keys(::Generator) pass-through

Let keys pass through to allow lightweigth/lazy find* and arg* functions

Author: rapus95

base/generator.jl

@@ -51,6 +51,7 @@ length(g::Generator) = length(g.iter)
 size(g::Generator) = size(g.iter)
 axes(g::Generator) = axes(g.iter)
 ndims(g::Generator) = ndims(g.iter)
+keys(g::Generator) = keys(g.iter)
 
 
 ## iterator traits

test/functional.jl

@@ -134,6 +134,9 @@ let gen = ((x,y) for x in 1:10, y in 1:10 if x % 2 == 0 && y % 2 == 0),
     @test collect(gen) == collect(gen2)
 end
 
+# keys of a generator for find* and arg* (see #34678)
+@test keys(x^2 for x in -1:0.5:1) == 1:5
+
 # inference on vararg generator of a type (see #22907 comments)
 let f(x) = collect(Base.Generator(=>, x, x))
     @test @inferred(f((1,2))) == [1=>1, 2=>2]