"Invalid declaration" inside a constant definition when loading a module

[frankwswang]

MWE (reproduced on 1.12.0-rc1 and 1.13.0-DEV.917):

First, define a module script "MyMod.jl" in the root directory:

module MyMod

const SubShellDict::Base.ImmutableDict{String, Int} = let
    key = ["S", "P", "D"]
    val = collect(0 : 2)
    mapreduce(Base.ImmutableDict, key, val, 
              init=Base.ImmutableDict{String, Int}()) do key, val
        key=>val
    end
end

end

Then try loading the script in a freshly started Julia REPL:

julia> include("./MyMod.jl")
ERROR: LoadError: method: invalid declaration
Stacktrace:
 [1] top-level scope
   @ .\MyMod.jl:6
 [2] include(mapexpr::Function, mod::Module, _path::String)
   @ Base .\Base.jl:312
 [3] top-level scope
   @ REPL[1]:1

[Keno]

julia> (@Meta.lower const SubShellDict::Base.ImmutableDict{String, Int} = let
           key = ["S", "P", "D"]
           val = collect(0 : 2)
           mapreduce(Base.ImmutableDict, key, val,
                     init=Base.ImmutableDict{String, Int}()) do key, val
               key=>val
           end
       end).args[1].code[11]
:($(Expr(:method, :(_2))))

Lowering gets confused about the gensym of the closure somehow if there's a type annotation on the const (which is unusual, but not illegal).