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

[Jason Merrill <jason@redhat.com>]

On 2/14/23 14:15, 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.

tsubst_maybe_ctad_type?

>> 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.
>>
>> gcc/cp/ChangeLog:
>>
>>     * cp-tree.h (enum tsubst_flags): Remove tf_keep_type_decl
>>     and tf_tst_ok.
>>     (make_typename_type): Add two trailing boolean parameters
>>     defaulted to false.
>>     * decl.cc (make_typename_type): Replace uses of
>>     tf_keep_type_decl and tf_tst_ok with the corresponding new
>>     boolean parameters.
>>     * pt.cc (tsubst_typename_type): New, factored out from tsubst
>>     and adjusted after removing tf_keep_type_decl and tf_tst_ok.
>>     (tsubst_decl) <case VAR_DECL>: Conditionally call
>>     tsubst_typename_type directly instead of using tf_tst_ok.
>>     (tsubst) <case TYPENAME_TYPE>: Call tsubst_typename_type.
>>     (tsubst_copy) <case CAST_EXPR>: Conditionally call
>>     tsubst_typename_type directly instead of using tf_tst_ok.
>>     (tsubst_copy_and_build) <case CAST_EXPR>: Likewise.
>>     <case CONSTRUCTOR>: Likewise.
>> ---
>>   gcc/cp/cp-tree.h |   9 +-
>>   gcc/cp/decl.cc   |  17 ++--
>>   gcc/cp/pt.cc     | 223 +++++++++++++++++++++++++----------------------
>>   3 files changed, 134 insertions(+), 115 deletions(-)
>>
>> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
>> index 06bc64a6b8d..a7c5765fc33 100644
>> --- a/gcc/cp/cp-tree.h
>> +++ b/gcc/cp/cp-tree.h
>> @@ -5573,8 +5573,7 @@ enum tsubst_flags {
>>     tf_error = 1 << 0,         /* give error messages  */
>>     tf_warning = 1 << 1,          /* give warnings too  */
>>     tf_ignore_bad_quals = 1 << 2,     /* ignore bad cvr qualifiers */
>> -  tf_keep_type_decl = 1 << 3,     /* retain typedef type decls
>> -                    (make_typename_type use) */
>> +  /* 1 << 3 available */
>>     tf_ptrmem_ok = 1 << 4,     /* pointers to member ok (internal
>>                       instantiate_type use) */
>>     tf_user = 1 << 5,         /* found template must be a user template
>> @@ -5594,8 +5593,7 @@ enum tsubst_flags {
>>                   (build_target_expr and friends) */
>>     tf_norm = 1 << 11,         /* Build diagnostic information during
>>                       constraint normalization.  */
>> -  tf_tst_ok = 1 << 12,         /* Allow a typename-specifier to name
>> -                    a template (C++17 or later).  */
>> +  /* 1 << 12 available */
>>     tf_dguide = 1 << 13,        /* Building a deduction guide from a 
>> ctor.  */
>>     /* Convenient substitution flags combinations.  */
>>     tf_warning_or_error = tf_warning | tf_error
>> @@ -6846,7 +6844,8 @@ extern tree declare_local_label            (tree);
>>   extern tree define_label            (location_t, tree);
>>   extern void check_goto                (tree);
>>   extern bool check_omp_return            (void);
>> -extern tree make_typename_type            (tree, tree, enum 
>> tag_types, tsubst_flags_t);
>> +extern tree make_typename_type            (tree, tree, enum 
>> tag_types, tsubst_flags_t,
>> +                         bool = false, bool = false);
>>   extern tree build_typename_type            (tree, tree, tree, 
>> tag_types);
>>   extern tree make_unbound_class_template        (tree, tree, tree, 
>> tsubst_flags_t);
>>   extern tree make_unbound_class_template_raw    (tree, tree, tree);
>> diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
>> index d606b31d7a7..430533606b0 100644
>> --- a/gcc/cp/decl.cc
>> +++ b/gcc/cp/decl.cc
>> @@ -4228,14 +4228,17 @@ build_typename_type (tree context, tree name, 
>> tree fullname,
>>   /* Resolve `typename CONTEXT::NAME'.  TAG_TYPE indicates the tag
>>      provided to name the type.  Returns an appropriate type, unless an
>>      error occurs, in which case error_mark_node is returned.  If we
