[PATCH] c++/modules: check mismatching exports for class tags [PR98885]

[Nathaniel Shead <nathanieloshead@gmail.com>]

I think the error message is still a little bit unclear but I couldn't
come up with something clearer that was similarly concise and matching
the existing style.

(Also I noticed that the linked PR was assigned to Nathan but there
hadn't been activity for a while, and I've been looking into these kinds
of issues recently anyway so I thought I'd give it a go.)

Bootstrapped and regtested on x86_64-pc-linux-gnu. I don't have write
access.

-- >8 --

Checks for exporting a declaration that was previously declared as not
exported is implemented in 'duplicate_decls', but this doesn't handle
declarations of classes. This patch adds these checks and slightly
adjusts the associated error messages for clarity.

	PR c++/98885

gcc/cp/ChangeLog:

	* decl.cc (duplicate_decls): Adjust error message.
	(xref_tag): Adjust error message. Check exporting decl that is
	already declared as non-exporting.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/export-1.C: Adjust error messages. Remove
	xfails for working case. Add new test case.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
---
 gcc/cp/decl.cc                          | 21 ++++++++++++++++++---
 gcc/testsuite/g++.dg/modules/export-1.C | 16 +++++++++-------
 2 files changed, 27 insertions(+), 10 deletions(-)

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 4a07c7e879b..bde9bd79d58 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -2236,8 +2236,10 @@ duplicate_decls (tree newdecl, tree olddecl, bool hiding, bool was_hidden)
 	  if (DECL_MODULE_EXPORT_P (STRIP_TEMPLATE (newdecl))
 	      && !DECL_MODULE_EXPORT_P (not_tmpl))
 	    {
-	      error ("conflicting exporting declaration %qD", newdecl);
-	      inform (olddecl_loc, "previous declaration %q#D here", olddecl);
+	      auto_diagnostic_group d;
+	      error ("conflicting exporting for declaration %qD", newdecl);
+	      inform (olddecl_loc,
+		      "previously declared here without exporting");
 	    }
 	}
       else if (DECL_MODULE_EXPORT_P (newdecl))
@@ -16249,11 +16251,24 @@ xref_tag (enum tag_types tag_code, tree name,
 	  tree decl = TYPE_NAME (t);
 	  if (!module_may_redeclare (decl))
 	    {
+	      auto_diagnostic_group d;
 	      error ("cannot declare %qD in a different module", decl);
-	      inform (DECL_SOURCE_LOCATION (decl), "declared here");
+	      inform (DECL_SOURCE_LOCATION (decl), "previously declared here");
 	      return error_mark_node;
 	    }
 
+	  tree not_tmpl = STRIP_TEMPLATE (decl);
+	  if (DECL_LANG_SPECIFIC (not_tmpl)
+	      && DECL_MODULE_ATTACH_P (not_tmpl)
+	      && !DECL_MODULE_EXPORT_P (not_tmpl)
+	      && module_exporting_p ())
+	    {
+	      auto_diagnostic_group d;
+	      error ("conflicting exporting for declaration %qD", decl);
+	      inform (DECL_SOURCE_LOCATION (decl),
+		      "previously declared here without exporting");
+	    }
+
 	  tree maybe_tmpl = decl;
 	  if (CLASS_TYPE_P (t) && CLASSTYPE_IS_TEMPLATE (t))
 	    maybe_tmpl = CLASSTYPE_TI_TEMPLATE (t);
diff --git a/gcc/testsuite/g++.dg/modules/export-1.C b/gcc/testsuite/g++.dg/modules/export-1.C
index 8ca696ebee0..3f93814d270 100644
--- a/gcc/testsuite/g++.dg/modules/export-1.C
+++ b/gcc/testsuite/g++.dg/modules/export-1.C
@@ -4,19 +4,21 @@ export module frob;
 // { dg-module-cmi !frob }
 
 int x ();
-export int x (); // { dg-error "conflicting exporting declaration" }
+export int x (); // { dg-error "conflicting exporting for declaration" }
 
 int y;
-export extern int y; // { dg-error "conflicting exporting declaration" }
+export extern int y; // { dg-error "conflicting exporting for declaration" }
 
 typedef int z;
-export typedef int z; // { dg-error "conflicting exporting declaration" }
+export typedef int z; // { dg-error "conflicting exporting for declaration" }
 
 template <typename T> int f (T);
-export template <typename T> int f (T); // { dg-error "conflicting exporting declaration" }
+export template <typename T> int f (T); // { dg-error "conflicting exporting for declaration" }
 
-// doesn't go via duplicate_decls so we miss this for now
 class A;
-export class A; // { dg-error "conflicting exporting declaration" "" { xfail *-*-* } }
+export class A; // { dg-error "conflicting exporting for declaration" }
 
-// { dg-warning  "due to errors" "" { target *-*-* } 0 }
+template <typename T> struct B;
+export template <typename T> struct B {};  // { dg-error "conflicting exporting for declaration" }
+
+// { dg-warning "due to errors" "" { target *-*-* } 0 }
-- 
2.42.0