c++: Check satisfaction before non-dep convs. [CWG2369]

It's very hard to use concepts to protect a template from hard errors due to
unwanted instantiation if constraints aren't checked until after doing all
substitution and checking of non-dependent conversions.

It was pretty straightforward to insert the satisfaction check into the
logic, but I needed to make the 3-parameter version of
satisfy_declaration_constraints call push_tinst_level like the 2-parameter
version already does.  For simplicity, I also made it add any needed outer
template arguments from the TEMPLATE_DECL to the args.

The testsuite changes are mostly because this change causes unsatisfaction
to cause deduction to fail rather than reject the candidate later in
overload resolution.

gcc/cp/ChangeLog:

	DR 2369
	* cp-tree.h (push_tinst_level, push_tinst_level_loc): Declare.
	* constraint.cc (satisfy_declaration_constraints):
	Use add_outermost_template_args and push_tinst_level.
	* pt.c (add_outermost_template_args): Handle getting
	a TEMPLATE_DECL as the first argument.
	(push_tinst_level, push_tinst_level_loc): No longer static.
	(fn_type_unification): Check satisfaction before non-dependent
	conversions.

gcc/testsuite/ChangeLog:

	DR 2369
	* g++.dg/concepts/diagnostic10.C: Adjust expexcted errors.
	* g++.dg/concepts/diagnostic13.C: Adjust expexcted errors.
	* g++.dg/concepts/diagnostic2.C: Adjust expexcted errors.
	* g++.dg/concepts/diagnostic3.C: Adjust expexcted errors.
	* g++.dg/concepts/diagnostic4.C: Adjust expexcted errors.
	* g++.dg/concepts/diagnostic5.C: Adjust expexcted errors.
	* g++.dg/concepts/diagnostic9.C: Adjust expexcted errors.
	* g++.dg/concepts/expression2.C: Adjust expexcted errors.
	* g++.dg/concepts/fn5.C: Adjust expexcted errors.
	* g++.dg/concepts/placeholder5.C: Adjust expexcted errors.
	* g++.dg/concepts/pr67595.C: Adjust expexcted errors.
	* g++.dg/cpp2a/concepts-pr78752-2.C: Adjust expexcted errors.
	* g++.dg/cpp2a/concepts-pr84140.C: Adjust expexcted errors.
	* g++.dg/cpp2a/concepts-recursive-sat3.C: Adjust expexcted errors.
	* g++.dg/cpp2a/concepts-requires18.C: Adjust expexcted errors.
	* g++.dg/cpp2a/concepts-requires19.C: Adjust expexcted errors.
	* g++.dg/cpp2a/concepts3.C: Adjust expexcted errors.
	* g++.dg/cpp2a/concepts-nondep1.C: New test.
	* g++.dg/cpp2a/concepts-nondep1a.C: New test.

3 files changed, 39 insertions, 10 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 48d52ec..7a2f3b9 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -2814,16 +2814,22 @@ satisfy_declaration_constraints (tree t, tree args, subst_info info)
   info.in_decl = t;
 
   gcc_assert (TREE_CODE (t) == TEMPLATE_DECL);
+
+  args = add_outermost_template_args (t, args);
+
+  tree result = boolean_true_node;
   if (tree norm = normalize_template_requirements (t, info.noisy ()))
     {
+      if (!push_tinst_level (t, args))
+	return result;
       tree pattern = DECL_TEMPLATE_RESULT (t);
       push_access_scope (pattern);
-      tree result = satisfy_associated_constraints (norm, args, info);
+      result = satisfy_associated_constraints (norm, args, info);
       pop_access_scope (pattern);
-      return result;
+      pop_tinst_level ();
     }
 
-  return boolean_true_node;
+  return result;
 }
 
 static tree
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 44531cd..3f3717a 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6981,7 +6981,9 @@ extern bool template_parm_object_p		(const_tree);
 extern tree tparm_object_argument		(tree);
 extern bool explicit_class_specialization_p     (tree);
 extern bool push_tinst_level                    (tree);
+extern bool push_tinst_level                    (tree, tree);
 extern bool push_tinst_level_loc                (tree, location_t);
+extern bool push_tinst_level_loc                (tree, tree, location_t);
 extern void pop_tinst_level                     (void);
 extern struct tinst_level *outermost_tinst_level(void);
 extern void init_template_processing		(void);
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index edaefcf..5dbdd37 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -586,13 +586,23 @@ add_to_template_args (tree args, tree extra_args)
    (EXTRA_ARGS) levels are added.  This function is used to combine
    the template arguments from a partial instantiation with the
    template arguments used to attain the full instantiation from the
-   partial instantiation.  */
+   partial instantiation.
+
+   If ARGS is a TEMPLATE_DECL, use its parameters as args.  */
 
 tree
 add_outermost_template_args (tree args, tree extra_args)
 {
   tree new_args;
 
+  if (!args)
+    return extra_args;
+  if (TREE_CODE (args) == TEMPLATE_DECL)
+    {
+      tree ti = get_template_info (DECL_TEMPLATE_RESULT (args));
+      args = TI_ARGS (ti);
+    }
+
   /* If there are more levels of EXTRA_ARGS than there are ARGS,
      something very fishy is going on.  */
   gcc_assert (TMPL_ARGS_DEPTH (args) >= TMPL_ARGS_DEPTH (extra_args));
@@ -10772,7 +10782,7 @@ static GTY(()) struct tinst_level *last_error_tinst_level;
 /* We're starting to instantiate D; record the template instantiation context
    at LOC for diagnostics and to restore it later.  */
 
-static bool
+bool
 push_tinst_level_loc (tree tldcl, tree targs, location_t loc)
 {
   struct tinst_level *new_level;
@@ -10826,7 +10836,7 @@ push_tinst_level_loc (tree tldcl, tree targs, location_t loc)
 /* We're starting substitution of TMPL<ARGS>; record the template
    substitution context for diagnostics and to restore it later.  */
 
-static bool
+bool
 push_tinst_level (tree tmpl, tree args)
 {
   return push_tinst_level_loc (tmpl, args, input_location);
@@ -21297,13 +21307,24 @@ fn_type_unification (tree fn,
       goto fail;
     }
 
+ deduced:
+
+  /* CWG2369: Check satisfaction before non-deducible conversions.  */
+  if (!constraints_satisfied_p (fn, targs))
+    {
+      if (explain_p)
+	diagnose_constraints (DECL_SOURCE_LOCATION (fn), fn, targs);
+      goto fail;
+    }
+
   /* DR 1391: All parameters have args, now check non-dependent parms for
-     convertibility.  */
-  if (check_non_deducible_conversions (parms, args, nargs, fn, strict, flags,
-				       convs, explain_p))
+     convertibility.  We don't do this if all args were explicitly specified,
+     as the standard says that we substitute explicit args immediately.  */
+  if (incomplete
+      && check_non_deducible_conversions (parms, args, nargs, fn, strict, flags,
+					  convs, explain_p))
     goto fail;
 
- deduced:
   /* All is well so far.  Now, check:
 
      [temp.deduct]