[flat.map.defn,flat.multimap.defn] Deduction guide needs to default `Allocator` to be useful LWG3786

[JohelEGP]

Is this right? What does it mean that the previous template parameter Compare is defaulted, but Allocator isn't, even though both corresponding function parameters are defaulted? Seems like you can't get this to work without defaulting this Allocator: https://godbolt.org/z/fYesrsbMz.

           class Allocator = allocator<ranges::range_value_t<R>>>

           class Allocator = allocator<@\exposid{range-mapped-type}@<R>>>

I think both suggestions fulfill the corresponding Constraints element.

           class Allocator = allocator<void>>

Originally posted by @JohelEGP in #5626 (comment)

See also https://github.com/cplusplus/draft/pull/5626/files#r922820452.

[cpplearner]

The affected deduction guides are

draft/source/containers.tex

Lines 14934 to 14937 in bc6d64c

	template<ranges::@\libconcept{input_range}@ R, class Compare = less<@\exposid{range-key-type}@<R>>,
	class Allocator>
	flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
	-> flat_map<@\exposid{range-key-type}@<R>, @\exposid{range-mapped-type}@<R>, Compare>;

draft/source/containers.tex

Lines 16039 to 16042 in bc6d64c

	template<ranges::@\libconcept{input_range}@ R, class Compare = less<@\exposid{range-key-type}@<R>>,
	class Allocator>
	flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
	-> flat_multimap<@\exposid{range-key-type}@<R>, @\exposid{range-mapped-type}@<R>, Compare>;

In these deduction guides, there's no Allocator on the right of ->, which means the Allocator does not affect the result of deduction. So IIUC it doesn't matter what the default is, as long as there is a default.

[jensmaurer]

Something is strange here, but the fix doesn't seem to be editorially-obvious. Maybe we want to pass the Allocator parameter to the vector containers that are inside the flat_map?

@jwakely , this looks like a library issue to me.

[jwakely]

@jwakely , this looks like a library issue to me.

Yes, I agree. Please submit one, @JohelEGP

[JohelEGP]

See https://cplusplus.github.io/LWG/issue3786.

[jwakely]

Thanks!

[JohelEGP]

Status: WP.