On Wed, Feb 25, 2026 at 11:24:50PM +1100, Nathaniel Shead wrote:
> Bootstrapped and regtested on x86_64-pc-linux-gnu. This isn't really a
> bugfix but I think it's a simple improvement, OK for trunk?
I think since it only affects code with REFLECTION_TYPE_P types, it's
OK for GCC 16 too.
> Or otherwise for GCC 17?
>
> -- >8 --
>
> This adds another case to static_assert to print a helpful diagnostic
> when comparing reflections. This makes it easier to see what's gone
> wrong when doing 'static_assert(some_query(^^a) == some_query(^^b));'
> by decomposing the resulting resulting reflection.
Nice!
> There's no need to do this decomposition in cases where both operands
> are already in their lowest form however, e.g. for cases like
> static_assert(^^int == ^^double).
>
> gcc/cp/ChangeLog:
>
> * constexpr.cc (diagnose_failing_condition): Also decompose
> comparisons of reflections.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/reflect/diag5.C: New test.
Patch looks good (but please wait for Jason's OK). I'm noticing that for
struct S {
void fn();
};
static_assert(&[:^^S::fn:] != &S::fn);
we say "the comparison reduces to '(1 == 0)'" but that has nothing
to do with this patch.
> Signed-off-by: Nathaniel Shead <[email protected]>
> ---
> gcc/cp/constexpr.cc | 10 +++++++---
> gcc/testsuite/g++.dg/reflect/diag5.C | 9 +++++++++
> 2 files changed, 16 insertions(+), 3 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/reflect/diag5.C
>
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 44e6b352d9a..4169f571285 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -3107,12 +3107,16 @@ diagnose_failing_condition (tree bad, location_t
> cloc, bool show_expr_p,
> else if (maybe_diagnose_standard_trait (cloc, bad))
> ;
> else if (COMPARISON_CLASS_P (bad)
> - && ARITHMETIC_TYPE_P (TREE_TYPE (TREE_OPERAND (bad, 0))))
> + && (ARITHMETIC_TYPE_P (TREE_TYPE (TREE_OPERAND (bad, 0)))
> + || REFLECTION_TYPE_P (TREE_TYPE (TREE_OPERAND (bad, 0)))))
> {
> tree op0 = fold_operand (TREE_OPERAND (bad, 0), ctx);
> tree op1 = fold_operand (TREE_OPERAND (bad, 1), ctx);
> - tree cond = build2 (TREE_CODE (bad), boolean_type_node, op0, op1);
> - inform (cloc, "the comparison reduces to %qE", cond);
> + if (op0 != TREE_OPERAND (bad, 0) || op1 != TREE_OPERAND (bad, 1))
> + {
> + tree cond = build2 (TREE_CODE (bad), boolean_type_node, op0, op1);
> + inform (cloc, "the comparison reduces to %qE", cond);
> + }
> }
> else if (show_expr_p)
> inform (cloc, "%qE evaluates to false", bad);
> diff --git a/gcc/testsuite/g++.dg/reflect/diag5.C
> b/gcc/testsuite/g++.dg/reflect/diag5.C
> new file mode 100644
> index 00000000000..44af952415d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/reflect/diag5.C
> @@ -0,0 +1,9 @@
> +// { dg-do compile { target c++26 } }
> +// { dg-additional-options "-freflection" }
> +// Test that we decompose reflections in static_asserts, where applicable.
> +
> +constexpr auto a = ^^int;
> +consteval auto b() { return ^^double; }
> +
> +static_assert(a == b()); // { dg-error "static assertion failed" }
> +// { dg-message "the comparison reduces to '(^^int == ^^double)'" "" {
> target *-*-* } .-1 }
> --
> 2.51.0
>
Marek
