c++/modules: Fix treatment of unnamed types

In r14-9530 we relaxed "depending on type with no-linkage" errors for
declarations that could actually be accessed from different TUs anyway.
However, this also enabled it for unnamed types, which never work.

In a normal module interface, an unnamed type is TU-local by
[basic.link] p15.2, and so cannot be exposed or the program is
ill-formed.  We don't yet implement this checking but we should assume
that we will later; currently supporting this actually causes ICEs when
attempting to create the mangled name in some situations.

For a header unit, by [module.import] p5.3 it is unspecified whether two
TUs importing a header unit providing such a declaration are importing
the same header unit.  In this case, we would require name mangling
changes to somehow allow the (anonymous) type exported by such a header
unit to correspond across different TUs in the presence of other
anonymous declarations, so for this patch just assume that this case
would be an ODR violation instead.

gcc/cp/ChangeLog:

	* tree.cc (no_linkage_check): Anonymous types can't be accessed
	in a different TU.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/linkage-1_a.C: Remove anonymous type test.
	* g++.dg/modules/linkage-1_b.C: Likewise.
	* g++.dg/modules/linkage-1_c.C: Likewise.
	* g++.dg/modules/linkage-2.C: Add note about anonymous types.

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

5 files changed, 7 insertions, 15 deletions

diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index 0485a61..fe3f034 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -2988,15 +2988,7 @@ no_linkage_check (tree t, bool relaxed_p)
       /* Only treat unnamed types as having no linkage if they're at
 	 namespace scope.  This is core issue 966.  */
       if (TYPE_UNNAMED_P (t) && TYPE_NAMESPACE_SCOPE_P (t))
-	{
-	  if (relaxed_p
-	      && TREE_PUBLIC (CP_TYPE_CONTEXT (t))
-	      && module_maybe_has_cmi_p ())
-	    /* This type could possibly be accessed outside this TU.  */
-	    return NULL_TREE;
-	  else
-	    return t;
-	}
+	return t;
 
       for (r = CP_TYPE_CONTEXT (t); ; )
 	{
diff --git a/gcc/testsuite/g++.dg/modules/linkage-1_a.C b/gcc/testsuite/g++.dg/modules/linkage-1_a.C
index 750e31f..1d81312 100644
--- a/gcc/testsuite/g++.dg/modules/linkage-1_a.C
+++ b/gcc/testsuite/g++.dg/modules/linkage-1_a.C
@@ -9,7 +9,3 @@ auto f() {
 }
 decltype(f()) g();  // { dg-warning "used but not defined" "" { target c++17_down } }
 export auto x = g();
-
-struct {} s;
-decltype(s) h();  // { dg-warning "used but not defined" "" { target c++17_down } }
-export auto y = h();
diff --git a/gcc/testsuite/g++.dg/modules/linkage-1_b.C b/gcc/testsuite/g++.dg/modules/linkage-1_b.C
index f23962d..5bc0d1b 100644
--- a/gcc/testsuite/g++.dg/modules/linkage-1_b.C
+++ b/gcc/testsuite/g++.dg/modules/linkage-1_b.C
@@ -3,4 +3,3 @@
 module M;
 
 decltype(f()) g() { return {}; }
-decltype(s) h() { return {}; }
diff --git a/gcc/testsuite/g++.dg/modules/linkage-1_c.C b/gcc/testsuite/g++.dg/modules/linkage-1_c.C
index f1406b9..9ff1491 100644
--- a/gcc/testsuite/g++.dg/modules/linkage-1_c.C
+++ b/gcc/testsuite/g++.dg/modules/linkage-1_c.C
@@ -5,5 +5,4 @@ import M;
 
 int main() {
   auto a = x;
-  auto b = y;
 }
diff --git a/gcc/testsuite/g++.dg/modules/linkage-2.C b/gcc/testsuite/g++.dg/modules/linkage-2.C
index eb4d7b0..d913d6a 100644
--- a/gcc/testsuite/g++.dg/modules/linkage-2.C
+++ b/gcc/testsuite/g++.dg/modules/linkage-2.C
@@ -23,4 +23,10 @@ export void use() {
   h();
 }
 
+// Additionally, unnamed types have no linkage but are also TU-local, and thus
+// cannot be exposed in a module interface unit.  The non-TU-local entity 's'
+// here is an exposure of this type, so this should be an error; we don't yet
+// implement this checking however.
+struct {} s;  // { dg-error "TU-local" "" { xfail *-*-* } }
+
 // { dg-prune-output "not writing module" }