c++/modules: Better handle no-linkage decls in unnamed namespaces [PR118799]

There are two issues with no-linkage decls (e.g. explicit type aliases)
in unnamed namespaces that this patch fixes.

Firstly, we don't currently handle exporting no-linkage decls in unnamed
namespaces.  This should be ill-formed in [module.export], since having
an exported declaration within a namespace-definition makes the
namespace definition exported (p2), but an unnamed namespace has
internal linkage thus violating p3.

Secondly, by the standard it appears to be possible to emit unnamed
namespaces from named modules in certain scenarios, for instance when
they contain a type alias (which is not itself an entity).  This patch
makes the adjustments needed to ensure we don't error in this scenario.

	PR c++/118799

gcc/cp/ChangeLog:

	* module.cc (depset:#️⃣:is_tu_local_entity): Only types,
	functions, variables, and template (specialisations) can be
	TU-local.  Explicit type aliases are TU-local iff the type they
	refer to are.
	(module_state::write_namespaces): Allow unnamed namespaces in
	named modules.
	(check_module_decl_linkage): Error for all exported declarations
	in an unnamed namespace.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/export-6.C: Adjust error message, add check for
	no-linkage decls in namespace.
	* g++.dg/modules/internal-4_b.C: Allow exposing a namespace with
	internal linkage.  Type aliases are not entities and so never
	exposures.
	* g++.dg/modules/using-30_a.C: New test.
	* g++.dg/modules/using-30_b.C: New test.
	* g++.dg/modules/using-30_c.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>

gcc/cp/module.cc

@@ -13478,16 +13478,35 @@ bool
 depset::hash::is_tu_local_entity (tree decl, bool explain/*=false*/)
 {
   gcc_checking_assert (DECL_P (decl));
+  location_t loc = DECL_SOURCE_LOCATION (decl);
+  tree type = TREE_TYPE (decl);
 
-  /* An explicit type alias is not an entity, and so is never TU-local.
-     Neither are the built-in declarations of 'int' and such.  */
+  /* Only types, functions, variables, and template (specialisations)
+     can be TU-local.  */
+  if (TREE_CODE (decl) != TYPE_DECL
+      && TREE_CODE (decl) != FUNCTION_DECL
+      && TREE_CODE (decl) != VAR_DECL
+      && TREE_CODE (decl) != TEMPLATE_DECL)
+    return false;
+
+  /* An explicit type alias is not an entity; we don't want to stream
+     such aliases if they refer to TU-local entities, so propagate this
+     from the original type. The built-in declarations of 'int' and such
+     are never TU-local.  */
   if (TREE_CODE (decl) == TYPE_DECL
       && !DECL_SELF_REFERENCE_P (decl)
       && !DECL_IMPLICIT_TYPEDEF_P (decl))
-    return false;
-
-  location_t loc = DECL_SOURCE_LOCATION (decl);
-  tree type = TREE_TYPE (decl);
+    {
+      tree orig = DECL_ORIGINAL_TYPE (decl);
+      if (orig && TYPE_NAME (orig))
+	{
+	  if (explain)
+	    inform (loc, "%qD is an alias of TU-local type %qT", decl, orig);
+	  return is_tu_local_entity (TYPE_NAME (orig), explain);
+	}
+      else
+	return false;
+    }
 
   /* Check specializations first for slightly better explanations.  */
   int use_tpl = -1;
@@ -16623,8 +16642,9 @@ module_state::write_namespaces (elf_out *to, vec<depset *> spaces,
       depset *b = spaces[ix];
       tree ns = b->get_entity ();
 
+      /* This could be an anonymous namespace even for a named module,
+	 since we can still emit no-linkage decls.  */
       gcc_checking_assert (TREE_CODE (ns) == NAMESPACE_DECL);
-      gcc_checking_assert (TREE_PUBLIC (ns) || header_module_p ());
 
       unsigned flags = 0;
       if (TREE_PUBLIC (ns))
@@ -20579,13 +20599,25 @@ check_module_decl_linkage (tree decl)
   /* An internal-linkage declaration cannot be generally be exported.
      But it's OK to export any declaration from a header unit, including
      internal linkage declarations.  */
-  if (!header_module_p ()
-      && DECL_MODULE_EXPORT_P (decl)
-      && decl_linkage (decl) == lk_internal)
+  if (!header_module_p () && DECL_MODULE_EXPORT_P (decl))
     {
-      error_at (DECL_SOURCE_LOCATION (decl),
-		"exporting declaration %qD with internal linkage", decl);
-      DECL_MODULE_EXPORT_P (decl) = false;
+      /* Let's additionally treat any exported declaration within an
+	 internal namespace as exporting a declaration with internal
+	 linkage, as this would also implicitly export the internal
+	 linkage namespace.  */
+      if (decl_internal_context_p (decl))
+	{
+	  error_at (DECL_SOURCE_LOCATION (decl),
+		    "exporting declaration %qD declared in unnamed namespace",
+		    decl);
+	  DECL_MODULE_EXPORT_P (decl) = false;
+	}
+      else if (decl_linkage (decl) == lk_internal)
+	{
+	  error_at (DECL_SOURCE_LOCATION (decl),
+		    "exporting declaration %qD with internal linkage", decl);
+	  DECL_MODULE_EXPORT_P (decl) = false;
+	}
     }
 }
 

gcc/testsuite/g++.dg/modules/export-6.C

@@ -16,27 +16,32 @@ export static auto [d] = S{};  // { dg-error "internal linkage" "" { target c++2
 #endif
 
 namespace {
-  export int y = 456;  // { dg-error "internal linkage" }
-  export void h();  // { dg-error "internal linkage" }
-  export void i() {}  // { dg-error "internal linkage" }
-  export template <typename T> void v(); // { dg-error "internal linkage" }
-  export template <typename T> void w() {} // { dg-error "internal linkage" }
-  export auto [e] = S{};  // { dg-error "internal linkage" }
+  export int y = 456;  // { dg-error "exporting" }
+  export void h();  // { dg-error "exporting" }
+  export void i() {}  // { dg-error "exporting" }
+  export template <typename T> void v(); // { dg-error "exporting" }
+  export template <typename T> void w() {} // { dg-error "exporting" }
+  export auto [e] = S{};  // { dg-error "exporting" }
 
