c++: Implement P2615 'Meaningful Exports' [PR107688]

This clarifies which kinds of declarations may and may not be exported
in various contexts. The patch additionally fixes up some small issues
that were clarified by the paper.

Most of the changes are with regards to export-declarations, which are
applied for all standards modes that we support '-fmodules-ts' for.
However there are also a couple of changes made to linkage specifiers
('extern "C"'); I've applied these as since C++20, to line up with when
modules were actually introduced.

	PR c++/107688

gcc/cp/ChangeLog:

	* name-lookup.cc (push_namespace): Error when exporting
	namespace with internal linkage.
	* parser.h (struct cp_parser): Add new flag
	'in_unbraced_export_declaration_p'.
	* parser.cc (cp_debug_parser): Print the new flag.
	(cp_parser_new): Initialise the new flag.
	(cp_parser_module_export): Set the new flag.
	(cp_parser_class_specifier): Clear and restore the new flag.
	(cp_parser_import_declaration): Imports can now appear directly
	in a linkage specification.
	(cp_parser_declaration): Categorise declarations as "name" or
	"special"; error on the later in contexts where the former is
	required.
	(cp_parser_class_head): Error when exporting a partial
	specialisation.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/contracts-1_a.C: Avoid now-illegal syntax.
	* g++.dg/modules/contracts-2_a.C: Likewise.
	* g++.dg/modules/contracts-3_a.C: Likewise.
	* g++.dg/modules/contracts-4_a.C: Likewise.
	* g++.dg/modules/lang-1_c.C: Clarify now-legal syntax.
	* g++.dg/modules/pr101582-1.C: Remove now-legal XFAILS.
	* g++.dg/template/crash71.C: Update error messages.
	* g++.dg/cpp2a/linkage-spec1.C: New test.
	* g++.dg/modules/export-3.C: New test.
	* g++.dg/modules/export-4_a.C: New test.
	* g++.dg/modules/export-4_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

gcc/cp/name-lookup.cc

@@ -9143,8 +9143,14 @@ push_namespace (tree name, bool make_inline)
     {
       /* A public namespace is exported only if explicitly marked, or
 	 it contains exported entities.  */
-      if (TREE_PUBLIC (ns) && module_exporting_p ())
-	DECL_MODULE_EXPORT_P (ns) = true;
+      if (module_exporting_p ())
+	{
+	  if (TREE_PUBLIC (ns))
+	    DECL_MODULE_EXPORT_P (ns) = true;
+	  else if (!header_module_p ())
+	    error_at (input_location,
+		      "exporting namespace with internal linkage");
+	}
       if (module_purview_p ())
 	DECL_MODULE_PURVIEW_P (ns) = true;
 

gcc/cp/parser.cc

@@ -560,6 +560,8 @@ cp_debug_parser (FILE *file, cp_parser *parser)
 			       & THIS_FORBIDDEN));
   cp_debug_print_flag (file, "In unbraced linkage specification",
 			      parser->in_unbraced_linkage_specification_p);
+  cp_debug_print_flag (file, "In unbraced export declaration",
+			      parser->in_unbraced_export_declaration_p);
   cp_debug_print_flag (file, "Parsing a declarator",
 			      parser->in_declarator_p);
   cp_debug_print_flag (file, "In template argument list",
@@ -4425,6 +4427,9 @@ cp_parser_new (cp_lexer *lexer)
   /* We are not processing an `extern "C"' declaration.  */
   parser->in_unbraced_linkage_specification_p = false;
 
+  /* We aren't parsing an export-declaration.  */
+  parser->in_unbraced_export_declaration_p = false;
+
   /* We are not processing a declarator.  */
   parser->in_declarator_p = false;
 
@@ -15249,10 +15254,6 @@ cp_parser_import_declaration (cp_parser *parser, module_parse mp_state,
 	goto skip_eol;
       cp_parser_require_pragma_eol (parser, token);
 
-      if (parser->in_unbraced_linkage_specification_p)
-	error_at (token->location, "import cannot appear directly in"
-		  " a linkage-specification");
-
       if (mp_state == MP_PURVIEW_IMPORTS || mp_state == MP_PRIVATE_IMPORTS)
 	{
 	  /* Module-purview imports must not be from source inclusion
@@ -15273,7 +15274,7 @@ cp_parser_import_declaration (cp_parser *parser, module_parse mp_state,
 
 /*  export-declaration.
 
