[icu-design] ICU4C MessageFormat 2.0: Function composition

[Tim Chevalier]

Dear ICU team & users,

I would like to propose the following changes to the MF2 API in the
MessageFormat 2.0 tech preview API in ICU 77.

Some people may remember the discussion of function composition in MF2
and the lack of definition of the concept of a "resolved value" in the
spec. Recently, the spec has changed to define this concept much more
precisely and provide clearer guidance for how to implement function
composition.

I've created a design doc at
https://docs.google.com/document/d/1nIYDyaTqB6nChhvoSVxBkRfBAiPchlN4anvaAma9WRc/edit


There is a fair amount of context needed from both the MF2 spec and
implementation in order to follow this proposal, so please don't
hesitate to comment on the doc with questions and I'll try to clarify
things as much as I can.

I would appreciate feedback before Thursday, October 17.

Thanks,

Tim