c++: Fix handling of extern templates in modules [PR112820]

Currently, extern templates are detected by looking for the
DECL_EXTERNAL flag on a TYPE_DECL. However, this is incorrect:
TYPE_DECLs don't actually set this flag, and it happens to work by
coincidence due to TYPE_DECL_SUPPRESS_DEBUG happening to use the same
underlying bit. This however causes issues with other TYPE_DECLs that
also happen to have suppressed debug information.

Instead, this patch reworks the logic so CLASSTYPE_INTERFACE_ONLY is
always emitted into the module BMI and can then be used to check for an
extern template correctly.

Otherwise, for other declarations we always want to redetermine this:
even for declarations from the GMF, we may change our mind on whether to
import or export depending on decisions made later in the TU after
importing so we shouldn't decide this now, or necessarily reuse what the
module we'd imported had decided.

Some of this may need to change in the future to account for
itanium-cxx-abi/cxx-abi#170.

	PR c++/112820
	PR c++/102607

gcc/cp/ChangeLog:

	* module.cc (trees_out::lang_type_bools): Write interface_only
	and interface_unknown.
	(trees_in::lang_type_bools): Read the above flags.
	(trees_in::decl_value): Reset CLASSTYPE_INTERFACE_* except for
	extern templates.
	(trees_in::read_class_def): Remove buggy extern template
	handling.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/debug-2_a.C: New test.
	* g++.dg/modules/debug-2_b.C: New test.
	* g++.dg/modules/debug-2_c.C: New test.
	* g++.dg/modules/debug-3_a.C: New test.
	* g++.dg/modules/debug-3_b.C: New test.

Signed-off-by: Nathaniel Shead <[email protected]>

gcc/cp/module.cc

@@ -5806,10 +5806,8 @@ trees_out::lang_type_bools (tree t)
 
   WB ((lang->gets_delete >> 0) & 1);
   WB ((lang->gets_delete >> 1) & 1);
-  // Interfaceness is recalculated upon reading.  May have to revisit?
-  // How do dllexport and dllimport interact across a module?
-  // lang->interface_only
-  // lang->interface_unknown
+  WB (lang->interface_only);
+  WB (lang->interface_unknown);
   WB (lang->contains_empty_class_p);
   WB (lang->anon_aggr);
   WB (lang->non_zero_init);
@@ -5877,9 +5875,8 @@ trees_in::lang_type_bools (tree t)
   v = b () << 0;
   v |= b () << 1;
   lang->gets_delete = v;
-  // lang->interface_only
-  // lang->interface_unknown
-  lang->interface_unknown = true; // Redetermine interface
+  RB (lang->interface_only);
+  RB (lang->interface_unknown);
   RB (lang->contains_empty_class_p);
   RB (lang->anon_aggr);
   RB (lang->non_zero_init);
@@ -8246,6 +8243,23 @@ trees_in::decl_value ()
 	/* Set the TEMPLATE_DECL's type.  */
 	TREE_TYPE (decl) = TREE_TYPE (inner);
 
+      /* Redetermine whether we need to import or export this declaration
+	 for this TU.  But for extern templates we know we must import:
+	 they'll be defined in a different TU.
+	 FIXME: How do dllexport and dllimport interact across a module?
+	 See also https://github.com/itanium-cxx-abi/cxx-abi/issues/170.
+	 May have to revisit?  */
+      if (type
+	  && CLASS_TYPE_P (type)
+	  && TYPE_LANG_SPECIFIC (type)
+	  && !(CLASSTYPE_EXPLICIT_INSTANTIATION (type)
+	       && CLASSTYPE_INTERFACE_KNOWN (type)
+	       && CLASSTYPE_INTERFACE_ONLY (type)))
+	{
+	  CLASSTYPE_INTERFACE_ONLY (type) = false;
+	  CLASSTYPE_INTERFACE_UNKNOWN (type) = true;
+	}
+
       /* Add to specialization tables now that constraints etc are
 	 added.  */
       if (mk == MK_partial)
@@ -12068,15 +12082,6 @@ trees_in::read_class_def (tree defn, tree maybe_template)
   bool installing = maybe_dup && !TYPE_SIZE (type);
   if (installing)
     {
-      if (DECL_EXTERNAL (defn) && TYPE_LANG_SPECIFIC (type))
-	{
-	  /* We don't deal with not-really-extern, because, for a
-	     module you want the import to be the interface, and for a
-	     header-unit, you're doing it wrong.  */
-	  CLASSTYPE_INTERFACE_UNKNOWN (type) = false;
-	  CLASSTYPE_INTERFACE_ONLY (type) = true;
-	}
-
       if (maybe_dup != defn)
 	{
 	  // FIXME: This is needed on other defns too, almost

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

@@ -0,0 +1,9 @@
+// PR c++/112820
+// { dg-additional-options "-fmodules-ts -g" }
+// { dg-module-cmi io }
+
+export module io;
+
+export struct error {
+  virtual const char* what() const noexcept;
+};

gcc/testsuite/g++.dg/modules/debug-2_b.C

@@ -0,0 +1,8 @@
+// PR c++/112820
+// { dg-additional-options "-fmodules-ts -g" }
+
+module io;
+
+const char* error::what() const noexcept {
+  return "bla";
+}

gcc/testsuite/g++.dg/modules/debug-2_c.C

@@ -0,0 +1,9 @@
+// PR c++/112820
+// { dg-module-do link }
+// { dg-additional-options "-fmodules-ts -g" }
+
+import io;
+
+int main() {
+  error{};
+}

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

@@ -0,0 +1,8 @@
+// PR c++/102607
+// { dg-additional-options "-fmodules-ts -g" }
+// { dg-module-cmi mod }
+
+export module mod;
+export struct B {
+  virtual ~B() = default;
+};

gcc/testsuite/g++.dg/modules/debug-3_b.C

@@ -0,0 +1,9 @@
+// PR c++/102607
+// { dg-module-do link }
+// { dg-additional-options "-fmodules-ts -g" }
+
+import mod;
+int main() {
+  struct D : B {};
+  (void)D{};
+}