inference: (slightly) improve type stability of capturing closures
As an idea to improve type stability for capturing closures, such as in
JuliaLang/julia#31909, I tried this idea of propagating the closure
object as a `PartialStruct` whose `fields` include captured variables
of which types are (partially) known. By performing const-prop on this
`closure::PartialStruct`, we can achieve a certain level of type
stability.
Specifically, I made some modifications to `abstract_eval_new` to allow
creating `PartialStruct` even for `:new` objects that are
`!isconcretedispatch` (since `PartialStruct` can now represent abstract
elements). I also adjusted `const_prop_argument_heuristic` to perform
aggressive constant propagation using such `closure::PartialStruct`.
As a result, the following code now achieves type stability:
```julia
julia> Base.infer_return_type((Bool,Int,)) do b, y
x = b ? 1 : missing
inner = y -> x + y
return inner(y)
end
Any # master
Union{Missing, Int64} # this commit
```
However, this alone was not enough to fully resolve JuliaLang/julia#31909.
The call graph of `map` is extremely complex, and simply applying
constant propagation everywhere does not achieve the type safety
requested in the issue.
Nevertheless this commit alone would still improve type stability for
some cases, so I will go ahead and submit it as a PR.
julia
fb76bfe1 - inference: (slightly) improve type stability of capturing closures
