On Tue, 29 Jul 2025, Nathaniel Shead wrote:
> Tested on x86_64-pc-linux-gnu, OK for trunk if full bootstrap+regtest
> passes?
>
> -- >8 --
>
> For the sake of determining if there are other errors in user code to
> report early, many trait functions don't return error_mark_node if not
> called in a SFINAE context (i.e., tf_error is set). This patch removes
> some assumptions on this behaviour I'd made when improving diagnostics
> of builtin traits.
>
> PR c++/121291
>
> gcc/cp/ChangeLog:
>
> * constraint.cc (diagnose_trait_expr): Remove assumption about
> failures returning error_mark_node.
> * method.cc (build_invoke): Adjust comment.
> (is_nothrow_convertible): Check for errorcount changes.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/ext/is_invocable7.C: New test.
> * g++.dg/ext/is_nothrow_convertible5.C: New test.
>
> Signed-off-by: Nathaniel Shead <nathanielosh...@gmail.com>
> ---
> gcc/cp/constraint.cc | 9 +++++---
> gcc/cp/method.cc | 9 ++++++--
> gcc/testsuite/g++.dg/ext/is_invocable7.C | 21 +++++++++++++++++++
> .../g++.dg/ext/is_nothrow_convertible5.C | 15 +++++++++++++
> 4 files changed, 49 insertions(+), 5 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/ext/is_invocable7.C
> create mode 100644 gcc/testsuite/g++.dg/ext/is_nothrow_convertible5.C
>
> diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> index d4a83e429e5..94238c9e8f1 100644
> --- a/gcc/cp/constraint.cc
> +++ b/gcc/cp/constraint.cc
> @@ -3176,8 +3176,7 @@ diagnose_trait_expr (location_t loc, tree expr, tree
> args)
> inform (loc, "%qT is not invocable, because", t1);
> else
> inform (loc, "%qT is not invocable by %qT, because", t1, t2);
> - tree call = build_invoke (t1, t2, tf_error);
> - gcc_assert (call == error_mark_node);
> + build_invoke (t1, t2, tf_error);
> }
> break;
> case CPTK_IS_LAYOUT_COMPATIBLE:
> @@ -3221,8 +3220,12 @@ diagnose_trait_expr (location_t loc, tree expr, tree
> args)
> inform (loc, "%qT is not nothrow invocable, because", t1);
> else
> inform (loc, "%qT is not nothrow invocable by %qT, because", t1, t2);
> + /* We can't rely on build_invoke returning error_mark_node, because
> + when passing tf_error in some cases a valid call may be returned
> + anyway. */
> + int errs = errorcount + sorrycount;
> tree call = build_invoke (t1, t2, tf_error);
> - if (call != error_mark_node)
> + if (call != error_mark_node && errs == errorcount + sorrycount)
> explain_not_noexcept (call);
What do you think about just checking expr_noexcept_p here instead (or
removing the assert in explain_not_noexcept)? So that if a call is not
nothrow invocable because the selected candidate is private _and_
because it's not noexcept, we'd report both errors. I see that
tsubst_compound_requirement already behaves this way.
> }
> break;
> diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
> index 62f8d80c781..cb2ea8db18d 100644
> --- a/gcc/cp/method.cc
> +++ b/gcc/cp/method.cc
> @@ -1952,7 +1952,8 @@ build_trait_object (tree type, tsubst_flags_t complain)
> }
>
> /* [func.require] Build an expression of INVOKE(FN_TYPE, ARG_TYPES...). If
> the
> - given is not invocable, returns error_mark_node. */
> + given is not invocable, returns error_mark_node, unless COMPLAIN includes
> + tf_error. */
>
> tree
> build_invoke (tree fn_type, const_tree arg_types, tsubst_flags_t complain)
> @@ -2598,11 +2599,15 @@ is_convertible (tree from, tree to, bool
> explain/*=false*/)
> bool
> is_nothrow_convertible (tree from, tree to, bool explain/*=false*/)
> {
> + /* as with is_nothrow_xible. */
> + int errs = errorcount + sorrycount;
> +
> tree expr = is_convertible_helper (from, to, explain);
> if (expr == NULL_TREE || expr == error_mark_node)
> return false;
> +
> bool is_noexcept = expr_noexcept_p (expr, tf_none);
> - if (explain)
> + if (explain && errs == (errorcount + sorrycount))
> {
> gcc_assert (!is_noexcept);
> explain_not_noexcept (expr);
> diff --git a/gcc/testsuite/g++.dg/ext/is_invocable7.C
> b/gcc/testsuite/g++.dg/ext/is_invocable7.C
> new file mode 100644
> index 00000000000..5c852fc8428
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/ext/is_invocable7.C
> @@ -0,0 +1,21 @@
> +// PR c++/121291
> +// { dg-do compile { target c++17 } }
> +
> +template <typename T>
> +constexpr bool is_invocable = __is_invocable(T);
> +
> +template <typename T>
> +constexpr bool is_nothrow_invocable = __is_nothrow_invocable(T);
> +
> +struct S {
> +private:
> + int operator()() noexcept; // { dg-message "here" }
> +};
> +
> +static_assert(is_invocable<S>); // { dg-error "assert" }
> +// { dg-message "not invocable" "" { target *-*-* } .-1 }
> +// { dg-error "private within this context" "" { target *-*-* } .-2 }
> +
> +static_assert(is_nothrow_invocable<S>); // { dg-error "assert" }
> +// { dg-message "not nothrow invocable" "" { target *-*-* } .-1 }
> +// { dg-error "private within this context" "" { target *-*-* } .-2 }
> diff --git a/gcc/testsuite/g++.dg/ext/is_nothrow_convertible5.C
> b/gcc/testsuite/g++.dg/ext/is_nothrow_convertible5.C
> new file mode 100644
> index 00000000000..0ce8fb82191
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/ext/is_nothrow_convertible5.C
> @@ -0,0 +1,15 @@
> +// PR c++/121291
> +// { dg-do compile { target c++17 } }
> +
> +template <typename T, typename U>
> +constexpr bool is_nothrow_convertible = __is_nothrow_convertible(T, U);
> +
> +struct A {};
> +struct B {
> +private:
> + operator A() noexcept; // { dg-message "here" }
> +};
> +
> +static_assert(is_nothrow_convertible<B, A>); // { dg-error "assert" }
> +// { dg-message "not nothrow convertible" "" { target *-*-* } .-1 }
> +// { dg-error "private within this context" "" { target *-*-* } .-2 }
> --
> 2.47.0
>
>
