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

[Patrick Palka <ppalka@redhat.com>]

On Mon, 21 Sep 2020, Jason Merrill wrote:

> On 9/19/20 3:49 PM, Patrick Palka wrote:
> > 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 this 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 is 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.
> > 
> > (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.)
> > 
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, and also tested on
> > cmcstl2 and range-v3.  Does this look OK for trunk?
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	PR c++/95310
> > 	* pt.c (keep_template_parm): 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.
> > 	* g++.dg/cpp2a/concepts-ttp2.C: New test.
> > ---
> >   gcc/cp/pt.c                                  | 19 +++++++++++++++++++
> >   gcc/testsuite/g++.dg/concepts/diagnostic15.C | 16 ++++++++++++++++
> >   2 files changed, 35 insertions(+)
> >   create mode 100644 gcc/testsuite/g++.dg/concepts/diagnostic15.C
> > 
> > diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> > index fe45de8d796..c2c70ff02b9 100644
> > --- a/gcc/cp/pt.c
> > +++ b/gcc/cp/pt.c
> > @@ -10550,6 +10550,25 @@ 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
> > +     always return that instead.  */
> > +  tree cparms = ftpi->ctx_parms;
> > +  while (TMPL_PARMS_DEPTH (cparms) > level)
> > +    cparms = TREE_CHAIN (cparms);
> > +  gcc_assert (TMPL_PARMS_DEPTH (cparms) == level);
> > +  if (TREE_VEC_LENGTH (TREE_VALUE (cparms)))
> > +    {
> > +      t = TREE_VALUE (TREE_VEC_ELT (TREE_VALUE (cparms), 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);
> > +    }
> 
> This seems like a useful separate function: given a parmlist and a single
> template parm (or index+level), return the corresponding parm from the
> parmlist.  Basically the reverse of canonical_type_parameter.

Sounds good.  Like this?

-- >8 --

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.
	* g++.dg/cpp2a/concepts-ttp2.C: New test.
---
 gcc/cp/pt.c                                  | 44 ++++++++++++++++++++
 gcc/testsuite/g++.dg/concepts/diagnostic15.C | 16 +++++++
 2 files changed, 60 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/concepts/diagnostic15.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 44ca14afc4e..bec8396f9f4 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -10244,6 +10244,42 @@ lookup_and_finish_template_variable (tree templ, tree targs,
   return convert_from_reference (templ);
 }
 
+/* If the set of template parameters PARMS contains a template with
+   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);
+}
+
 \f
 struct pair_fn_data
 {
@@ -10550,6 +10586,14 @@ 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.  */
diff --git a/gcc/testsuite/g++.dg/concepts/diagnostic15.C b/gcc/testsuite/g++.dg/concepts/diagnostic15.C
new file mode 100644
index 00000000000..3acd9f67968
--- /dev/null
+++ b/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" }
-- 
2.28.0.497.g54e85e7af1