Re: [PATCH 1/2] c++: factor out TYPENAME_TYPE substitution

[Jason Merrill <jason@redhat.com>]

On 2/15/23 09:21, Patrick Palka wrote:
> On Tue, 14 Feb 2023, Jason Merrill wrote:
> 
>> On 2/13/23 09:23, Patrick Palka wrote:
>>> [N.B. this is a corrected version of
>>> https://gcc.gnu.org/pipermail/gcc-patches/2022-November/607443.html ]
>>>
>>> This patch factors out the TYPENAME_TYPE case of tsubst into a separate
>>> function tsubst_typename_type.  It also factors out the two tsubst flags
>>> controlling TYPENAME_TYPE substitution, tf_keep_type_decl and tf_tst_ok,
>>> into distinct boolean parameters of this new function (and of
>>> make_typename_type).  Consequently, callers which used to pass tf_tst_ok
>>> to tsubst now instead must directly call tsubst_typename_type when
>>> appropriate.
>>
>> Hmm, I don't love how that turns 4 lines into 8 more complex lines in each
>> caller.  And the previous approach of saying "a CTAD placeholder is OK" seem
>> like better abstraction than repeating the specific TYPENAME_TYPE handling in
>> each place.
> 
> Ah yeah, I see what you mean.  I was thinking since tf_tst_ok is
> specific to TYPENAME_TYPE handling and isn't propagated (i.e. it only
> affects top-level TYPENAME_TYPEs), it seemed cleaner to encode the flag
> as a bool parameter "template_ok" of tsubst_typename_type instead of as
> a global tsubst_flag that gets propagated freely.
> 
>>
>>> In a subsequent patch we'll add another flag to
>>> tsubst_typename_type controlling whether we want to ignore non-types
>>> during the qualified lookup.
> 
> As mentioned above, the second patch in this series would just add
> another flag "type_only" alongside "template_ok", since this flag will
> also only affects top-level TYPENAME_TYPEs and doesn't need to propagate
> like tsubst_flags.
> 
> Except, it turns it, this new flag _does_ need to propagate, namely when
> expanding a variadic using:
> 
>    using typename Ts::type::m...; // from typename25a.C below
> 
> Here we have a USING_DECL whose USING_DECL_SCOPE is a
> TYPE_PACK_EXPANSION over TYPENAME_TYPE.  In order to correctly
> substitute this TYPENAME_TYPE, the USING_DECL case of tsubst_decl needs
> to pass an appropriate tsubst_flag to tsubst_pack_expansion to be
> propagated to tsubst (to be propagated to make_typename_type).
> 
> So in light of this case it seems adding a new tsubst_flag is the
> way to go, which means we can avoid this refactoring patch entirely.
> 
> Like so?  Bootstrapped and regtested on x86_64-pc-linux-gnu.

OK, though I still wonder about adding a tsubst_scope function that 
would add the tf_qualifying_scope.

> -- >8 --
> 
> Subject: [PATCH] c++: TYPENAME_TYPE lookup ignoring non-types [PR107773]
> 
> Currently when resolving a TYPENAME_TYPE for 'typename T::m' via
> make_typename_type, we consider only type bindings of 'm' and ignore
> non-type ones.  But [temp.res.general]/3 says, in a note, "the usual
> qualified name lookup ([basic.lookup.qual]) applies even in the presence
> of 'typename'", and qualified name lookup doesn't discriminate between
> type and non-type bindings.  So when resolving such a TYPENAME_TYPE
> we want the lookup to consider all bindings.
> 
> An exception is when we have a TYPENAME_TYPE corresponding to the
> qualifying scope of the :: scope resolution operator, such as
> 'T::type' in 'typename T::type::m'.  In that case, [basic.lookup.qual]/1
> applies, and lookup for such a TYPENAME_TYPE must ignore non-type bindings.
> So in order to correctly handle all cases, make_typename_type needs an
> additional flag controlling whether lookup should ignore non-types or not.
> 
> To that end this patch adds a new tsubst flag tf_qualifying_scope to
> communicate to make_typename_type whether we want to ignore non-type
> bindings during the lookup (by default we don't want to ignore them).
> In contexts where we do want to ignore non-types (when substituting
> into the scope of TYPENAME_TYPE, SCOPE_REF or USING_DECL) we simply
> pass tf_qualifying_scope to the relevant tsubst / tsubst_copy call.
> This flag is intended to apply only to top-level TYPENAME_TYPEs so
> we must be careful to clear the flag to avoid propagating it during
> substitution of sub-trees.
