c++/modules: Ensure deduction guides are always reachable [PR115231]

Deduction guides are represented as 'normal' functions currently, and
have no special handling in modules.  However, this causes some issues;
by [temp.deduct.guide] a deduction guide is not found by normal name
lookup and instead all reachable deduction guides for a class template
should be considered, but this does not happen currently.

To solve this, this patch ensures that all deduction guides are
considered exported to ensure that they are always visible to importers
if they are reachable.  Another alternative here would be to add a new
kind of "all reachable" flag to name lookup, but that is complicated by
some difficulties in handling GM entities; this may be a better way to
go if more kinds of entities end up needing this handling, however.

Another issue here is that because deduction guides are "unrelated"
functions, they will usually get discarded from the GMF, so this patch
ensures that when finding dependencies, GMF deduction guides will also
have bindings created.  We do this in find_dependencies so that we don't
unnecessarily create bindings for GMF deduction guides that are never
reached; for consistency we do this for *all* deduction guides, not just
GM ones.  We also make sure that the opposite (a deduction guide being
the only purview reference to a GMF template) correctly marks it as
reachable.

Finally, when merging deduction guides from multiple modules, the name
lookup code may now return two-dimensional overload sets, so update
callers to match.

As a small drive-by improvement this patch also updates the error pretty
printing code to add a space before the '->' when printing a deduction
guide, so we get 'S(int) -> S<int>' instead of 'S(int)-> S<int>'.

	PR c++/115231

gcc/cp/ChangeLog:

	* error.cc (dump_function_decl): Add a space before '->' when
	printing deduction guides.
	* module.cc (depset::hash::add_binding_entity): Don't create
	bindings for guides here, only mark dependencies.
	(depset::hash::add_deduction_guides): New.
	(depset::hash::find_dependencies): Add deduction guide
	dependencies for a class template.
	(module_state::write_cluster): Always consider deduction guides
	as exported.
	* pt.cc (deduction_guides_for): Use 'lkp_iterator' instead of
	'ovl_iterator'.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/dguide-1_a.C: New test.
	* g++.dg/modules/dguide-1_b.C: New test.
	* g++.dg/modules/dguide-2_a.C: New test.
	* g++.dg/modules/dguide-2_b.C: New test.
	* g++.dg/modules/dguide-3_a.C: New test.
	* g++.dg/modules/dguide-3_b.C: New test.
	* g++.dg/modules/dguide-3_c.C: New test.
	* g++.dg/modules/dguide-3_d.C: New test.

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

1 files changed, 65 insertions, 0 deletions

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 7130faf..723f089 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -2589,6 +2589,9 @@ public:
     void add_partial_entities (vec<tree, va_gc> *);
     void add_class_entities (vec<tree, va_gc> *);
 
+  private:
+    void add_deduction_guides (tree decl);
+
   public:    
     void find_dependencies (module_state *);
     bool finalize_dependencies ();
@@ -13172,6 +13175,15 @@ depset::hash::add_binding_entity (tree decl, WMB_Flags flags, void *data_)
 	/* Ignore NTTP objects.  */
 	return false;
 
+      if (deduction_guide_p (decl))
+	{
+	  /* Ignore deduction guides, bindings for them will be created within
+	     find_dependencies for their class template.  But still build a dep
+	     for them so that we don't discard them.  */
+	  data->hash->make_dependency (decl, EK_FOR_BINDING);
+	  return false;
+	}
+
       if (!(flags & WMB_Using) && CP_DECL_CONTEXT (decl) != data->ns)
 	{
 	  /* An unscoped enum constant implicitly brought into the containing
@@ -13600,6 +13612,50 @@ find_pending_key (tree decl, tree *decl_p = nullptr)
   return ns;
 }
 
+/* Creates bindings and dependencies for all deduction guides of
+   the given class template DECL as needed.  */
+
+void
+depset::hash::add_deduction_guides (tree decl)
+{
+  /* Alias templates never have deduction guides.  */
+  if (DECL_ALIAS_TEMPLATE_P (decl))
+    return;
+
+  /* We don't need to do anything for class-scope deduction guides,
+     as they will be added as members anyway.  */
+  if (!DECL_NAMESPACE_SCOPE_P (decl))
+    return;
+
+  tree ns = CP_DECL_CONTEXT (decl);
+  tree name = dguide_name (decl);
+
+  /* We always add all deduction guides with a given name at once,
+     so if there's already a binding there's nothing to do.  */
+  if (find_binding (ns, name))
+    return;
+
+  tree guides = lookup_qualified_name (ns, name, LOOK_want::NORMAL,
+				       /*complain=*/false);
+  if (guides == error_mark_node)
+    return;
+
+  /* We have bindings to add.  */
+  depset *binding = make_binding (ns, name);
+  add_namespace_context (binding, ns);
+
+  depset **slot = binding_slot (ns, name, /*insert=*/true);
+  *slot = binding;
+
+  for (lkp_iterator it (guides); it; ++it)
+    {
+      gcc_checking_assert (!TREE_VISITED (*it));
+      depset *dep = make_dependency (*it, EK_FOR_BINDING);
+      binding->deps.safe_push (dep);
+      dep->deps.safe_push (binding);
+    }
+}
+
 /* Iteratively find dependencies.  During the walk we may find more
    entries on the same binding that need walking.  */
 
@@ -13660,6 +13716,10 @@ depset::hash::find_dependencies (module_state *module)
 	      walker.end ();
 
 	      if (!walker.is_key_order ()
+		  && DECL_CLASS_TEMPLATE_P (decl))
+		add_deduction_guides (decl);
+
+	      if (!walker.is_key_order ()
 		  && TREE_CODE (decl) == TEMPLATE_DECL
 		  && !DECL_UNINSTANTIATED_TEMPLATE_FRIEND_P (decl))
 		{
@@ -15158,6 +15218,11 @@ module_state::write_cluster (elf_out *to, depset *scc[], unsigned size,
 		      flags |= cbf_hidden;
 		    else if (DECL_MODULE_EXPORT_P (STRIP_TEMPLATE (bound)))
 		      flags |= cbf_export;
+		    else if (deduction_guide_p (bound))
+		      /* Deduction guides are always exported so that they are
+			 visible to name lookup whenever their class template
+			 is reachable.  */
+		      flags |= cbf_export;
 		  }
 
 		gcc_checking_assert (DECL_P (bound));