Discovered while dealing with #4389.

The use of _Fake_copy_init here (determining whether the implicit coversion is non-throwing) looks superfluous.

In this place:

• The type of _Left._Outer and _Right._Outer models forward_iterator (definition can be found here), and thus decltype(_Left._Outer == _Right._Outer) should already model boolean-testable, and
• _Left._Inner and _Right._Inner are _Defaultaboxes (definition can be found here) for which operator== always returns bool ([1], [2]).

As indicated by the use of boolean-testable, the && operator in _Left._Outer == _Right._Outer && _Left._Inner == _Right._Inner should always has the built-in meaning and thus the result should always be a bool prvalue. Otherwise, the program is already ill-formed, possibly no diagnostic required.

So, I think we can directly use noexcept(_Left._Outer == _Right._Outer && _Left._Inner == _Right._Inner) instead, which possibly slightly improves compiler throughput.

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