diag(::Diagonal) now returns a copy of the underlying diagonal vector

[gasagna]

JuliaLang/julia#24324 introduced a breaking change whereby diag(::Diagonal) returns a copy of the underlying vector, rather than the vector itself as it used to be in 0.6 (@fredrikekre ). For code that relies on diag(::Diagonal) as an accessor function, this now produces large memory allocations.

This was a somehow breaking change but no warnings are emitted. Perhaps, the NEWS.md should be updated to reflect the new behaviour and probably the docs of diag(::Diagonal) too. What is the recommended way of accessing the underlying field?

[fredrikekre]

See JuliaLang/julia#24324 for the rationale of the change.

For code that relies on diag(::Diagonal) as an accessor function, this now produces large memory allocations.

Use D.diag?

[oscardssmith]

does that work in generic code?

[fredrikekre]

No, there you should use diag(A).

[oscardssmith]

so then the issue is that previously, diag(A) would convert if necessary, but would not copy of it didn't need to, while now it copies no matter what making generic efficient code harder.

[fredrikekre]

No? Now you can use x = diag(A) and freely change x without modifying A, making generic code easier. This was one of the motivations for the change.

[gasagna]

IIUC, the rationale of JuliaLang/julia#24324 was that to have diag(A::Diagonal) return a copy vector, like when A is a dense matrix. While I understand why this would useful in generic code, this remains a breaking change that should be documented, at least in NEWS.md. Possibly a deprecation warning should be emitted.