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Deduction failures in pointer interconvertibility trait tests #5730

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Merged
Stephan T. Lavavej (StephanTLavavej) merged 1 commit into from
Sep 22, 2025
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Deduction failures in pointer interconvertibility trait tests #5730

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5 changes: 0 additions & 5 deletions tests/std/tests/P0466R5_layout_compatibility_and_pointer_interconvertibility_traits/test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -176,8 +176,6 @@ constexpr bool test() {
}
};

struct NS : S1, S2 {}; // Non-standard layout

ASSERT(is_corresponding_member(&S1::v1, &S::v1));
ASSERT(is_corresponding_member(&S1::v2, &S::v2));
ASSERT(is_corresponding_member(&S1::v1, &S1::v1));
Expand All @@ -199,7 +197,6 @@ constexpr bool test() {
ASSERT(!is_corresponding_member(&S5::v1, &S6::v2));
ASSERT(!is_corresponding_member(&S5::v2, &S6::v1));
ASSERT(!is_corresponding_member(&S5::v3, &S6::v3));
ASSERT(!is_corresponding_member<NS, NS>(&NS::v1, &NS::w1));

@cpplearner S. B. Tam (cpplearner) Sep 18, 2025

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This assertion is directly translated from the example in the standard's [meta.member]. We might need to tell the committee that this example is problematic.

@joemmett Jonathan Emmett (joemmett) Sep 18, 2025

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The example does note that it fails, but not why it fails:

static_assert( is_corresponding_member<C, C>( &C::a, &C::b ) );
  // Forces the use of class C, and fails.

It's reasonable to assume that this would fail the static_assert, but it actually fails deduction.

@frederick-vs-ja A. Jiang (frederick-vs-ja) Sep 19, 2025

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I guess it's better to use is_corresponding_member<C, C, int, int>( &C::a, &C::b ) to achieve implicit conversion. Correspondingly, it's perhaps better to modify the line in the test file to ASSERT(!is_corresponding_member<NS, NS, int, int>(&NS::v1, &NS::w1));.

ASSERT(!is_corresponding_member(&S7::f1, &S7::f1));
ASSERT(!is_corresponding_member(static_cast<int S1::*>(nullptr), static_cast<int S2::*>(nullptr)));
ASSERT(!is_corresponding_member(&S1::v1, static_cast<int S2::*>(nullptr)));
Expand Down Expand Up @@ -233,8 +230,6 @@ constexpr bool test() {
ASSERT(is_pointer_interconvertible_with_class(&U::v1));
ASSERT(is_pointer_interconvertible_with_class(&U::v2));

ASSERT(!is_pointer_interconvertible_with_class<NS>(&NS::a));
ASSERT(!is_pointer_interconvertible_with_class<NS>(&NS::b));
ASSERT(!is_pointer_interconvertible_with_class(&C::f1));
ASSERT(!is_pointer_interconvertible_with_class(static_cast<int A::*>(nullptr)));
}
Expand Down