>> -   locate a non-artificial TYPE_DECL and TF_KEEP_TYPE_DECL is set, we
>> +   locate a non-artificial TYPE_DECL and KEEP_TYPE_DECL is true, we
>>      return that, rather than the _TYPE it corresponds to, in other
>> -   cases we look through the type decl.  If TF_ERROR is set, complain
>> -   about errors, otherwise be quiet.  */
>> +   cases we look through the type decl.  If TEMPLATE_OK is true and
>> +   we found a TEMPLATE_DECL then we return a CTAD placeholder for the
>> +   TEMPLATE_DECL.  If TF_ERROR is set, complain about errors, otherwise
>> +   be quiet.  */
>>   tree
>>   make_typename_type (tree context, tree name, enum tag_types tag_type,
>> -            tsubst_flags_t complain)
>> +            tsubst_flags_t complain, bool keep_type_decl /* = false */,
>> +            bool template_ok /* = false */)
>>   {
>>     tree fullname;
>>     tree t;
>> @@ -4352,8 +4355,8 @@ make_typename_type (tree context, tree name, 
>> enum tag_types tag_type,
>>       }
>>     if (!want_template && TREE_CODE (t) != TYPE_DECL)
>>       {
>> -      if ((complain & tf_tst_ok) && cxx_dialect >= cxx17
>> -      && DECL_TYPE_TEMPLATE_P (t))
>> +      if (template_ok && DECL_TYPE_TEMPLATE_P (t)
>> +      && cxx_dialect >= cxx17)
>>       /* The caller permits this typename-specifier to name a template
>>          (because it appears in a CTAD-enabled context).  */;
>>         else
>> @@ -4383,7 +4386,7 @@ make_typename_type (tree context, tree name, 
>> enum tag_types tag_type,
>>         t = TYPE_NAME (t);
>>       }
>> -  if (DECL_ARTIFICIAL (t) || !(complain & tf_keep_type_decl))
>> +  if (DECL_ARTIFICIAL (t) || !keep_type_decl)
>>       t = TREE_TYPE (t);
>>     maybe_record_typedef_use (t);
>> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
>> index e89dbf47097..bc47bf15d38 100644
>> --- a/gcc/cp/pt.cc
>> +++ b/gcc/cp/pt.cc
>> @@ -13949,6 +13949,94 @@ tsubst_aggr_type_1 (tree t,
>>       return t;
>>   }
>> +/* Substitute ARGS into the TYPENAME_TYPE T.  The flag TEMPLATE_OK
>> +   is passed to make_typename_type.  */
>> +
>> +static tree
>> +tsubst_typename_type (tree t, tree args, tsubst_flags_t complain, 
>> tree in_decl,
>> +              bool template_ok = false)
>> +{
>> +  tree ctx = TYPE_CONTEXT (t);
>> +  if (TREE_CODE (ctx) == TYPE_PACK_EXPANSION)
>> +    {
>> +      ctx = tsubst_pack_expansion (ctx, args, complain, in_decl);
>> +      if (ctx == error_mark_node
>> +      || TREE_VEC_LENGTH (ctx) > 1)
>> +    return error_mark_node;
>> +      if (TREE_VEC_LENGTH (ctx) == 0)
>> +    {
>> +      if (complain & tf_error)
>> +        error ("%qD is instantiated for an empty pack",
>> +           TYPENAME_TYPE_FULLNAME (t));
>> +      return error_mark_node;
>> +    }
>> +      ctx = TREE_VEC_ELT (ctx, 0);
>> +    }
>> +  else
>> +    ctx = tsubst_aggr_type (ctx, args, complain, in_decl,
>> +                /*entering_scope=*/1);
>> +  if (ctx == error_mark_node)
>> +    return error_mark_node;
>> +
>> +  tree f = tsubst_copy (TYPENAME_TYPE_FULLNAME (t), args,
>> +            complain, in_decl);
>> +  if (f == error_mark_node)
>> +    return error_mark_node;
>> +
>> +  if (!MAYBE_CLASS_TYPE_P (ctx))
>> +    {
>> +      if (complain & tf_error)
>> +    error ("%qT is not a class, struct, or union type", ctx);
>> +      return error_mark_node;
>> +    }
>> +  else if (!uses_template_parms (ctx) && !TYPE_BEING_DEFINED (ctx))
>> +    {
>> +      /* Normally, make_typename_type does not require that the CTX
>> +     have complete type in order to allow things like:
>> +
>> +       template <class T> struct S { typename S<T>::X Y; };
>> +
>> +     But, such constructs have already been resolved by this
>> +     point, so here CTX really should have complete type, unless
>> +     it's a partial instantiation.  */
>> +      if (!complete_type_or_maybe_complain (ctx, NULL_TREE, complain))
>> +    return error_mark_node;
