Improper resolution of constraint requirements when using CRTP

[llvmbot]

Bugzilla Link	48074
Version	11.0
OS	Windows NT
Reporter	LLVM Bugzilla Contributor
CC	@ldalessa,@zygoloid,@jwakely

Extended Description

Using libstdc++'s header with clang 11 doesn't compile even the most basic examples:

#include
auto w = std::views::iota(0);

The failure reduces to the following:

template T declval();

template using begin_t = decltype(declval<T&>().begin());
template concept C = true;

template
struct simple_view_interface
{
auto also_begin() requires
#ifdef TYPE_TRAIT
C<begin_t>
#else
requires (D d) { {d.begin()} -> C; }
#endif
{
return static_cast<D&>(*this).begin();
}
};

struct wat_view : simple_view_interface<wat_view>
{
auto begin() -> int* { return nullptr; }
};

auto w = wat_view().also_begin();

Both spellings of the requirement on also_begin() should be equivalent, and should be valid in this case. The checking of the requirements should be deferred until instantiation, at which point, wat_view is complete.

If TYPE_TRAIT is defined, the requirement is eagerly instantiated, an even the definition of wat_view fails to compile:

:3:62: error: no member named 'begin' in 'wat_view' template using begin_t = decltype(declval().begin()); ~~~~~~~~~~~~~ ^ :11:11: note: in instantiation of template type alias 'begin_t' requested here C> ^ :20:19: note: in instantiation of template class 'simple_view_interface' requested here struct wat_view : simple_view_interface ^

If TYPE_TRAIT is not defined, wat_view is properly defined, but usage fails:

:25:21: error: invalid reference to function 'also_begin': constraints not satisfied auto w = wat_view().also_begin(); ^ :13:29: note: because 'd.begin()' would be invalid: no member named 'begin' in 'wat_view' requires (D d) { {d.begin()} -> C; } ^

Even though there is a member named 'begin' in 'wat_view'.

The program should be valid either way. gcc accepts both.

[royjacobson]

Confirmed to work after https://reviews.llvm.org/D126907, so duplicate of #44178. Closing.

[llvmbot]

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