-    export declaration
+    export name-declaration
     export { declaration-seq-opt }  */
 
 static void
@@ -15315,7 +15316,13 @@ cp_parser_module_export (cp_parser *parser)
 	  || cp_lexer_next_token_is_keyword (parser->lexer, RID__EXPORT))
 	error_at (token->location, "%<export%> not part of following"
 		  " module-directive");
+
+      bool saved_in_unbraced_export_declaration_p
+	= parser->in_unbraced_export_declaration_p;
+      parser->in_unbraced_export_declaration_p = true;
       cp_parser_declaration (parser, NULL_TREE);
+      parser->in_unbraced_export_declaration_p
+	= saved_in_unbraced_export_declaration_p;
     }
 
   module_kind = mk;
@@ -15346,27 +15353,29 @@ cp_parser_declaration_seq_opt (cp_parser* parser)
     }
 }
 
-/* Parse a declaration.
+/* Parse a declaration.  The distinction between name-declaration
+   and special-declaration is only since C++20.
 
    declaration:
+     name-declaration
+     special-declaration
+
+   name-declaration:
      block-declaration
+     nodeclspec-function-declaration
      function-definition
      template-declaration
-     explicit-instantiation
-     explicit-specialization
+     deduction-guide  (C++17)
      linkage-specification
      namespace-definition
+     empty-declaration
+     attribute-declaration
+     module-import-declaration  (modules)
 
-   C++17:
-     deduction-guide
-
-   modules:
-     (all these are only allowed at the outermost level, check
-   	that semantically, for better diagnostics)
-     module-declaration
-     module-export-declaration
-     module-import-declaration
-     export-declaration
+   special-declaration:
+     explicit-instantiation
+     explicit-specialization
+     export-declaration   (modules)
 
    GNU extension:
 
@@ -15389,6 +15398,13 @@ cp_parser_declaration (cp_parser* parser, tree prefix_attrs)
       return;
     }
 
+  /* P2615: Determine if we're parsing a name-declaration specifically,
+     or if special-declarations are OK too.  */
+  bool require_name_decl_p
+    = (parser->in_unbraced_export_declaration_p
+       || (parser->in_unbraced_linkage_specification_p
+	   && cxx_dialect >= cxx20));
+
   /* Try to figure out what kind of declaration is present.  */
   cp_token *token1 = cp_lexer_peek_token (parser->lexer);
   cp_token *token2 = (token1->type == CPP_EOF
@@ -15496,13 +15512,30 @@ cp_parser_declaration (cp_parser* parser, tree prefix_attrs)
       /* `template <>' indicates a template specialization.  */
       if (token2->type == CPP_LESS
 	  && cp_lexer_peek_nth_token (parser->lexer, 3)->type == CPP_GREATER)
-	cp_parser_explicit_specialization (parser);
+	{
+	  if (require_name_decl_p)
+	    {
+	      auto_diagnostic_group d;
+	      cp_token *token3 = cp_lexer_peek_nth_token (parser->lexer, 3);
+	      location_t loc = make_location (token1, token1, token3);
+	      error_at (loc, "explicit specializations are not permitted here");
+	      if (parser->in_unbraced_export_declaration_p)
+		inform (loc, "a specialization is always exported alongside "
+			"its primary template");
+	    }
+	  cp_parser_explicit_specialization (parser);
+	}
       /* `template <' indicates a template declaration.  */
       else if (token2->type == CPP_LESS)
 	cp_parser_template_declaration (parser, /*member_p=*/false);
       /* Anything else must be an explicit instantiation.  */
       else
-	cp_parser_explicit_instantiation (parser);
+	{
+	  if (require_name_decl_p)
+	    error_at (token1->location,
+		     "explicit instantiations are not permitted here");
+	  cp_parser_explicit_instantiation (parser);
+	}
     }
   /* If the next token is `export', it's new-style modules or
      old-style template.  */
@@ -15511,7 +15544,14 @@ cp_parser_declaration (cp_parser* parser, tree prefix_attrs)
       if (!modules_p ())
 	cp_parser_template_declaration (parser, /*member_p=*/false);
       else
-	cp_parser_module_export (parser);
+	{
+	  /* We check for nested exports in cp_parser_module_export.  */
+	  if (require_name_decl_p
+	      && !parser->in_unbraced_export_declaration_p)
+	    error_at (token1->location,
+		      "export-declarations are not permitted here");
+	  cp_parser_module_export (parser);
+	}
     }
   else if (cp_token_is_module_directive (token1))
     {
@@ -16836,7 +16876,7 @@ cp_parser_function_specifier_opt (cp_parser* parser,
 