>> +    }
>> +
>> +  f = make_typename_type (ctx, f, typename_type, complain,
>> +              /*keep_type_decl=*/true, template_ok);
>> +  if (f == error_mark_node)
>> +    return f;
>> +  if (TREE_CODE (f) == TYPE_DECL)
>> +    {
>> +      complain |= tf_ignore_bad_quals;
>> +      f = TREE_TYPE (f);
>> +    }
>> +
>> +  if (TREE_CODE (f) != TYPENAME_TYPE)
>> +    {
>> +      if (TYPENAME_IS_ENUM_P (t) && TREE_CODE (f) != ENUMERAL_TYPE)
>> +    {
>> +      if (complain & tf_error)
>> +        error ("%qT resolves to %qT, which is not an enumeration type",
>> +           t, f);
>> +      else
>> +        return error_mark_node;
>> +    }
>> +      else if (TYPENAME_IS_CLASS_P (t) && !CLASS_TYPE_P (f))
>> +    {
>> +      if (complain & tf_error)
>> +        error ("%qT resolves to %qT, which is not a class type",
>> +           t, f);
>> +      else
>> +        return error_mark_node;
>> +    }
>> +    }
>> +
>> +  return cp_build_qualified_type
>> +    (f, cp_type_quals (f) | cp_type_quals (t), complain);
>> +}
>> +
>>   /* Map from a FUNCTION_DECL to a vec of default argument 
>> instantiations,
>>      indexed in reverse order of the parameters.  */
>> @@ -15193,10 +15281,13 @@ tsubst_decl (tree t, tree args, 
>> tsubst_flags_t complain)
>>           && VAR_HAD_UNKNOWN_BOUND (t)
>>           && type != error_mark_node)
>>             type = strip_array_domain (type);
>> -        tsubst_flags_t tcomplain = complain;
>> -        if (VAR_P (t))
>> -          tcomplain |= tf_tst_ok;
>> -        type = tsubst (type, args, tcomplain, in_decl);
>> +        if (VAR_P (t)
>> +        && TREE_CODE (type) == TYPENAME_TYPE
>> +        && !typedef_variant_p (type))
>> +          type = tsubst_typename_type (type, args, complain, in_decl,
>> +                       /*template_ok=*/true);
>> +        else
>> +          type = tsubst (type, args, complain, in_decl);
>>           /* Substituting the type might have recursively instantiated 
>> this
>>              same alias (c++/86171).  */
>>           if (gen_tmpl && DECL_ALIAS_TEMPLATE_P (gen_tmpl)
>> @@ -15889,9 +15980,6 @@ 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
>> @@ -16424,89 +16512,7 @@ tsubst (tree t, tree args, tsubst_flags_t 
>> complain, tree in_decl)
>>         }
>>       case TYPENAME_TYPE:
>> -      {
>> -    tree ctx = TYPE_CONTEXT (t);
>> -    if (TREE_CODE (ctx) == TYPE_PACK_EXPANSION)
>> -      {
>> -        ctx = tsubst_pack_expansion (ctx, args, complain, in_decl);
>> -        if (ctx == error_mark_node
>> -        || TREE_VEC_LENGTH (ctx) > 1)
>> -          return error_mark_node;
>> -        if (TREE_VEC_LENGTH (ctx) == 0)
>> -          {
>> -        if (complain & tf_error)
>> -          error ("%qD is instantiated for an empty pack",
>> -             TYPENAME_TYPE_FULLNAME (t));
>> -        return error_mark_node;
>> -          }
>> -        ctx = TREE_VEC_ELT (ctx, 0);
>> -      }
>> -    else
>> -      ctx = tsubst_aggr_type (ctx, args, complain, in_decl,
>> -                  /*entering_scope=*/1);
>> -    if (ctx == error_mark_node)
>> -      return error_mark_node;
>> -
>> -    tree f = tsubst_copy (TYPENAME_TYPE_FULLNAME (t), args,
>> -                  complain, in_decl);
>> -    if (f == error_mark_node)
>> -      return error_mark_node;
>> -
>> -    if (!MAYBE_CLASS_TYPE_P (ctx))
>> -      {
>> -        if (complain & tf_error)
>> -          error ("%qT is not a class, struct, or union type", ctx);
>> -        return error_mark_node;
>> -      }
>> -    else if (!uses_template_parms (ctx) && !TYPE_BEING_DEFINED (ctx))
>> -      {
>> -        /* Normally, make_typename_type does not require that the CTX
>> -           have complete type in order to allow things like:
>> -
>> -         template <class T> struct S { typename S<T>::X Y; };
>> -
>> -           But, such constructs have already been resolved by this
