fix #46778, precompile() for abstract but compileable signatures

[JeffBezanson]

fix #46778

[KristofferC]

Bump! Good to go?

[KristofferC]

Something doesn't seem right with the caching of these signatures in a sysimage.

As an example, I add a specific precompile statement for the problematic method in #46778:

diff --git a/contrib/generate_precompile.jl b/contrib/generate_precompile.jl
index 295b24d22e..abc887d94d 100644
--- a/contrib/generate_precompile.jl
+++ b/contrib/generate_precompile.jl
@@ -186,6 +186,18 @@ if Libdl !== nothing
     """
 end
 
+Serialization = get(Base.loaded_modules,
+          Base.PkgId(Base.UUID("9e88b42a-f829-5b0c-bbe9-9e923198166b"), "Serialization"),
+          nothing)
+if Serialization !== nothing
+    @info "Precompiling deserialization..."
+    hardcoded_precompile_statements *= """
+        precompile(Tuple{typeof(Serialization.deserialize), Serialization.Serializer{Base.GenericIOBuffer{Array{UInt8, 1}}}, DataType})
+
+        """
+end
+
+
 Test = get(Base.loaded_modules,
           Base.PkgId(Base.UUID("8dfed614-e22c-5e08-85e1-65c5234f0b40"), "Test"),
           nothing)

I build a Julia with this, and then I create a serialized file:

using Serialization
struct MyStruct
    x::String
end
s = MyStruct("foo")
serialize("file.jls", s)

Now, I run Julia with --trace-compile=stderr:

julia> using Serialization

julia> struct MyStruct
           x::String
       end

julia> @time deserialize(IOBuffer(read("file.jls")));
precompile(Tuple{typeof(Serialization.deserialize), Serialization.Serializer{Base.GenericIOBuffer{Array{UInt8, 1}}}, DataType})
  0.022934 seconds (11.89 k allocations: 800.115 KiB, 98.08% compilation time)

julia> @time deserialize(IOBuffer(read("file.jls")));
  0.000084 seconds (28 allocations: 2.008 KiB)

The compile time is not removed and the signature printed is exactly the one that was supposed to be cached in the generate_precompile.jl file.

[JeffBezanson]

Oh, it looks like generate_precompile has an explicit check for this:

            ms = length(ps) == 1 ? Base._methods_by_ftype(ps[1], 1, Base.get_world_counter()) : Base.methods(ps...)
            ms isa Vector || continue

[KristofferC]

Oh...