Worldage error with @generated in let block only in assignments

[simeonschaub]

This one is really weird. It happens only if the let block in question is on the rhs of an assignment, otherwise it works just fine:

julia> a = let
       @generated f(x) = 1
       f(1)
       end
ERROR: MethodError: no method matching #s24#27(::Type{var"#f#28"}, ::Type{Int64})
The applicable method may be too new: running in world age 28483, while current world is 28484.
Closest candidates are:
  #s24#27(::Any, ::Any) at none:0 (method too new to be called from this world context.)
Stacktrace:
 [1] (::Core.GeneratedFunctionStub)(::Any, ::Vararg{Any, N} where N)
   @ Core ./boot.jl:556
 [2] top-level scope
   @ REPL[14]:3

julia> let
       @generated f(x) = 1
       f(1)
       end
1

It's also interesting what happens if I use if @generated for f:

julia> a = let
       f(x) = if @generated() 1 else error() end
       f(1)
       end
ERROR: 
Stacktrace:
 [1] error()
   @ Base ./error.jl:42
 [2] (::var"#f#24")(x::Int64)
   @ Main ./REPL[12]:2
 [3] top-level scope
   @ REPL[12]:3

julia> let
       f(x) = if @generated() 1 else error() end
       f(1)
       end
1

That it's a worldage error is what probably confuses me the most. I would have thought this could be an issue during lowering, but perhaps it occurs later on?
cc: @mcabbott

[simeonschaub]

@JeffBezanson Any ideas why the produced lowered code would be so different here? (left one is just with a = in front, right one without) It seems to happen somewhere after form expansion at least.