type assertion doesn't help type inference within method body, causing run time dispatch

[nsajko]

Reproducer:

using JET

struct MyStruct
    alist::Union{Nothing,Vector{Int}}
end

function f(m)
    a = m.alist
    a::Vector{Int}
    a[1]   
end


report_opt(f, Tuple{MyStruct})  # run time dispatch

This is with Julia v1.11, as JET doesn't work on nightly Julia:

julia> versioninfo()
Julia Version 1.11.0-beta2
Commit edb3c92d6a6 (2024-05-29 09:37 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 8 × AMD Ryzen 3 5300U with Radeon Graphics
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver2)
Threads: 1 default, 0 interactive, 1 GC (on 8 virtual cores)

If this:

    a::Vector{Int}
    a[1]

... is changed to:

    (a::Vector{Int})[1]

... then there's no run time dispatch!

[KristofferC]

Dup of #38274