On Tue, 23 Sep 2025, Nathaniel Shead wrote:
> Bootstrapped and regtested (so far just dg.exp and modules.exp) on
> x86_64-pc-linux-gnu, OK for trunk (and maybe 15) if full regtest passes?
>
> -- >8 --
>
> comp_template_parms_position uses whether a TEMPLATE_TYPE_PARM is a pack
> to determine equivalency. This in turn affects whether
> canonical_type_parameter finds a pre-existing auto type as equivalent.
>
> When generating the 'auto...' type for a lambda pack capture, we only
> mark it as a pack after generating the node (and calculating its
> canonical); this means that later when comparing a version streamed in
> from a module we think that two equivalent types have different
> TYPE_CANONICAL, because the latter already had
> TEMPLATE_PARM_PARAMETER_PACK set before calculating its canonical.
>
> This patch fixes this by using a new 'make_auto_pack' function to ensure
> that packness is set before the canonical is looked up.
>
> PR c++/122015
>
> gcc/cp/ChangeLog:
>
> * cp-tree.h (make_auto_pack): Declare.
> * lambda.cc (lambda_capture_field_type): Use make_auto_pack to
> ensure TYPE_CANONICAL is set correctly.
> * pt.cc (make_auto_pack): New function.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/modules/lambda-11.h: New test.
> * g++.dg/modules/lambda-11_a.H: New test.
> * g++.dg/modules/lambda-11_b.C: New test.
>
> Signed-off-by: Nathaniel Shead <[email protected]>
> ---
> gcc/cp/cp-tree.h | 1 +
> gcc/cp/lambda.cc | 25 +++++++++++++---------
> gcc/cp/pt.cc | 12 +++++++++++
> gcc/testsuite/g++.dg/modules/lambda-11.h | 6 ++++++
> gcc/testsuite/g++.dg/modules/lambda-11_a.H | 5 +++++
> gcc/testsuite/g++.dg/modules/lambda-11_b.C | 5 +++++
> 6 files changed, 44 insertions(+), 10 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/modules/lambda-11.h
> create mode 100644 gcc/testsuite/g++.dg/modules/lambda-11_a.H
> create mode 100644 gcc/testsuite/g++.dg/modules/lambda-11_b.C
>
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index d531bcd833b..54156cb0d23 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -7835,6 +7835,7 @@ extern tree make_constrained_auto (tree,
> tree);
> extern tree make_constrained_decltype_auto (tree, tree);
> extern tree make_template_placeholder (tree);
> extern tree make_cast_auto (void);
> +extern tree make_auto_pack (void);
> extern bool template_placeholder_p (tree);
> extern bool ctad_template_p (tree);
> extern bool unparenthesized_id_or_class_member_access_p (tree);
> diff --git a/gcc/cp/lambda.cc b/gcc/cp/lambda.cc
> index bb0aa7d0a02..575daaa9d71 100644
> --- a/gcc/cp/lambda.cc
> +++ b/gcc/cp/lambda.cc
> @@ -220,18 +220,23 @@ lambda_capture_field_type (tree expr, bool
> explicit_init_p,
>
> if (explicit_init_p)
> {
> - tree auto_node = make_auto ();
> -
> - type = auto_node;
> - if (by_reference_p)
> - /* Add the reference now, so deduction doesn't lose
> - outermost CV qualifiers of EXPR. */
> - type = build_reference_type (type);
> if (uses_parameter_packs (expr))
> - /* Stick with 'auto' even if the type could be deduced. */
> - TEMPLATE_TYPE_PARAMETER_PACK (auto_node) = true;
> + {
> + /* Stick with 'auto...' even if the type could be deduced. */
> + type = make_auto_pack ();
> + if (by_reference_p)
> + type = build_reference_type (type);
> + }
> else
> - type = do_auto_deduction (type, expr, auto_node);
> + {
> + tree auto_node = make_auto ();
> + type = auto_node;
> + if (by_reference_p)
> + /* Add the reference now, so deduction doesn't lose
> + outermost CV qualifiers of EXPR. */
> + type = build_reference_type (type);
> + type = do_auto_deduction (type, expr, auto_node);
> + }
> }
> else if (!type_deducible_expression_p (expr))
> {
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 494b6be7a24..06287f0b045 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -30493,6 +30493,18 @@ make_constrained_decltype_auto (tree con, tree args)
> return make_constrained_placeholder_type (type, con, args);
> }
>
> +/* Create an "auto..." type-specifier. */
> +
> +tree
> +make_auto_pack ()
> +{
> + tree type = make_auto_1 (auto_identifier, false);
> + TEMPLATE_TYPE_PARAMETER_PACK (type) = true;
> + /* Our canonical type depends on being a pack. */
> + TYPE_CANONICAL (type) = canonical_type_parameter (type);
> + return type;
> +}
> +
> /* Returns true if the placeholder type constraint T has any dependent
> (explicit) template arguments. */
>
> diff --git a/gcc/testsuite/g++.dg/modules/lambda-11.h
> b/gcc/testsuite/g++.dg/modules/lambda-11.h
> new file mode 100644
> index 00000000000..e4a07f22cf0
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/lambda-11.h
> @@ -0,0 +1,6 @@
> +// PR c++/122015
> +
> +template <typename> auto declval();
> +template <typename... _BindArgs> void bind_front() {
> + [... __bound_args(_BindArgs{})] {};
> +}
> diff --git a/gcc/testsuite/g++.dg/modules/lambda-11_a.H
> b/gcc/testsuite/g++.dg/modules/lambda-11_a.H
> new file mode 100644
> index 00000000000..371157bee54
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/lambda-11_a.H
> @@ -0,0 +1,5 @@
> +// PR c++/122015
> +// { dg-additional-options "-fmodule-header -std=c++20" }
'dg-do compile { target c++20 }' instead -std=c++20?
> +// { dg-module-cmi {} }
> +
> +#include "lambda-11.h"
> diff --git a/gcc/testsuite/g++.dg/modules/lambda-11_b.C
> b/gcc/testsuite/g++.dg/modules/lambda-11_b.C
> new file mode 100644
> index 00000000000..490dfcfa3cc
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/lambda-11_b.C
> @@ -0,0 +1,5 @@
> +// PR c++/122015
> +// { dg-additional-options "-fmodules -std=c++20" }
I think this needs -fno-module-lazy? The test doesn't ICE for me
without that.
Looks OK to me with these changes!
> +
> +#include "lambda-11.h"
> +import "lambda-11_a.H";
> --
> 2.51.0
>
>
