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c++: lambda in pack expansion using pack in constraint [PR103706]

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Here when expanding the pack expansion pattern containing a constrained
lambda, the template argument for each Ts is an ARGUMENT_PACK_SELECT,
which we store inside the lambda's LAMBDA_EXPR_REGEN_INFO.  Then during
satisfaction of the lambda's constraint C<Ts> the satisfaction cache
uses this argument as part of the key to the corresponding sat_entry, but
iterative_hash_template_arg and template_args_equal deliberately don't
handle ARGUMENT_PACK_SELECT.

Since it's wrong to preserve ARGUMENT_PACK_SELECT inside a hash table
due to its instability (as documented in iterative_hash_template_arg),
this patch helps make sure the satisfaction cache doesn't see such trees
by resolving ARGUMENT_PACK_SELECT arguments before adding them to
LAMBDA_EXPR_REGEN_INFO.

	PR c++/103706

gcc/cp/ChangeLog:

	* pt.cc (preserve_args): New function.
	(tsubst_lambda_expr): Use it when setting LAMBDA_EXPR_REGEN_INFO.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-lambda19.C: New test.
This commit is contained in:
Patrick Palka 2022-02-08 08:46:32 -05:00
parent 34ba3d9a2b
commit db5f1c1703
2 changed files with 49 additions and 2 deletions
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40
gcc/cp/pt.cc
View file

@ -3743,6 +3743,41 @@ argument_pack_select_arg (tree t)
return arg;
}
/* Return a modification of ARGS that's suitable for preserving inside a hash
table. In particular, this replaces each ARGUMENT_PACK_SELECT with its
underlying argument. ARGS is copied (upon modification) iff COW_P. */
static tree
preserve_args (tree args, bool cow_p = true)
{
if (!args)
return NULL_TREE;
for (int i = 0, len = TREE_VEC_LENGTH (args); i < len; ++i)
{
tree t = TREE_VEC_ELT (args, i);
tree r;
if (!t)
r = NULL_TREE;
else if (TREE_CODE (t) == ARGUMENT_PACK_SELECT)
r = argument_pack_select_arg (t);
else if (TREE_CODE (t) == TREE_VEC)
r = preserve_args (t, cow_p);
else
r = t;
if (r != t)
{
if (cow_p)
{
args = copy_template_args (args);
cow_p = false;
}
TREE_VEC_ELT (args, i) = r;
}
}
return args;
}
/* True iff FN is a function representing a built-in variadic parameter
pack. */
@ -19511,10 +19546,11 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
LAMBDA_EXPR_MUTABLE_P (r) = LAMBDA_EXPR_MUTABLE_P (t);
if (tree ti = LAMBDA_EXPR_REGEN_INFO (t))
LAMBDA_EXPR_REGEN_INFO (r)
= build_template_info (t, add_to_template_args (TI_ARGS (ti), args));
= build_template_info (t, add_to_template_args (TI_ARGS (ti),
preserve_args (args)));
else
LAMBDA_EXPR_REGEN_INFO (r)
= build_template_info (t, args);
= build_template_info (t, preserve_args (args));
gcc_assert (LAMBDA_EXPR_THIS_CAPTURE (t) == NULL_TREE
&& LAMBDA_EXPR_PENDING_PROXIES (t) == NULL);

11
gcc/testsuite/g++.dg/cpp2a/concepts-lambda19.C Normal file
View file

@ -0,0 +1,11 @@
// PR c++/103706
// { dg-do compile { target c++20 } }
template<class T> concept C = __is_same(T, int);
template<class... Ts> void f() {
([]() requires C<Ts> { return Ts(); }(), ...); // { dg-error "no match" }
}
template void f<int, int, int>(); // { dg-bogus "" }
template void f<int, int, char>(); // { dg-message "required from here" }