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[flat.map.defn,flat.multimap.defn] Deduction guide needs to default Allocator to be useful LWG3786 #5800

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JohelEGP opened this issue Aug 29, 2022 · 6 comments · Fixed by #6116
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[flat.map.defn,flat.multimap.defn] Deduction guide needs to default Allocator to be useful LWG3786 #5800

JohelEGP opened this issue Aug 29, 2022 · 6 comments · Fixed by #6116
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lwg Issue must be reviewed by LWG. not-editorial Issue is not deemed editorial; the editorial issue is kept open for tracking.

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@JohelEGP
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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
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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 jensmaurer added the lwg Issue must be reviewed by LWG. label Aug 30, 2022
@jensmaurer
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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.

@JohelEGP JohelEGP mentioned this issue Sep 7, 2022
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@jwakely
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jwakely commented Sep 24, 2022

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

Yes, I agree. Please submit one, @JohelEGP

@jensmaurer jensmaurer added the not-editorial Issue is not deemed editorial; the editorial issue is kept open for tracking. label Sep 24, 2022
@JohelEGP JohelEGP changed the title [flat.map.defn,flat.multimap.defn] Deduction guide needs to default Allocator to be useful [flat.map.defn,flat.multimap.defn] Deduction guide needs to default Allocator to be useful LWG3786 Sep 25, 2022
@JohelEGP
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See https://cplusplus.github.io/LWG/issue3786.

@jwakely
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jwakely commented Sep 25, 2022

Thanks!

@JohelEGP JohelEGP mentioned this issue Sep 25, 2022
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@JohelEGP
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Status: WP.

@jensmaurer jensmaurer mentioned this issue Mar 14, 2023
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[LWG 3] P2790R0 Immediate issues except 3441 cplusplus/draft
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@jwakely @cpplearner @JohelEGP @jensmaurer