Re: [PATCH] c++: 'typename T::X' vs 'struct T::X' lookup [PR109420]

[Andrew Pinski <pinskia@gmail.com>]

On Wed, Apr 5, 2023 at 10:32 AM Patrick Palka via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Wed, 5 Apr 2023, Patrick Palka wrote:
>
> > r13-6098-g46711ff8e60d64 made make_typename_type no longer ignore
> > non-types during the lookup, unless the TYPENAME_TYPE in question was
> > followed by the :: scope resolution operator.  But there is another
> > exception to this rule: we need to ignore non-types during the lookup
> > also if the TYPENAME_TYPE was named with a tag other than 'typename',
> > such as 'struct' or 'enum', as per [dcl.type.elab]/5.
> >
> > This patch implements this additional exception.  It occurred to me that
> > the tf_qualifying_scope flag is probably unnecessary if we'd use the
> > scope_type tag more thoroughly, but that requires parser changes that
> > are probably too risky at this stage.  (I'm working on addressing the
> > FIXME/TODOs here for GCC 14.)
> >
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > trunk?
> >
> >       PR c++/109420
> >
> > gcc/cp/ChangeLog:
> >
> >       * decl.cc (make_typename_type): Also ignore non-types during
> >       the lookup if tag_type is something other than none_type or
> >       typename_type.
> >       * pt.cc (tsubst) <case TYPENAME_TYPE>: Pass class_type or
> >       enum_type as tag_type to make_typename_type as appropriate
> >       instead of always passing typename_type.
> >
> > gcc/testsuite/ChangeLog:
> >
> >       * g++.dg/template/typename27.C: New test.
> > ---
> >  gcc/cp/decl.cc                             |  9 ++++++++-
> >  gcc/cp/pt.cc                               |  9 ++++++++-
> >  gcc/testsuite/g++.dg/template/typename27.C | 19 +++++++++++++++++++
> >  3 files changed, 35 insertions(+), 2 deletions(-)
> >  create mode 100644 gcc/testsuite/g++.dg/template/typename27.C
> >
> > diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> > index 5369714f9b3..a0a20c5accc 100644
> > --- a/gcc/cp/decl.cc
> > +++ b/gcc/cp/decl.cc
> > @@ -4307,7 +4307,14 @@ make_typename_type (tree context, tree name, enum tag_types tag_type,
> >       lookup will stop when we hit a dependent base.  */
> >    if (!dependent_scope_p (context))
> >      {
> > -      bool want_type = (complain & tf_qualifying_scope);
> > +      /* As per [dcl.type.elab]/5 and [temp.res.general]/3, ignore non-types if
> > +      the tag corresponds to a class-key or 'enum' (or is scope_type), or if
> > +      this typename is followed by :: as per [basic.lookup.qual.general]/1.
> > +      TODO: If we'd set the scope_type tag accurately on all TYPENAME_TYPEs
> > +      that are followed by :: then we wouldn't need the tf_qualifying_scope
> > +      flag.  */
> > +      bool want_type = (tag_type != none_type && tag_type != typename_type)
> > +     || (complain & tf_qualifying_scope);
>
> Here's v2 which just slightly improves this comment.  I reckon [basic.lookup.elab]
> is a better reference than [dcl.type.elab]/5 for justifying why the
> lookup should be type-only for class-key and 'enum' TYPENAME_TYPEs.
>
> -- >8 --
>
>         PR c++/109420
>
> gcc/cp/ChangeLog:
>
>         * decl.cc (make_typename_type): Also ignore non-types during the
>         lookup if tag_type corresponds to an elaborated-type-specifier.
>         * pt.cc (tsubst) <case TYPENAME_TYPE>: Pass class_type or
>         enum_type as tag_type to make_typename_type as appropriate
>         instead of always passing typename_type.
>
> gcc/testsuite/ChangeLog:
>
>         * g++.dg/template/typename27.C: New test.
> ---
>  gcc/cp/decl.cc                             | 12 +++++++++++-
>  gcc/cp/pt.cc                               |  9 ++++++++-
>  gcc/testsuite/g++.dg/template/typename27.C | 19 +++++++++++++++++++
>  3 files changed, 38 insertions(+), 2 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/template/typename27.C
>
> diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> index 5369714f9b3..772c059dc2c 100644
> --- a/gcc/cp/decl.cc
> +++ b/gcc/cp/decl.cc
> @@ -4307,7 +4307,17 @@ make_typename_type (tree context, tree name, enum tag_types tag_type,
>       lookup will stop when we hit a dependent base.  */
>    if (!dependent_scope_p (context))
>      {
> -      bool want_type = (complain & tf_qualifying_scope);
> +      /* We generally don't ignore non-types during TYPENAME_TYPE lookup
> +        (as per [temp.res.general]/3), unless
> +          - the tag corresponds to a class-key or 'enum' so
> +            [basic.lookup.elab] applies, or
> +          - the tag corresponds to scope_type or tf_qualifying_scope is
> +            set so [basic.lookup.qual]/1 applies.
> +        TODO: If we'd set/track the scope_type tag thoroughly on all
> +        TYPENAME_TYPEs that are followed by :: then we wouldn't need the
> +        tf_qualifying_scope flag.  */
> +      bool want_type = (tag_type != none_type && tag_type != typename_type)
> +       || (complain & tf_qualifying_scope);
>        t = lookup_member (context, name, /*protect=*/2, want_type, complain);
>      }
>    else
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 821e0035c08..09559c88f29 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -16580,9 +16580,16 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>               return error_mark_node;
>           }
>
> +       /* FIXME: TYPENAME_IS_CLASS_P conflates 'union' vs 'struct' vs 'class'
> +          tags.  TYPENAME_TYPE should probably remember the exact tag that
> +          was written.  */

Yes I had a patch for that but I submitted during stage 4 of GCC 12
and never updated again during stage 1 of GCC 13. I hope to submit it
again with updated changes post this patch.

Thanks,
Andrew


> +       enum tag_types tag
> +         = TYPENAME_IS_CLASS_P (t) ? class_type
> +         : TYPENAME_IS_ENUM_P (t) ? enum_type
> +         : typename_type;
>         tsubst_flags_t tcomplain = complain | tf_keep_type_decl;
>         tcomplain |= tst_ok_flag | qualifying_scope_flag;
> -       f = make_typename_type (ctx, f, typename_type, tcomplain);
> +       f = make_typename_type (ctx, f, tag, tcomplain);
>         if (f == error_mark_node)
>           return f;
>         if (TREE_CODE (f) == TYPE_DECL)
> diff --git a/gcc/testsuite/g++.dg/template/typename27.C b/gcc/testsuite/g++.dg/template/typename27.C
> new file mode 100644
> index 00000000000..61b3efd998e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/typename27.C
> @@ -0,0 +1,19 @@
> +// PR c++/109420
> +
> +struct A {
> +  struct X { };
> +  int X;
> +};
> +
> +struct B {
> +  enum E { };
> +  enum F { E };
> +};
> +
> +template<class T, class U>
> +void f() {
> +  struct T::X x; // OK, lookup ignores the data member 'int A::X'
> +  enum U::E e;   // OK, lookup ignores the enumerator 'B::F::E'
> +}
> +
> +template void f<A, B>();
> --
> 2.40.0.153.g6369acd968
>