c++: Return only in-scope tparms in keep_template_parm [PR95310]

In the testcase below, the dependent specializations iter_reference_t<F>
and iter_reference_t<Out> share the same tree due to specialization
caching.  So when find_template_parameters walks through the
requires-expression (as part of normalization), it sees and includes the
out-of-scope template parameter F in the list of template parameters
it found within the requires-expression (along with Out and N).

From a correctness perspective this is harmless since the parameter mapping
routines only care about the level and index of each parameter, so F is
no different from Out in that sense.  And it's also harmless that two
parameters in the parameter mapping have the same level and index.

But having both Out and F in the parameter mapping means extra work for
hash_atomic_constrant, tsubst_parameter_mapping and get_mapped_args; and
it also means we print this irrelevant template parameter in the
testcase's diagnostics (via pp_cxx_parameter_mapping):

  in requirements with ‘Out o’ [with N = (const int&)&a; F = const int*; Out = const int*]

This patch makes keep_template_parm return only in-scope template
parameters by looking into ctx_parms for the corresponding in-scope
one, through a new helper function corresponding_template_parameter.

(That we sometimes print irrelevant template parameters in diagnostics
is also the subject of PR99 and PR66968, so the above diagnostic issue
could likely be fixed in a more general way, but this targeted fix to
keep_template_parm is perhaps worthwhile on its own.)

gcc/cp/ChangeLog:

	PR c++/95310
	* pt.c (corresponding_template_parameter): Define.
	(keep_template_parm): Use it to adjust the given template
	parameter to the corresponding in-scope one from ctx_parms.

gcc/testsuite/ChangeLog:

	PR c++/95310
	* g++.dg/concepts/diagnostic15.C: New test.

gcc/cp/pt.c

@@ -10243,6 +10243,42 @@ lookup_and_finish_template_variable (tree templ, tree targs,
   return convert_from_reference (templ);
 }
 
+/* If the set of template parameters PARMS contains a template parameter
+   at the given LEVEL and INDEX, then return this parameter.  Otherwise
+   return NULL_TREE.  */
+
+static tree
+corresponding_template_parameter (tree parms, int level, int index)
+{
+  while (TMPL_PARMS_DEPTH (parms) > level)
+    parms = TREE_CHAIN (parms);
+
+  if (TMPL_PARMS_DEPTH (parms) != level
+      || TREE_VEC_LENGTH (TREE_VALUE (parms)) <= index)
+    return NULL_TREE;
+
+  tree t = TREE_VALUE (TREE_VEC_ELT (TREE_VALUE (parms), index));
+  /* As in template_parm_to_arg.  */
+  if (TREE_CODE (t) == TYPE_DECL || TREE_CODE (t) == TEMPLATE_DECL)
+    t = TREE_TYPE (t);
+  else
+    t = DECL_INITIAL (t);
+
+  gcc_assert (TEMPLATE_PARM_P (t));
+  return t;
+}
+
+/* Return the template parameter from PARMS that positionally corresponds
+   to the template parameter PARM, or else return NULL_TREE.  */
+
+static tree
+corresponding_template_parameter (tree parms, tree parm)
+{
+  int level, index;
+  template_parm_level_and_index (parm, &level, &index);
+  return corresponding_template_parameter (parms, level, index);
+}
+
 
 struct pair_fn_data
 {
@@ -10549,6 +10585,14 @@ keep_template_parm (tree t, void* data)
        BOUND_TEMPLATE_TEMPLATE_PARM itself.  */
     t = TREE_TYPE (TEMPLATE_TEMPLATE_PARM_TEMPLATE_DECL (t));
 
+  /* This template parameter might be an argument to a cached dependent
+     specalization that was formed earlier inside some other template, in
+     which case the parameter is not among the ones that are in-scope.
+     Look in CTX_PARMS to find the corresponding in-scope template
+     parameter, and use it instead.  */
+  if (tree in_scope = corresponding_template_parameter (ftpi->ctx_parms, t))
+    t = in_scope;
+
   /* Arguments like const T yield parameters like const T. This means that
      a template-id like X<T, const T> would yield two distinct parameters:
      T and const T. Adjust types to their unqualified versions.  */

gcc/testsuite/g++.dg/concepts/diagnostic15.C

@@ -0,0 +1,16 @@
+// PR c++/95310
+// { dg-do compile { target concepts } }
+
+template <class T>
+using iter_reference_t = decltype(*T{});
+
+template <typename F>
+struct result { using type = iter_reference_t<F>; };
+
+template <class Out, const int& N>
+concept indirectly_writable = requires(Out o) { // { dg-bogus "F =" }
+  iter_reference_t<Out>(*o) = N;
+};
+
+const int a = 0;
+static_assert(indirectly_writable<const int*, a>); // { dg-error "assert" }