Bug 23844 – chain(only) doesn't support immutable structs

Consider this code: ``` void main() { import std.array : array; import std.range : chain, only; struct S { immutable int value; } auto range = chain(only(S(5)), only(S(6))); assert(range.array == [S(5), S(6)]); } ``` It errors with a flood of ``` std/range/package.d(1172): Error: cannot modify struct instance `result.__source_field_0` of type `OnlyResult!(S)` because it contains `const` or `immutable` members ``` If I remove `immutable`, it works.

@FeepingCreature updated dlang/phobos pull request #8736 "Fix issue 23844: Support ranges with immutable fields (like `only` with `immutable struct`) in `chain`." fixing this issue: - Fix issue 23844: Support ranges with immutable fields (like `only` with `immutable struct`) in `chain`. `only` is a range that may be *mutable*, but not *assignable*. `chain` falls over here because it assumes it can make a struct with ranges, and reassign them with new values, which isn't necessarily the case even if the ranges are not `const`. Solved by creating a separate tuple of `Rebindable` ranges for this case. https://github.com/dlang/phobos/pull/8736

dlang/phobos pull request #8736 "Fix issue 23844: Support ranges with immutable fields (like `only` with `immutable struct`) in `chain`." was merged into master: - b9c6e3ca4b6fec799964394ad3e080d7b347f658 by Mathis Beer: Fix issue 23844: Support ranges with immutable fields (like `only` with `immutable struct`) in `chain`. `only` is a range that may be *mutable*, but not *assignable*. `chain` falls over here because it assumes it can make a struct with ranges, and reassign them with new values, which isn't necessarily the case even if the ranges are not `const`. Solved by creating a separate tuple of `Rebindable` ranges for this case. https://github.com/dlang/phobos/pull/8736