloses the current
+    // block, that declaration is hidden as soon as the inner declarator is
+    // completed*.)
+    // [...]
+    // *) That means, that the outer declaration is not visible for the
+    // initializer.
+    auto Range = SourceRange(LParenLoc, RParenLoc);
+    const auto *Tag = literalType->castAs<TagType>();
+    const auto &TagRange = Tag->getDecl()->getSourceRange();
+
+    // We should diagnose underspecified declaration, unless the identifier has
+    // been diagnosed as being a redefinition, since the tag is made anonymous.
+    if (Range.fullyContains(TagRange) && Tag->getDecl()->getIdentifier()) {
+      Diag(TagRange.getBegin(), diag::err_c23_underspecified_object_declaration)
+          << (unsigned)Tag->getDecl()->getTagKind() << Tag->getDecl()->getName()
+          << TagRange;
+      return ExprError();
+    }
   } else if (!literalType->isDependentType() &&
-             RequireCompleteType(LParenLoc, literalType,
-               diag::err_typecheck_decl_incomplete_type,
-               SourceRange(LParenLoc, LiteralExpr->getSourceRange().getEnd())))
+             RequireCompleteType(
+                 LParenLoc, literalType,
+                 diag::err_typecheck_decl_incomplete_type,
+                 SourceRange(LParenLoc,
+                             LiteralExpr->getSourceRange().getEnd())))
     return ExprError();
 
   InitializedEntity Entity

clang/test/C/C23/n3006.c

@@ -0,0 +1,66 @@
+// RUN: %clang_cc1 -std=c2x -verify %s
+
+/* WG14 N3006: Full
+ * Underspecified object declarations
+ */
+
+struct S1 { int x, y; };        // expected-note {{previous definition is here}}
+union U1 { int a; double b; };  // expected-note {{previous definition is here}}
+enum E1 { FOO, BAR };           // expected-note {{previous definition is 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 };           // expected-error {{'struct S2' is defined as an underspecified object initializer}}
+auto underspecified_struct_redef = (struct S1 { char x, y; }){ 'A', 'B'}; // expected-error {{redefinition of 'S1'}}
+auto underspecified_empty_struct = (struct S3 { }){ };                    // expected-error {{'struct S3' is defined as an underspecified object initializer}}
+
+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 };         // expected-error {{'union U2' is defined as an underspecified object initializer}}
+auto underspecified_union_redef = (union U1 { char a; double b; }){ .a = 'A' }; // expected-error {{redefinition of 'U1'}}
+auto underspecified_empty_union = (union U3 {  }){  };                          // expected-error {{'union U3' is defined as an underspecified object initializer}}
+
+auto normal_enum_foo = (enum E1){ FOO };
+auto normal_enum_bar = (enum E1){ BAR };
+auto underspecified_enum = (enum E2 { BAZ, QUX }){ BAZ };       // expected-error {{'enum E2' is defined as an underspecified object initializer}}
+auto underspecified_enum_redef = (enum E1 { ONE, TWO }){ ONE }; // expected-error {{redefinition of 'E1'}}
+auto underspecified_empty_enum = (enum E3 {  }){ };             // expected-error {{'enum E3' is defined as an underspecified object initializer}} \
+                                                                     expected-error {{use of empty enum}}
+void constexpr_test() {
+  constexpr auto ce_struct = (struct S1){ 1, 2 };
+  constexpr auto ce_union = (union U1){ .a = 12 };
+  constexpr auto ce_enum = (enum E1){ FOO };
+}
+
+void trivial_test() {
+  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 double_definition_test() {
+  const struct S { int x; } s;  // expected-note {{previous definition is here}}
+  constexpr struct S s = {0};   // expected-error {{redefinition of 's'}}
+}
+
+void declaring_an_underspecified_defied_object_test() {
+  struct S { int x, y; };
+  constexpr int i = (struct T { int a, b; }){0, 1}.a;  // expected-error {{'struct T' is defined as an underspecified object initializer}} \
+                                                          FIXME: `constexpr variable 'i' must be initialized by a constant expression` shoud appear
+
+  struct T t = { 1, 2 };                               // TODO: Should this be diagnosed as an invalid declaration?
+}
+
+void constexpr_complience_test() {
+  int x = (struct Foo { int x; }){ 0 }.x;           // expected-error {{'struct Foo' is defined as an underspecified object initializer}}
+  constexpr int y = (struct Bar { int x; }){ 0 }.x; // expected-error {{'struct Bar' is defined as an underspecified object initializer}}
+}
+
+void special_test() {
+  constexpr typeof(struct s *) x = 0;               // FIXME: declares `s` which is not an ordinary identifier
+  constexpr struct S { int a, b; } y = { 0 };    // FIXME: declares `S`, `a`, and `b`, none of which are ordinary identifiers
+  constexpr int a = 0, b = 0;
+  auto c = (struct T { int x, y; }){0, 0};          // expected-error {{'struct T' is defined as an underspecified object initializer}}
+  constexpr int (*fp)(struct X { int x; } val) = 0; // expected-warning {{declaration of 'struct X' will not be visible outside of this function}} \
+                                                       FIXME: declares `X` and `x` which are not ordinary identifiers
+}

clang/test/Parser/c23-underspecified-decls.c

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,c23 -std=c23 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,c17 -std=c17 %s
+
+auto underspecified_struct = (struct S1 { int x, y; }){ 1, 2 };         // c23-error {{'struct S1' is defined as an underspecified object initializer}} \
+                                                                           c17-error {{type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int}} \
+                                                                           c17-error {{illegal storage class on file-scoped variable}}
+auto underspecified_union = (union U1 { int a; double b; }){ .a = 34 }; // c23-error {{'union U1' is defined as an underspecified object initializer}} \
+                                                                           c17-error {{type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int}} \
+                                                                           c17-error {{illegal storage class on file-scoped variable}}
+auto underspecified_enum = (enum E1 { FOO, BAR }){ BAR };               // c23-error {{'enum E1' is defined as an underspecified object initializer}} \
+                                                                           c17-error {{type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int}} \
+                                                                           c17-error {{illegal storage class on file-scoped variable}}

clang/www/c_status.html

@@ -797,7 +797,7 @@ <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">Clang 21</td>
     </tr>
     <tr>
       <td>Type inference for object declarations</td>