STL: "Effects: Equivalent to" implies Constraints

[StephanTLavavej]

The Standard frequently uses "Effects: Equivalent to" when specifying things. This is awesome because it provides an implementation that can be used in production code, nearly unchanged, 99% of the time. (Occasionally, a more efficient implementation is possible.)

For example, shared_ptr's assignment from unique_ptr is specified as:

Effects: Equivalent to shared_­ptr(std​::​move(r)).swap(*this).

However, shared_ptr's constructor from unique_ptr is constrained (currently with old-style wording):

Remarks: This constructor shall not participate in overload resolution unless Y* is compatible with T* and unique_­ptr<Y, D>​::​pointer is convertible to element_­type*.

Finally, the Standard's Library-wide wording says:

Whenever the Effects element specifies that the semantics of some function F are Equivalent to some code sequence, then the various elements are interpreted as follows. If F's semantics specifies any Constraints or Mandates elements, then those requirements are logically imposed prior to the equivalent-to semantics.

We've never implemented that. For example, we constrain the constructor:

STL/stl/inc/memory

Lines 1109 to 1113 in 6b0238d

	template <class _Ux, class _Dx,
	enable_if_t<conjunction_v<_SP_pointer_compatible<_Ux, _Ty>,
	is_convertible<typename unique_ptr<_Ux, _Dx>::pointer, element_type*>>,
	int> = 0>
	shared_ptr(unique_ptr<_Ux, _Dx>&& _Other) {

But we don't constrain the assignment operator:

STL/stl/inc/memory

Lines 1162 to 1166 in 6b0238d

	template <class _Ux, class _Dx>
	shared_ptr& operator=(unique_ptr<_Ux, _Dx>&& _Right) { // move from unique_ptr
	shared_ptr(_STD move(_Right)).swap(*this);
	return *this;
	}

This is a bug because the difference is detectable through is_assignable.

The extent of this problem is unknown. I don't know of a better way to thoroughly fix this issue than going through every occurrence of "Effects: Equivalent to" in the Standard and looking for expressions that involve constrained machinery.