> 
> 	PR c++/107773
> 
> gcc/cp/ChangeLog:
> 
> 	* cp-tree.h (enum tsubst_flags): New flag tf_qualifying_scope.
> 	* decl.cc (make_typename_type): Use lookup_member instead of
> 	lookup_field.  If tf_qualifying_scope is set, pass want_type=true
> 	instead of =false to lookup_member.  Generalize format specifier
> 	in diagnostic to handle both type and non-type bindings.
> 	* pt.cc (tsubst_aggr_type_1): Clear tf_qualifying_scope.  Tidy
> 	the function.
> 	(tsubst_decl) <case USING_DECL>: Set tf_qualifying_scope when
> 	substituting USING_DECL_SCOPE.
> 	(tsubst): Clear tf_qualifying_scope right away and remember if
> 	it was set.  Do the same for tf_tst_ok sooner.
> 	<case TYPENAME_TYPE>: Set tf_qualifying_scope when substituting
> 	TYPE_CONTEXT.  Pass tf_qualifying_scope to make_typename_type
> 	if it was set.
> 	(tsubst_qualified_id): Set tf_qualifying_scope when substituting
> 	the scope.
> 	(tsubst_copy): Clear tf_qualifying_scope and remember if it was
> 	set.
> 	<case SCOPE_REF>: Set tf_qualifying_scope when substituting the
> 	scope.
> 	<case *_TYPE>: Pass tf_qualifying_scope to tsubst if it was set.
> 	* search.cc (lookup_member): Document default argument.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/template/typename24.C: New test.
> 	* g++.dg/template/typename25.C: New test.
> 	* g++.dg/template/typename25a.C: New test.
> 	* g++.dg/template/typename26.C: New test.
> ---
>   gcc/cp/cp-tree.h                            |  3 ++
>   gcc/cp/decl.cc                              |  9 ++--
>   gcc/cp/pt.cc                                | 58 ++++++++++++---------
>   gcc/cp/search.cc                            |  2 +-
>   gcc/testsuite/g++.dg/template/typename24.C  | 18 +++++++
>   gcc/testsuite/g++.dg/template/typename25.C  | 33 ++++++++++++
>   gcc/testsuite/g++.dg/template/typename25a.C | 37 +++++++++++++
>   gcc/testsuite/g++.dg/template/typename26.C  | 20 +++++++
>   8 files changed, 150 insertions(+), 30 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/template/typename24.C
>   create mode 100644 gcc/testsuite/g++.dg/template/typename25.C
>   create mode 100644 gcc/testsuite/g++.dg/template/typename25a.C
>   create mode 100644 gcc/testsuite/g++.dg/template/typename26.C
> 
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index be8775ed0f8..891fcf521a8 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -5597,6 +5597,9 @@ enum tsubst_flags {
>     tf_tst_ok = 1 << 12,		 /* Allow a typename-specifier to name
>   				    a template (C++17 or later).  */
>     tf_dguide = 1 << 13,		/* Building a deduction guide from a ctor.  */
> +  tf_qualifying_scope = 1 << 14, /* Substituting the LHS of the :: operator.
> +				    Controls TYPENAME_TYPE resolution from
> +				    make_typename_type.  */
>     /* Convenient substitution flags combinations.  */
>     tf_warning_or_error = tf_warning | tf_error
>   };
> diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> index d606b31d7a7..2f6412d04e6 100644
> --- a/gcc/cp/decl.cc
> +++ b/gcc/cp/decl.cc
> @@ -4305,9 +4305,10 @@ 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);
> +    {
> +      bool want_type = (complain & tf_qualifying_scope);
> +      t = lookup_member (context, name, /*protect=*/2, want_type, complain);
> +    }
>     else
>       t = NULL_TREE;
>   
> @@ -4359,7 +4360,7 @@ 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/pt.cc b/gcc/cp/pt.cc
> index e89dbf47097..d11d540ab44 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -13919,8 +13919,7 @@ tsubst_aggr_type_1 (tree t,
>   {
>     if (TYPE_TEMPLATE_INFO (t) && uses_template_parms (t))
>       {
> -      tree argvec;
> -      tree r;
> +      complain &= ~tf_qualifying_scope;
>   
>         /* Figure out what arguments are appropriate for the
>   	 type we are trying to find.  For example, given:
> @@ -13931,18 +13930,14 @@ tsubst_aggr_type_1 (tree t,
