[PATCH v2] c++: ->template and implicit typedef [PR104608]

[Jason Merrill]

On 3/1/22 17:12, Marek Polacek wrote:
> On Mon, Feb 28, 2022 at 12:31:37PM -0400, Jason Merrill wrote:
>> On 2/22/22 17:46, Marek Polacek wrote:
>>> Here we have a forward declaration of Parameter for which we create
>>> an implicit typedef, which is a TYPE_DECL.  Then, when looking it up
>>> at template definition time, cp_parser_template_id gets (since r12-6754)
>>> this TYPE_DECL which it can't handle.
>>
>> Hmm, getting that global TYPE_DECL from lookup seems like a bug; isn't the
>> lookup earlier in cp_parser_template_name in object scope?
> 
> Yes, it is (in Function), but I think we do the pre-DR1835 lookup.  For
> 
>    this->template Parameter<R>();
> 
> we don't find Parameter in the object expression's type (Function), so we do
> unqualified lookup in the enclosing context and find the global Parameter
> TYPE_DECL.  This is implemented in cp_parser_lookup_name:
> 
>    decl = lookup_member (object_type, name, ...);
>    if (!decl)
>       decl = lookup_name (name, ...);
> 
> [basic.lookup.qual.general] now says that we only perform unqualified lookup
> if the object type isn't dependent.  But I don't think we can fix this by
> implementing DR1835 because that would only help in C++23(?).
> 
> My v1 patch is wrong in any case; I've come up with template-keyword4.C
> where we find a TYPE_DECL which is not an implicit typedef.
> 
> Since cp_parser_template_id is only able to handle these TYPE_DECLs:
> 
> 18353   else if (TREE_CODE (templ) == TYPE_DECL
> 18354            && TREE_CODE (TREE_TYPE (templ)) == TYPENAME_TYPE)
> 
> this v2 patch fixes the problem by repeating lookup of TYPE_DECLs whose
> TREE_TYPE is *not* TYPENAME_TYPE.  That fixes my testcases and doesn't
> regress any.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

> -- >8 --
> Here we have a forward declaration of Parameter for which we create
> an implicit typedef, which is a TYPE_DECL.  Then, when looking it up
> at template definition time, cp_parser_template_id gets (since r12-6754)
> this TYPE_DECL which it can't handle.
> 
> This patch defers lookup for TYPE_DECLs that cp_parser_template_id can't
> handle, a la r12-6879.
> 
> 	PR c++/104608
> 
> gcc/cp/ChangeLog:
> 
> 	* parser.cc (cp_parser_template_name): Repeat lookup of
> 	TYPE_DECLs.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/parse/template-keyword3.C: New test.
> 	* g++.dg/parse/template-keyword4.C: New test.
> ---
>   gcc/cp/parser.cc                               |  5 ++++-
>   gcc/testsuite/g++.dg/parse/template-keyword3.C | 12 ++++++++++++
>   gcc/testsuite/g++.dg/parse/template-keyword4.C | 17 +++++++++++++++++
>   3 files changed, 33 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/parse/template-keyword3.C
>   create mode 100644 gcc/testsuite/g++.dg/parse/template-keyword4.C
> 
> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> index 03d99aba13e..02c34bf964e 100644
> --- a/gcc/cp/parser.cc
> +++ b/gcc/cp/parser.cc
> @@ -18681,7 +18681,10 @@ cp_parser_template_name (cp_parser* parser,
>   	  return error_mark_node;
>   	}
>         else if ((!DECL_P (decl) && !is_overloaded_fn (decl))
> -	       || TREE_CODE (decl) == USING_DECL)
> +	       || TREE_CODE (decl) == USING_DECL
> +	       /* cp_parser_template_id can only handle some TYPE_DECLs.  */
> +	       || (TREE_CODE (decl) == TYPE_DECL
> +		   && TREE_CODE (TREE_TYPE (decl)) != TYPENAME_TYPE))
>   	/* Repeat the lookup at instantiation time.  */
>   	decl = identifier;
>       }
> diff --git a/gcc/testsuite/g++.dg/parse/template-keyword3.C b/gcc/testsuite/g++.dg/parse/template-keyword3.C
> new file mode 100644
> index 00000000000..59fe0fc180b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/parse/template-keyword3.C
> @@ -0,0 +1,12 @@
> +// PR c++/104608
> +
> +class Parameter;
> +template <typename R> class Function
> +: public R
> +{
> +    Function();
> +};
> +template <typename R>
> +Function<R>::Function() {
> +    this->template Parameter<R>();
> +}
> diff --git a/gcc/testsuite/g++.dg/parse/template-keyword4.C b/gcc/testsuite/g++.dg/parse/template-keyword4.C
> new file mode 100644
> index 00000000000..c688094bcf2
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/parse/template-keyword4.C
> @@ -0,0 +1,17 @@
> +// PR c++/104608
> +// { dg-do compile { target c++11 } }
> +
> +class S;
> +using Parameter = S;
> +typedef S Parameter2;
> +
> +template <typename R> class Function
> +: public R
> +{
> +    Function();
> +};
> +template <typename R>
> +Function<R>::Function() {
> +    this->template Parameter<R>();
> +    this->template Parameter2<R>();
> +}
> 
> base-commit: 4a1c20df82c9e14478d79fbe1ae9690a36285ac1