Drop use of something in reflection.jl (#38067)

* Drop use of `something` in reflection.jl

reflection.jl is included in Core.Compiler, but the `something` function is not, so we cannot use it here.
Throw an explicit error instead. I considered having it not throw the error when `err==false`, but it
seems to me like passing such a UnionAll is just a user error, rather than an acceptable query, so
I left it as is and simply did the bug fix.

* Update base/reflection.jl

Author: Keno

base/reflection.jl

@@ -638,7 +638,13 @@ function fieldindex(T::DataType, name::Symbol, err::Bool=true)
     return Int(ccall(:jl_field_index, Cint, (Any, Any, Cint), T, name, err)+1)
 end
 
-fieldindex(t::UnionAll, name::Symbol, err::Bool=true) = fieldindex(something(argument_datatype(t)), name, err)
+function fieldindex(t::UnionAll, name::Symbol, err::Bool=true)
+    t = argument_datatype(t)
+    if t === nothing
+        throw(ArgumentError("type does not have definite fields"))
+    end
+    return fieldindex(t, name, err)
+end
 
 argument_datatype(@nospecialize t) = ccall(:jl_argument_datatype, Any, (Any,), t)