CWG Poll 2b: P3921R0 issue 1670

[eisenwave]

Fixes #8456.
Fixes FI-4 (C++14 CD).
Also fixes cplusplus/papers#2409

Note that this PR also fixes an example in [class.conv.fct] that becomes clearly incorrect after CWG1670 is applied.

[eisenwave]

You know what, I'm just going to submit an LWG issue for this.

[Dani-Hub]

We have now LWG 4468 for this.

[tkoeppe]

Thanks, @Dani-Hub!

[t3nsor]

It seems to me that the entire paragraph "A conversion function template shall not have a deduced return type" should go away.

Technically, that might (?) actually be a normative change in that the current rule makes the template operator auto clearly ill-formed whereas if that rule is struck then it might be IFNDR under [temp.res.general], but there seems to be little value in having this special case.

[eisenwave]

It seems to me that the entire paragraph "A conversion function template shall not have a deduced return type" should go away.

I wouldn't be opposed, but that seems more like CWG issue material than a drive-by fix in this PR which applies the motions.

[tkoeppe]

Is "Fixes FI-4 (C++14 CD)." correct?

[eisenwave]

CWG1670 links to "N3690 comment FI 4".

I'm not 100% sure about the hyphenation rules here; perhaps it should be "FI 4".

[tkoeppe]

Right, yes, I was thinking the same. Indeed, it fixes a long-standing issue, though I guess not relevant to the C++14 CD record of response...