>   	 and supposing that we are instantiating f<int, double>,
>   	 then our ARGS will be {int, double}, but, when looking up
>   	 S we only want {double}.  */
> -      argvec = tsubst_template_args (TYPE_TI_ARGS (t), args,
> -				     complain, in_decl);
> +      tree argvec = tsubst_template_args (TYPE_TI_ARGS (t), args,
> +					  complain, in_decl);
>         if (argvec == error_mark_node)
> -	r = error_mark_node;
> -      else
> -	{
> -	  r = lookup_template_class (t, argvec, in_decl, NULL_TREE,
> -				     entering_scope, complain);
> -	  r = cp_build_qualified_type (r, cp_type_quals (t), complain);
> -	}
> +	return error_mark_node;
>   
> -      return r;
> +      tree r = lookup_template_class (t, argvec, in_decl, NULL_TREE,
> +				      entering_scope, complain);
> +      return cp_build_qualified_type (r, cp_type_quals (t), complain);
>       }
>     else
>       /* This is not a template type, so there's nothing to do.  */
> @@ -15003,11 +14998,15 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
>   	  tree scope = USING_DECL_SCOPE (t);
>   	  if (PACK_EXPANSION_P (scope))
>   	    {
> -	      scope = tsubst_pack_expansion (scope, args, complain, in_decl);
> +	      scope = tsubst_pack_expansion (scope, args,
> +					     complain | tf_qualifying_scope,
> +					     in_decl);
>   	      variadic_p = true;
>   	    }
>   	  else
> -	    scope = tsubst_copy (scope, args, complain, in_decl);
> +	    scope = tsubst_copy (scope, args,
> +				 complain | tf_qualifying_scope,
> +				 in_decl);
>   
>   	  tree name = DECL_NAME (t);
>   	  if (IDENTIFIER_CONV_OP_P (name)
> @@ -15821,6 +15820,12 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>         || TREE_CODE (t) == TRANSLATION_UNIT_DECL)
>       return t;
>   
> +  tsubst_flags_t tst_ok_flag = (complain & tf_tst_ok);
> +  complain &= ~tf_tst_ok;
> +
> +  tsubst_flags_t qualifying_scope_flag = (complain & tf_qualifying_scope);
> +  complain &= ~tf_qualifying_scope;
> +
>     if (DECL_P (t))
>       return tsubst_decl (t, args, complain);
>   
> @@ -15889,9 +15894,6 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>     bool fndecl_type = (complain & tf_fndecl_type);
>     complain &= ~tf_fndecl_type;
>   
> -  bool tst_ok = (complain & tf_tst_ok);
> -  complain &= ~tf_tst_ok;
> -
>     if (type
>         && code != TYPENAME_TYPE
>         && code != TEMPLATE_TYPE_PARM
> @@ -16428,7 +16430,9 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>   	tree ctx = TYPE_CONTEXT (t);
>   	if (TREE_CODE (ctx) == TYPE_PACK_EXPANSION)
>   	  {
> -	    ctx = tsubst_pack_expansion (ctx, args, complain, in_decl);
> +	    ctx = tsubst_pack_expansion (ctx, args,
> +					 complain | tf_qualifying_scope,
> +					 in_decl);
>   	    if (ctx == error_mark_node
>   		|| TREE_VEC_LENGTH (ctx) > 1)
>   	      return error_mark_node;
> @@ -16442,8 +16446,9 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>   	    ctx = TREE_VEC_ELT (ctx, 0);
>   	  }
>   	else
> -	  ctx = tsubst_aggr_type (ctx, args, complain, in_decl,
> -				  /*entering_scope=*/1);
> +	  ctx = tsubst_aggr_type (ctx, args,
> +				  complain | tf_qualifying_scope,
> +				  in_decl, /*entering_scope=*/1);
>   	if (ctx == error_mark_node)
>   	  return error_mark_node;
>   
> @@ -16473,8 +16478,7 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>   	  }
>   
>   	tsubst_flags_t tcomplain = complain | tf_keep_type_decl;
> -	if (tst_ok)
> -	  tcomplain |= tf_tst_ok;
> +	tcomplain |= tst_ok_flag | qualifying_scope_flag;
>   	f = make_typename_type (ctx, f, typename_type, tcomplain);
>   	if (f == error_mark_node)
>   	  return f;
> @@ -16879,7 +16883,7 @@ tsubst_qualified_id (tree qualified_id, tree args,
>     scope = TREE_OPERAND (qualified_id, 0);
>     if (args)
>       {
> -      scope = tsubst (scope, args, complain, in_decl);
> +      scope = tsubst (scope, args, complain | tf_qualifying_scope, in_decl);
