Array constructors ignored in `precompile(...)` statements

[topolarity]

If you create a package that tries to precompile an Array constructor:

module PrecompileVector

struct Foo end
precompile(Tuple{Type{Array{Foo, 1}}, UndefInitializer, Tuple{Int64}})
export Foo

end # module PrecompileVector

This doesn't seem to "stick":

$ julia +nightly --project=PrecompileVector --trace-compile=stderr -e "using PrecompileVector; Vector{Foo}(undef, (3,))"
precompile(Tuple{Type{Array{PrecompileVector.Foo, 1}}, UndefInitializer, Tuple{Int64}})

The problem seems to be that this doesn't make it into our "newly-inferred" list:

julia> using PrecompileVector
julia> tt = Tuple{Type{Vector{Foo}}, UndefInitializer, Tuple{Int}}
julia> mi = ccall(:jl_method_lookup_by_tt, Any,
                   (Any, Csize_t, Any),
                   tt, Base.get_world_counter(), #= mt =# nothing)
julia> isdefined(mi, :cache)
false

julia> inferred = Any[]; ccall(:jl_set_newly_inferred, Cvoid, (Any,), inferred);
julia> Vector{Foo}(undef, (3,)); # trigger compilation
julia> ccall(:jl_set_newly_inferred, Cvoid, (Any,), nothing)

julia> mi.cache
CodeInstance for MethodInstance for Vector{Foo}(::UndefInitializer, ::Tuple{Int64})

julia> mi.cache in inferred
false

Vector{Foo}(::UndefInitializer, ::Tuple{Int64}) was compiled (and inferred) for the first time, but it didn't make it into the list due to this m.module != Core check:

julia/Compiler/src/cicache.jl

Line 17 in 6f52a98

if isa(m, Method) && m.module != Core

I'm not sure why we have that guard, but it has the un-intended side effect that precompile(...) statements are less effective over re-exported Core methods in Base.

[Keno]

I'm not sure why we have that guard

I don't know either, the condition goes back to the original introduction of this code in df81bf9. We should try removing it - if something breaks, we can at least add a comment that explains it. Or maybe @timholy can offer some historical context.

[timholy]

I don't remember the reason for that check, sadly. I like the idea of seeing what happens if we remove it. A lot about Core has changed, anyway.