relax dispatch for the IteratorSize method for Generator (#58110)

Fixes #58109

(cherry picked from commit 805f85f)

Author: nsajko

base/generator.jl

@@ -97,7 +97,7 @@ IteratorSize(::Type{Any}) = SizeUnknown()
 
 IteratorSize(::Type{<:Tuple}) = HasLength()
 IteratorSize(::Type{<:AbstractArray{<:Any,N}})  where {N} = HasShape{N}()
-IteratorSize(::Type{Generator{I,F}}) where {I,F} = IteratorSize(I)
+IteratorSize(::Type{<:Generator{I}}) where {I} = (@isdefined I) ? IteratorSize(I) : SizeUnknown()
 
 haslength(iter) = IteratorSize(iter) isa Union{HasShape, HasLength}
 

test/iterators.jl

@@ -959,6 +959,12 @@ end
     @test Iterators.tail_if_any((1,)) == ()
 end
 
+@testset "issue #58109" begin
+    i = Iterators.map(identity, 3)
+    j = Iterators.map(sqrt, 7)
+    @test (@inferred Base.IteratorSize(i)) === @inferred Base.IteratorSize(eltype([i, j]))
+end
+
 @testset "IteratorSize trait for zip" begin
     @test Base.IteratorSize(zip()) == Base.IsInfinite()                     # for zip of empty tuple
     @test Base.IteratorSize(zip((1,2,3), repeated(0))) == Base.HasLength()  # for zip of ::HasLength and ::IsInfinite