    linkage-specification:
      extern string-literal { declaration-seq [opt] }
-     extern string-literal declaration  */
+     extern string-literal name-declaration  */
 
 static void
 cp_parser_linkage_specification (cp_parser* parser, tree prefix_attr)
@@ -26795,6 +26835,7 @@ cp_parser_class_specifier (cp_parser* parser)
   unsigned char in_statement;
   bool in_switch_statement_p;
   bool saved_in_unbraced_linkage_specification_p;
+  bool saved_in_unbraced_export_declaration_p;
   tree old_scope = NULL_TREE;
   tree scope = NULL_TREE;
   cp_token *closing_brace;
@@ -26846,6 +26887,10 @@ cp_parser_class_specifier (cp_parser* parser)
   saved_in_unbraced_linkage_specification_p
     = parser->in_unbraced_linkage_specification_p;
   parser->in_unbraced_linkage_specification_p = false;
+  /* Or in an export-declaration.  */
+  saved_in_unbraced_export_declaration_p
+    = parser->in_unbraced_export_declaration_p;
+  parser->in_unbraced_export_declaration_p = false;
   /* 'this' from an enclosing non-static member function is unavailable.  */
   tree saved_ccp = current_class_ptr;
   tree saved_ccr = current_class_ref;
@@ -27228,6 +27273,8 @@ cp_parser_class_specifier (cp_parser* parser)
     = saved_num_template_parameter_lists;
   parser->in_unbraced_linkage_specification_p
     = saved_in_unbraced_linkage_specification_p;
+  parser->in_unbraced_export_declaration_p
+    = saved_in_unbraced_export_declaration_p;
   current_class_ptr = saved_ccp;
   current_class_ref = saved_ccr;
 
@@ -27515,6 +27562,20 @@ cp_parser_class_head (cp_parser* parser,
 	permerror (nested_name_specifier_token_start->location,
 		   "extra qualification not allowed");
     }
+  /* The name-declaration of an export-declaration shall not declare
+     a partial specialization.  */
+  if (template_id_p
+      && parser->in_unbraced_export_declaration_p
+      && !processing_specialization
+      && !processing_explicit_instantiation)
+    {
+      auto_diagnostic_group d;
+      location_t loc = type_start_token->location;
+      error_at (loc, "declaration of partial specialization in unbraced "
+		"export-declaration");
+      inform (loc, "a specialization is always exported alongside its "
+	      "primary template");
+    }
   /* An explicit-specialization must be preceded by "template <>".  If
      it is not, try to recover gracefully.  */
   if (at_namespace_scope_p ()

gcc/cp/parser.h

@@ -312,9 +312,13 @@ struct GTY(()) cp_parser {
 
   /* TRUE if the declaration we are parsing is part of a
      linkage-specification of the form `extern string-literal
-     declaration'.  */
+     name-declaration'.  */
   bool in_unbraced_linkage_specification_p;
 
+  /* TRUE if the declaration we are parsing is part of an
+     export-declaration of the form 'export name-declaration'.  */
+  bool in_unbraced_export_declaration_p;
+
   /* TRUE if we are presently parsing a declarator, after the
      direct-declarator.  */
   bool in_declarator_p;

gcc/testsuite/g++.dg/cpp2a/linkage-spec1.C

@@ -0,0 +1,22 @@
+// PR c++/107688
+// P2615R1: Meaningful exports: Newly invalid declarations
+// { dg-do compile { target c++11 } }
+
+extern "C++" template <typename T> struct A {};
+
+extern "C++" template <typename T> struct A<T*> {};
+
+extern "C++" template <> struct A<int*> {};
+// { dg-error "explicit specializations are not permitted here" "" { target c++20 } .-1 }
+
+extern "C++" template struct A<int>;
+// { dg-error "explicit instantiations are not permitted here" "" { target c++20 } .-1 }
+
+
+// These should all still be valid, though
+extern "C++" {
+  template <typename T> struct B {};
+  template <typename T> struct B<T*> {};
+  template <> struct B<int*> {};
+  template struct B<int>;
+}

gcc/testsuite/g++.dg/modules/contracts-1_a.C

@@ -8,7 +8,7 @@ export module foo;
 // { dg-module-cmi foo }
 
 export int violation_count{0};
-extern "C++" export void handle_contract_violation(const std::experimental::contract_violation &violation)
+export extern "C++" void handle_contract_violation(const std::experimental::contract_violation &violation)
 {
   violation_count++;
   printf("violation_count: %d\n", violation_count);