>         expr = tsubst_copy (name, args, complain, in_decl);
>       }
>     else
> @@ -17125,6 +17129,9 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>     if (t == NULL_TREE || t == error_mark_node || args == NULL_TREE)
>       return t;
>   
> +  tsubst_flags_t qualifying_scope_flag = (complain & tf_qualifying_scope);
> +  complain &= ~tf_qualifying_scope;
> +
>     if (tree d = maybe_dependent_member_ref (t, args, complain, in_decl))
>       return d;
>   
> @@ -17598,7 +17605,8 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>   
>       case SCOPE_REF:
>         {
> -	tree op0 = tsubst_copy (TREE_OPERAND (t, 0), args, complain, in_decl);
> +	tree op0 = tsubst_copy (TREE_OPERAND (t, 0), args,
> +				complain | tf_qualifying_scope, in_decl);
>   	tree op1 = tsubst_copy (TREE_OPERAND (t, 1), args, complain, in_decl);
>   	return build_qualified_name (/*type=*/NULL_TREE, op0, op1,
>   				     QUALIFIED_NAME_IS_TEMPLATE (t));
> @@ -17713,7 +17721,7 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>       case TYPEOF_TYPE:
>       case DECLTYPE_TYPE:
>       case TYPE_DECL:
> -      return tsubst (t, args, complain, in_decl);
> +      return tsubst (t, args, complain | qualifying_scope_flag, in_decl);
>   
>       case USING_DECL:
>         t = DECL_NAME (t);
> diff --git a/gcc/cp/search.cc b/gcc/cp/search.cc
> index f3f19cafec6..e472a97679d 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..8b2b3718442
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/typename24.C
> @@ -0,0 +1,18 @@
> +// PR c++/107773
> +// Verify lookup for a non-neste TYPENAME_TYPE correctly considers
> +// non-types.
> +
> +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..04e48e11724
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/typename25.C
> @@ -0,0 +1,33 @@
> +// PR c++/107773
> +// Verify lookup for TYPENAME_TYPE appearing to the left of the ::
> +// scope resolution operator correctly ignores non-types.
> +
> +struct a {
> +  typedef void type;
> +};
> +
> +struct c {
> +  struct b : a {
> +    typedef b self;
> +    static int m;
> +  };
> +  int b;
> +};
> +
> +template<class T>
> +void f() {
> +  // A TYPENAME_TYPE whose TYPE_CONTEXT is a nested TYPENAME_TYPE.
> +  typedef typename T::b::type type;
> +  // A SCOPE_REF whose first operand is a TYPENAME_TYPE.
> +  int m = T::b::m;
> +}
> +
> +template void f<c>();
> +
> +template<class T>
> +struct d : T::b::self {
> +  // A USING_DECL whose USING_DECL_SCOPE is a TYPENAME_TYPE.
> +  using typename T::b::type;
> +};
> +
> +template struct d<c>;
> diff --git a/gcc/testsuite/g++.dg/template/typename25a.C b/gcc/testsuite/g++.dg/template/typename25a.C
> new file mode 100644
> index 00000000000..ecb34aada34
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/typename25a.C
> @@ -0,0 +1,37 @@
> +// PR c++/107773
> +// A variadic version of typename25.C
> +// { dg-do compile { target c++11 } }
> +
> +struct a {
> +  typedef void type;
> +};
> +
> +struct c {
> +  struct b : a {
> +    typedef b self;
> +    static int m;
> +  };
> +  int b;
> +};
> +
> +template<class...> void sink(...);
> +
> +template<class... Ts>
> +void f() {
> +  // A TYPENAME_TYPE whose TYPE_CONTEXT is a nested TYPENAME_TYPE.
> +  sink<typename Ts::b::type...>();
> +  // A SCOPE_REF whose first operand is a TYPENAME_TYPE.
> +  sink(Ts::b::m...);
> +}
> +
> +template void f<c>();
> +
> +template<class... Ts>
> +struct d : Ts::b::self... {
> +#if __cpp_variadic_using
> +  // A USING_DECL whose USING_DECL_SCOPE is a TYPENAME_TYPE.
> +  using typename Ts::b::type...;
> +#endif
> +};
> +
> +template struct d<c>;
> diff --git a/gcc/testsuite/g++.dg/template/typename26.C b/gcc/testsuite/g++.dg/template/typename26.C
> new file mode 100644
> index 00000000000..4e6b764a97b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/typename26.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 }
> +}