>> -           point, so here CTX really should have complete type, unless
>> -           it's a partial instantiation.  */
>> -        if (!complete_type_or_maybe_complain (ctx, NULL_TREE, complain))
>> -          return error_mark_node;
>> -      }
>> -
>> -    tsubst_flags_t tcomplain = complain | tf_keep_type_decl;
>> -    if (tst_ok)
>> -      tcomplain |= tf_tst_ok;
>> -    f = make_typename_type (ctx, f, typename_type, tcomplain);
>> -    if (f == error_mark_node)
>> -      return f;
>> -    if (TREE_CODE (f) == TYPE_DECL)
>> -      {
>> -        complain |= tf_ignore_bad_quals;
>> -        f = TREE_TYPE (f);
>> -      }
>> -
>> -    if (TREE_CODE (f) != TYPENAME_TYPE)
>> -      {
>> -        if (TYPENAME_IS_ENUM_P (t) && TREE_CODE (f) != ENUMERAL_TYPE)
>> -          {
>> -        if (complain & tf_error)
>> -          error ("%qT resolves to %qT, which is not an enumeration 
>> type",
>> -             t, f);
>> -        else
>> -          return error_mark_node;
>> -          }
>> -        else if (TYPENAME_IS_CLASS_P (t) && !CLASS_TYPE_P (f))
>> -          {
>> -        if (complain & tf_error)
>> -          error ("%qT resolves to %qT, which is not a class type",
>> -             t, f);
>> -        else
>> -          return error_mark_node;
>> -          }
>> -      }
>> -
>> -    return cp_build_qualified_type
>> -      (f, cp_type_quals (f) | cp_type_quals (t), complain);
>> -      }
>> +      return tsubst_typename_type (t, args, complain, in_decl);
>>       case UNBOUND_CLASS_TEMPLATE:
>>         {
>> @@ -17391,10 +17397,14 @@ tsubst_copy (tree t, tree args, 
>> tsubst_flags_t complain, tree in_decl)
>>       case IMPLICIT_CONV_EXPR:
>>       CASE_CONVERT:
>>         {
>> -    tsubst_flags_t tcomplain = complain;
>> -    if (code == CAST_EXPR)
>> -      tcomplain |= tf_tst_ok;
>> -    tree type = tsubst (TREE_TYPE (t), args, tcomplain, in_decl);
>> +    tree type = TREE_TYPE (t);
>> +    if (code == CAST_EXPR
>> +        && TREE_CODE (type) == TYPENAME_TYPE
>> +        && !typedef_variant_p (type))
>> +      type = tsubst_typename_type (type, args, complain, in_decl,
>> +                       /*template_ok=*/true);
>> +    else
>> +      type = tsubst (type, args, complain, in_decl);
>>       tree op0 = tsubst_copy (TREE_OPERAND (t, 0), args, complain, 
>> in_decl);
>>       return build1 (code, type, op0);
>>         }
>> @@ -20430,13 +20440,16 @@ tsubst_copy_and_build (tree t,
>>       case DYNAMIC_CAST_EXPR:
>>       case STATIC_CAST_EXPR:
>>         {
>> -    tree type;
>> +    tree type = TREE_TYPE (t);
>>       tree op, r = NULL_TREE;
>> -    tsubst_flags_t tcomplain = complain;
>> -    if (TREE_CODE (t) == CAST_EXPR)
>> -      tcomplain |= tf_tst_ok;
>> -    type = tsubst (TREE_TYPE (t), args, tcomplain, in_decl);
>> +    if (TREE_CODE (t) == CAST_EXPR
>> +        && TREE_CODE (type) == TYPENAME_TYPE
>> +        && !typedef_variant_p (type))
>> +      type = tsubst_typename_type (type, args, complain, in_decl,
>> +                       /*template_ok=*/true);
>> +    else
>> +      type = tsubst (type, args, complain, in_decl);
>>       op = RECUR (TREE_OPERAND (t, 0));
>> @@ -21421,10 +21434,14 @@ tsubst_copy_and_build (tree t,
>>           bool need_copy_p = false;
>>       tree r;
>> -    tsubst_flags_t tcomplain = complain;
>> -    if (COMPOUND_LITERAL_P (t))
>> -      tcomplain |= tf_tst_ok;
>> -    tree type = tsubst (TREE_TYPE (t), args, tcomplain, in_decl);
>> +    tree type = TREE_TYPE (t);
>> +    if (COMPOUND_LITERAL_P (t)
>> +        && TREE_CODE (type) == TYPENAME_TYPE
>> +        && !typedef_variant_p (type))
>> +      type = tsubst_typename_type (type, args, complain, in_decl,
>> +                       /*template_ok=*/true);
>> +    else
>> +      type = tsubst (type, args, complain, in_decl);
>>       if (type == error_mark_node)
>>         RETURN (error_mark_node);
>