[utility.arg.requirements]/2 is useless and technically incorrect

[cpplearner]

[utility.arg.requirements]/2:

In general, a default constructor is not required. Certain container class member function signatures specify T() as a default argument. T() shall be a well-defined expression ([dcl.init]) if one of those signatures is called using the default argument.

This is restating the obvious, and does so incorrectly: the signature (as defined in [intro.defs]) does not include the default arguments, and, assuming that T means the container's element type, containers no longer have T() as a default argument.

[jwakely]

And even the container members which insert a default-constructed element actually do so via allocator_traits::construct so they require Cpp17DefaultInsertable not Cpp17DefaultConstructible.

[jwakely]

Although container member functions do often have const Alloc& a = Alloc() and Cpp17DefaultConstructible is required there.

A type must be Cpp17DefaultConstructible to meet the Cpp17NullablePointer requirements (which applies to allocator pointers). unique_ptr deleters might need to be default constructed, and comparison functions and hashers and equality predicates in containers, and forward iterators, ...

But all those places explicitly say that Cpp17DefaultConstructible is required. I don't think this paragraph adds any value whatsoever. It's true that in many places, a default constructor is not required, but sometimes it is. In those places, we say so. But that's the case for all requirements. We don't require copyability in general, but we say when do require it.

So I think we could strike the whole paragraph and nothing of value would be lost.

[jensmaurer]

Good riddance, then. See #8987