Re: [PATCH] c++: TYPENAME_TYPE lookup ignoring non-types [PR107773]

[Patrick Palka <ppalka@redhat.com>]

On Mon, 28 Nov 2022, Patrick Palka wrote:

> [temp.res.general]/3 says, in a note, "the usual qualified name lookup
> ([basic.lookup.qual]) applies even in the presence of typename".  Thus
> when resolving a TYPENAME_TYPE, it seems we shouldn't be looking past
> non-type members.
> 
> This patch fixes this by passing want_type=false instead of =true during
> the member lookup from make_typename_type.  An old nearby comment
> mentions that we want to continue to set want_type=true when resolving a
> nested typename type, but it appears that the nested case is handled by
> resolve_typename_type instead (which passes want_type=true appropriately).

Whoops, it seems this isn't true -- not all nested TYPENAME_TYPEs are
handled by resolve_typename_type, e.g. for T::b in

  struct a {
    struct b { typedef void get; };
    int b;
  };

  template<class T>
  void f() {
    typedef typename T::b::get type;
  }

  template void f<a>();

Passing want_type=false in make_typename_type causes us to incorrectly
reject the TYPENAME_TYPE for T::b here because qualified lookup now
finds the data member a::b instead of the nested class of the same name.
So it looks like we need a flag to control whether we're dealing with a
nested TYPENAME_TYPE or not and to pass want_type=true/false appropriately,
I'll poke more tomorrow.

> 
> In passing, use lookup_member instead of lookup_field so that we give a
> better diagnostic when a member function is found, and generalize the T
> format specifier to D in the diagnostic.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
> 
> 	PR c++/107773
> 
> gcc/cp/ChangeLog:
> 
> 	* decl.cc (make_typename_type): Use lookup_member instead of
> 	lookup_field.  Pass want_type=false instead of =true.  Use D
> 	instead of T format specifier.
> 	* search.cc (lookup_member): Document default argument.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/template/typename24.C: New test.
> 	* g++.dg/template/typename25.C: New test.
> ---
>  gcc/cp/decl.cc                             |  7 +++----
>  gcc/cp/search.cc                           |  2 +-
>  gcc/testsuite/g++.dg/template/typename24.C | 16 ++++++++++++++++
>  gcc/testsuite/g++.dg/template/typename25.C | 20 ++++++++++++++++++++
>  4 files changed, 40 insertions(+), 5 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/template/typename24.C
>  create mode 100644 gcc/testsuite/g++.dg/template/typename25.C
> 
> diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> index 238e72f90da..673e10801a6 100644
> --- a/gcc/cp/decl.cc
> +++ b/gcc/cp/decl.cc
> @@ -4303,9 +4303,8 @@ make_typename_type (tree context, tree name, enum tag_types tag_type,
>       member of the current instantiation or a non-dependent base;
>       lookup will stop when we hit a dependent base.  */
>    if (!dependent_scope_p (context))
> -    /* We should only set WANT_TYPE when we're a nested typename type.
> -       Then we can give better diagnostics if we find a non-type.  */
> -    t = lookup_field (context, name, 2, /*want_type=*/true);
> +    t = lookup_member (context, name, /*protect=*/2, /*want_type=*/false,
> +		       complain);
>    else
>      t = NULL_TREE;
>  
> @@ -4357,7 +4356,7 @@ make_typename_type (tree context, tree name, enum tag_types tag_type,
>        else
>  	{
>  	  if (complain & tf_error)
> -	    error ("%<typename %T::%D%> names %q#T, which is not a type",
> +	    error ("%<typename %T::%D%> names %q#D, which is not a type",
>  		   context, name, t);
>  	  return error_mark_node;
>  	}
> diff --git a/gcc/cp/search.cc b/gcc/cp/search.cc
> index 0dbb3be1ee7..e5848ebc620 100644
> --- a/gcc/cp/search.cc
> +++ b/gcc/cp/search.cc
> @@ -1109,7 +1109,7 @@ build_baselink (tree binfo, tree access_binfo, tree functions, tree optype)
>  
>  tree
>  lookup_member (tree xbasetype, tree name, int protect, bool want_type,
> -	       tsubst_flags_t complain, access_failure_info *afi)
> +	       tsubst_flags_t complain, access_failure_info *afi /* = NULL */)
>  {
>    tree rval, rval_binfo = NULL_TREE;
>    tree type = NULL_TREE, basetype_path = NULL_TREE;
> diff --git a/gcc/testsuite/g++.dg/template/typename24.C b/gcc/testsuite/g++.dg/template/typename24.C
> new file mode 100644
> index 00000000000..4b1d5e5271b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/typename24.C
> @@ -0,0 +1,16 @@
> +// PR c++/107773
> +
> +struct a {
> +  typedef void get;
> +};
> +
> +struct b : a {
> +  int get(int i) const;
> +};
> +
> +template<class T>
> +void f() {
> +  typedef typename T::get type; // { dg-error "'int b::get\\(int\\) const', which is not a type" }
> +}
> +
> +template void f<b>();
> diff --git a/gcc/testsuite/g++.dg/template/typename25.C b/gcc/testsuite/g++.dg/template/typename25.C
> new file mode 100644
> index 00000000000..4e6b764a97b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/typename25.C
> @@ -0,0 +1,20 @@
> +// Example 4 from [temp.res.general]/3.
> +
> +struct A {
> +  struct X { };
> +  int X;
> +};
> +struct B {
> +  struct X { };
> +};
> +template<class T> void f(T t) {
> +  typename T::X x; // { dg-error "'int A::X', which is not a type" }
> +}
> +void foo() {
> +  A a;
> +  B b;
> +  f(b); // OK, T::X refers to B::X
> +  // { dg-bogus "" "" { target *-*-* } .-1 }
> +  f(a); // error: T::X refers to the data member A::X not the struct A::X
> +  // { dg-message "required from here" "" { target *-*-* } .-1 }
> +}
> -- 
> 2.39.0.rc0.33.g815c1e8202
> 
>