-  export namespace ns {}  // { dg-error "internal linkage" }
-  export namespace alias = global;  // { dg-error "internal linkage" }
+  export namespace ns {}  // { dg-error "exporting" }
+  export namespace alias = global;  // { dg-error "exporting" }
 
-  export struct A {};  // { dg-error "internal linkage" }
-  export template <typename T> struct B {};  // { dg-error "internal linkage" }
+  export struct A {};  // { dg-error "exporting" }
+  export template <typename T> struct B {};  // { dg-error "exporting" }
 
-  export enum E {};  // { dg-error "internal linkage" }
-  export enum class F {};  // { dg-error "internal linkage" }
+  export enum E {};  // { dg-error "exporting" }
+  export enum class F {};  // { dg-error "exporting" }
 
-  export template <typename T> using U = int;  // { dg-error "internal linkage" }
+  export template <typename T> using U = int;  // { dg-error "exporting" }
 
 #if __cplusplus >= 202002L
-  export template <typename T> concept C = true;  // { dg-error "internal linkage" "" { target c++20 } }
+  export template <typename T> concept C = true;  // { dg-error "exporting" "" { target c++20 } }
 #endif
+
+  // Also complain about exporting no-linkage decls in an unnamed namespace
+  export typedef int T;  // { dg-error "exporting" }
+  export typedef struct {} *PC;  // { dg-error "exporting" }
+  export using V = int;  // { dg-error "exporting" }
 }
 
 export namespace {}  // { dg-error "exporting unnamed namespace" }

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

@@ -32,8 +32,8 @@ inline void expose_header_decl() {  // { dg-error "exposes TU-local entity" }
   header_f();
 }
 
-// We additionally consider a namespace with internal linkage as TU-local
-namespace expose_ns = internal_ns;  // { dg-error "exposes TU-local entity" }
+// A namespace with internal linkage is not inherently an exposure.
+namespace expose_ns = internal_ns;  // { dg-bogus "exposes TU-local entity" }
 
 // But we don't consider a weakref as being TU-local, despite being
 // marked static; this is to support uses of weakrefs in header files
@@ -56,9 +56,12 @@ static auto get_local_type() {
 static auto get_local_lambda() {
   return []{};
 }
-using T = decltype(get_local_ok());  // OK
-using U = decltype(get_local_type());  // { dg-error "exposes TU-local entity" }
-using V = decltype(get_local_lambda());  // { dg-error "exposes TU-local entity" }
+using T = decltype(get_local_ok());
+T* expose_t;   // OK
+using U = decltype(get_local_type());
+U* expose_u;  // { dg-error "exposes TU-local entity" }
+using V = decltype(get_local_lambda());
+V* expose_v;  // { dg-error "exposes TU-local entity" }
 
 static auto internal_lambda = []{ internal_f(); };  // OK
 auto expose_lambda = internal_lambda;  // { dg-error "exposes TU-local entity" }
@@ -73,7 +76,8 @@ int not_in_tu_local
 
 struct {} no_name;  // { dg-error "exposes TU-local entity" }
 enum {} e;  // { dg-error "exposes TU-local entity" }
-using not_an_initializer = class {};  // { dg-error "exposes TU-local entity" }
+using not_an_initializer = class {};
+not_an_initializer expose_nai;  // { dg-error "exposes TU-local entity" }
 
 class in_class_specifier { struct {} x; };  // OK
 void in_function_body() { struct {} x; }  // OK
@@ -81,7 +85,8 @@ auto in_initializer = []{};  // OK
 
 #if __cplusplus >= 202002L
 decltype([]{}) d_lambda;  // { dg-error "exposes TU-local entity" "" { target c++20 } }
-using alias_lambda = decltype([]{});  // { dg-error "exposes TU-local entity" "" { target c++20 } }
+using alias_lambda = decltype([]{});
+alias_lambda expose_lam;  // { dg-error "exposes TU-local entity" "" { target c++20 } }
 
 template <typename T>
 concept in_constraint_expression = requires {

gcc/testsuite/g++.dg/modules/using-30_a.C

@@ -0,0 +1,13 @@
+// { dg-additional-options "-fmodules" }
+// { dg-module-cmi M }
+
+export module M;
+
+namespace {
+  using A = int;
+  typedef int B;
+
+  struct Internal {};
+  using C = Internal;
+  typedef Internal D;
+}

gcc/testsuite/g++.dg/modules/using-30_b.C

@@ -0,0 +1,10 @@
+// { dg-additional-options "-fmodules" }
+
+module M;
+
+A x;
+B y;
+
+// Aliases that refer to TU-local entities should not be visible.
+C c;  // { dg-error "does not name a type" }
+D d;  // { dg-error "does not name a type" }

gcc/testsuite/g++.dg/modules/using-30_c.C

@@ -0,0 +1,17 @@
+// { dg-additional-options "-fmodules" }
+// The different declarations in the anonymous namespace shouldn't clash with
+// those in M.
+
+namespace {
+  using A = double;
+  typedef double B;
+  using C = double;
+  typedef double D;
+}
+import M;
+int main() {
+  A a = 1.0;
+  B b = 2.0;
+  C c = 3.0;
+  D d = 4.0;
+}