Optional generator expansion in code_lowered (#22979) broken by #23168

[jrevels]

Found by @martinholters here (I figured I should make an actual issue for it).

Previously, you could pass false to code_lowered's expand_generated argument to get the CodeInfo for a function's unexpanded generator. Now, however:

julia> @generated f(x) = :(x+1)
f (generic function with 1 method)

julia> code_lowered(f, Tuple{Int}, false)
ERROR: Method is @generated; try `code_lowered` instead.
Stacktrace:
 [1] error at ./error.jl:33 [inlined]
 [2] uncompressed_ast(::Method) at ./reflection.jl:753
 [3] uncompressed_ast(::Core.MethodInstance) at ./reflection.jl:758
 [4] (::getfield(Base, Symbol("##12#13")){DataType,Bool})(::Core.MethodInstance) at ./reflection.jl:634
 [5] next at ./generator.jl:47 [inlined]
 [6] _collect(::Array{Union{Method, Core.MethodInstance},1}, ::Base.Generator{Array{Union{Method, Core.MethodInstance},1},getfield(Base, Symbol("##12#13")){DataType,Bool}}, ::Base.EltypeUnknown, ::Base.HasShape) at ./array.jl:538
 [7] collect_similar(::Array{Union{Method, Core.MethodInstance},1}, ::Base.Generator{Array{Union{Method, Core.MethodInstance},1},getfield(Base, Symbol("##12#13")){DataType,Bool}}) at ./array.jl:475
 [8] map(::Function, ::Array{Union{Method, Core.MethodInstance},1}) at ./abstractarray.jl:1923
 [9] code_lowered(::Any, ::Any, ::Bool) at ./reflection.jl:624
 [10] top-level scope

This appears to have been broken by 80a2c2f#diff-ca03aefbdf6018c4da887b6ec35e0027L740.

There was a test that checked to make sure this functionality didn't regress, but it looks like that test got deleted as part of #23168: 80a2c2f#diff-2ea596942a7d251702840ffcaab1e763L773

[martinholters]

Ah, actually the docstring of code_lowered tells:

  If expand_generated is false, the returned CodeInfo instances will correspond to fallback implementations. An error is thrown if no fallback
  implementation exists. If expand_generated is true, these CodeInfo instances will correspond to the method bodies yielded by expanding the
  generators.

For the @generated function f, no fallback implementation exists, so having the error is to be expected as by the documentation. (Although the error message isn't particularly helpful.)

However, over in #25055 it would be helpful to have a way of retrieving the lowered generator.

[JeffBezanson]

This is intentional. The generator is a separate function, which can be accessed as method.generator.