Re: [PATCH] c++: class-head parsing and CPP_TEMPLATE_ID access [PR108275]

[Jason Merrill <jason@redhat.com>]

On 1/5/23 12:20, Patrick Palka wrote:
> When tentatively parsing what is really an elaborated-type-specifier
> first as a class-specifier, we may form a CPP_TEMPLATE_ID token that
> later gets reused in the fallback parse if the tentative parse fails.
> These special tokens also capture the access checks that have been
> deferred while parsing the template-id.  But here, we form such a token
> when the access check state is dk_no_check, and so the token captures
> no access checks.  This effectively bypasses access checking for the
> template-id during the subsequent parse as an elaborated-type-specifier.
> 
> This patch fixes this by using dk_deferred instead of dk_no_check when
> parsing the class name.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?

OK.

> 	PR c++/108275
> 
> gcc/cp/ChangeLog:
> 
> 	* parser.cc (cp_parser_class_head): Use dk_deferred instead of
> 	dk_no_check when parsing the class name.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/parse/access14.C: New test.
> ---
>   gcc/cp/parser.cc                      | 23 +++++++++++++++++------
>   gcc/testsuite/g++.dg/parse/access14.C | 18 ++++++++++++++++++
>   2 files changed, 35 insertions(+), 6 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/parse/access14.C
> 
> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> index bfd8aeae39f..8b1658decba 100644
> --- a/gcc/cp/parser.cc
> +++ b/gcc/cp/parser.cc
> @@ -26559,7 +26559,23 @@ cp_parser_class_head (cp_parser* parser,
>     if (cp_parser_global_scope_opt (parser, /*current_scope_valid_p=*/false))
>       qualified_p = true;
>   
> -  push_deferring_access_checks (dk_no_check);
> +  /* It is OK to define an inaccessible class; for example:
> +
> +       class A { class B; };
> +       class A::B {};
> +
> +     So we want to ignore access when parsing the class name.
> +     However, we might be tentatively parsing what is really an
> +     elaborated-type-specifier naming a template-id, e.g.
> +
> +       struct C<&D::m> c;
> +
> +     In this case the tentative parse as a class-head will fail, but not
> +     before cp_parser_template_id splices in a CPP_TEMPLATE_ID token.
> +     Since dk_no_check is sticky, we must instead use dk_deferred so that
> +     any such CPP_TEMPLATE_ID token created during this tentative parse
> +     will correctly capture the access checks imposed by the template-id . */
> +  push_deferring_access_checks (dk_deferred);
>   
>     /* Determine the name of the class.  Begin by looking for an
>        optional nested-name-specifier.  */
> @@ -26586,11 +26602,6 @@ cp_parser_class_head (cp_parser* parser,
>   	 The proposed resolution for Core Issue 180 says that wherever
>   	 you see `class T::X' you should treat `X' as a type-name.
>   
> -	 It is OK to define an inaccessible class; for example:
> -
> -	   class A { class B; };
> -	   class A::B {};
> -
>   	 We do not know if we will see a class-name, or a
>   	 template-name.  We look for a class-name first, in case the
>   	 class-name is a template-id; if we looked for the
> diff --git a/gcc/testsuite/g++.dg/parse/access14.C b/gcc/testsuite/g++.dg/parse/access14.C
> new file mode 100644
> index 00000000000..bdbc7f6ee2b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/parse/access14.C
> @@ -0,0 +1,18 @@
> +// PR c++/108275
> +
> +struct A {
> +  int i;
> +private:
> +  int j;
> +};
> +
> +template<int A::* V>
> +struct B {
> +  struct C { };
> +private:
> +  template<int N> struct D { };
> +};
> +
> +struct B<&A::j> b;       // { dg-error "private" }
> +struct B<&A::j>::C c;    // { dg-error "private" }
> +struct B<&A::i>::D<0> d; // { dg-error "private" }