P1684: Avoid users needing to specify required span size twice

[mhoemmen]

This comes from 1684R2 LEWG review on 2022/04/19.

If users want to use std::array as mdarray's Container, they currently need to specify the size as part of the Container's type. If the size is too small, it's UB. For common use cases (layout_{left,right} with all compile-time extents), mdarray can compute the correct minimum size (via required_span_size()) at compile time. Users shouldn't have to repeat themselves (Don't Repeat Yourself principle).

One suggested fix was to provide a template type alias that deduces the correct std::array type from ElementType and SizeTypes..., at least for common layouts (perhaps layout_{left,right} only). A more comprehensive fix would make the default value of Container a function of all the previous template parameters. There would be no need to make that policy customizable, as users could always explicitly specify the Container type (and write their own policy to deduce it).

[crtrott]

Why is it easier to write: array<T,static_extent_function(N0,N1,N2,N3)> then array<T,N0*N1*N2*N3> ? in particular since this would only work for layout_right, layout_left.