On Thu, 5 Sep 2024, Patrick Palka wrote:
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
> for trunk/14?
>
> -- >8 --
>
> A lambda within a default template argument used in some template-id
> may have a smaller template depth than the context of the template-id.
> For example, the lambda in v1's default template argument has template
> depth 1, and in v2's has template depth 2, but the template-ids v1<0>
> and v2<0> which uses these default arguments appear in a depth 3 template
> context. So add_extra_args will ultimately return args with depth 3 --
> too many args for the lambda, leading to a bogus substitution.
>
> This patch fixes this by trimming the result of add_extra_args to match
> the template depth of the lambda. A new LAMBDA_EXPR_TEMPLATE_DEPTH field
> is added that tracks the template-ness of a lambda;
Oops, disregard this last sentence, it's a vestige from an earlier
version of this patch before it occurred to me we can freely obtain the
template depth of a lambda through its operator()'s TEMPLATE_DECL.
>
> PR c++/116567
>
> gcc/cp/ChangeLog:
>
> * pt.cc (tsubst_lambda_expr): For a deferred-substitution lambda,
> trim the augmented template arguments to match the template depth
> of the lambda.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp2a/lambda-targ7.C: New test.
> ---
> gcc/cp/pt.cc | 11 +++++++++
> gcc/testsuite/g++.dg/cpp2a/lambda-targ7.C | 30 +++++++++++++++++++++++
> 2 files changed, 41 insertions(+)
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-targ7.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 747e627f547..c49a26b4f5e 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -19699,6 +19699,17 @@ tsubst_lambda_expr (tree t, tree args,
> tsubst_flags_t complain, tree in_decl)
> LAMBDA_EXPR_EXTRA_ARGS (t) = build_extra_args (t, args, complain);
> return t;
> }
> + if (LAMBDA_EXPR_EXTRA_ARGS (t))
> + {
> + /* If we deferred substitution into this lambda, then it's probably
> from
> + a context (e.g. default template argument context) which may have fewer
> + levels than the current context it's embedded in. Adjust the result of
> + add_extra_args accordingly. */
> + tree ctx_parms = DECL_TEMPLATE_PARMS (DECL_TI_TEMPLATE (oldfn));
> + if (generic_lambda_fn_p (oldfn))
> + ctx_parms = TREE_CHAIN (ctx_parms);
> + args = get_innermost_template_args (args, TMPL_PARMS_DEPTH
> (ctx_parms));
> + }
>
> tree r = build_lambda_expr ();
>
> diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-targ7.C
> b/gcc/testsuite/g++.dg/cpp2a/lambda-targ7.C
> new file mode 100644
> index 00000000000..c5c0525908e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-targ7.C
> @@ -0,0 +1,30 @@
> +// PR c++/116567
> +// { dg-do compile { target c++20 } }
> +
> +template<auto N, auto F = []{}>
> +bool v1 = true;
> +
> +template<auto N, auto F = [](auto){}>
> +bool v1g = true;
> +
> +template<class T>
> +struct A {
> + template<auto N, auto F = []{}>
> + static inline bool v2 = true;
> +
> + template<auto N, auto F = [](auto){}>
> + static inline bool v2g = true;
> +
> + template<class U>
> + struct B {
> + template<class V>
> + static void f() {
> + v1<0> && v1g<0>;
> + v2<0> && v2g<0>;
> + }
> + };
> +};
> +
> +auto main() -> int {
> + A<int>::B<int>::f<int>();
> +}
> --
> 2.46.0.519.g2e7b89e038
>
>
