[Bug c++/94124] [10 Regression] conversion from ‘<brace-enclosed initializer list>’ to ‘F’ is ambiguous since r10-6388-ge98ebda074bf8fc5

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94124

--- Comment #5 from Martin Liška <marxin at gcc dot gnu.org> ---
commit r10-7139-g4069adf4bbc90d16b603e0308b48499c36b2b637
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Mar 12 08:28:05 2020 +0100

    c++: Tweak reshape_init_array_1 [PR94124]

    Isn't it wasteful to first copy perhaps a large constructor (recursively)
    and then truncate it to very few elts (zero in this case)?

    > We should certainly avoid copying if they're the same.  The code above
for
    > only copying the bits that aren't going to be thrown away seems pretty
    > straightforward, might as well use it even if the savings aren't likely
to
    > be large.

    Calling vec_safe_truncate with the same number of elts the vector already
    has is a nop, so IMHO we just should make sure we only unshare if it
    changed.

    2020-03-12  Jakub Jelinek  <jakub@redhat.com>

            PR c++/94124
            * decl.c (reshape_init_array_1): Don't unshare constructor if there
            aren't any trailing zero elts, otherwise only unshare the first
            nelts.