gcc/testsuite/g++.dg/modules/contracts-2_a.C

@@ -12,7 +12,7 @@ export module foo;
 
 export int violation_count{0};
 export int violation_line_sum{0};
-extern "C++" export void handle_contract_violation(const std::experimental::contract_violation &violation)
+export extern "C++" void handle_contract_violation(const std::experimental::contract_violation &violation)
 {
   violation_count++;
   violation_line_sum += violation.line_number () * violation_count;

gcc/testsuite/g++.dg/modules/contracts-3_a.C

@@ -8,7 +8,7 @@ export module foo;
 // { dg-module-cmi foo }
 
 export int violation_count{0};
-extern "C++" export void handle_contract_violation(const std::experimental::contract_violation &violation)
+export extern "C++" void handle_contract_violation(const std::experimental::contract_violation &violation)
 {
   violation_count++;
   printf("violation_count: %d\n", violation_count);

gcc/testsuite/g++.dg/modules/contracts-4_a.C

@@ -9,7 +9,7 @@ export module foo;
 // { dg-module-cmi foo }
 
 export int violation_count{0};
-extern "C++" export void handle_contract_violation(const std::experimental::contract_violation &violation)
+export extern "C++" void handle_contract_violation(const std::experimental::contract_violation &violation)
 {
   violation_count++;
   printf("violation_count: %d\n", violation_count);

gcc/testsuite/g++.dg/modules/export-3.C

@@ -0,0 +1,30 @@
+// P2615R1 invalid declarations
+// PR c++/107688
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi !bad }
+
+export module bad;
+
+extern "C++" export void foo();  // { dg-error "export-declarations are not permitted here" "" { target c++20 } }
+
+export template <typename T> struct S {};
+
+export template <typename T> struct S<T*> {};  // { dg-error "partial specialization in unbraced export-declaration" }
+
+export template <> struct S<int*> {};  // { dg-error "explicit specializations are not permitted here" }
+
+export template struct S<int>;  // { dg-error "explicit instantiations are not permitted here" }
+
+template <> export struct S<double>;  // { dg-error "expected unqualified-id" }
+
+export export int x;  // { dg-error ".export. may only occur once" }
+
+export { export int y; }  // { dg-error ".export. may only occur once" }
+
+namespace {
+  export namespace ns {}  // { dg-error "internal linkage" }
+}
+
+export namespace {}  // { dg-error "internal linkage" }
+
+// { dg-prune-output "not writing module" }

gcc/testsuite/g++.dg/modules/export-4_a.C

@@ -0,0 +1,23 @@
+// P2615R1 valid declarations
+// PR c++/107688
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi M }
+
+export module M;
+
+export {}
+export { static_assert(true); }
+
+export namespace Empty {}
+export using namespace Empty;
+
+export {
+  template <typename T> struct S {};
+  template <typename T> struct S<T*> { using a = int; };
+  template <> struct S<int*> { using b = int; };
+  template struct S<int>;
+}
+
+extern "C++" {
+  export void foo();
+}

gcc/testsuite/g++.dg/modules/export-4_b.C

@@ -0,0 +1,13 @@
+// PR c++/107688
+// { dg-additional-options "-fmodules-ts" }
+
+import M;
+
+using namespace Empty;
+
+int main() {
+  S<int> x;
+  S<int*>::b y;
+  S<int**>::a z;
+  foo();
+}

gcc/testsuite/g++.dg/modules/lang-1_c.C

@@ -6,7 +6,7 @@ extern "C++"
 }
 
 extern "C"
-import "lang-1_a.H"; // { dg-error "cannot appear directly" }
+  import "lang-1_a.H"; // OK since p2615r1
 
 extern "C" int cfunc (int); // { dg-error "conflicting declaration" }
 extern "C" int cxxfunc (int);

gcc/testsuite/g++.dg/modules/pr101582-1.C

@@ -2,8 +2,10 @@
 // { dg-additional-options "-fmodules-ts" }
 export module pr101582;
 // { dg-module-cmi "pr101582" }
-export ;			// { dg-error "export declaration does not declare anything" "" { xfail *-*-* } }
-export [[]];			// { dg-error "export declaration does not declare anything" "" { xfail *-*-* } }
-export				// { dg-error "export declaration does not declare anything" "" { xfail *-*-* } }
+
+// These are all legal since P2615R1.
+export ;
+export [[]];
+export
 {
 }