use inferred type instead of Union{} for empty comprehensions

fixes #17811

base/array.jl

@@ -249,14 +249,10 @@ function _collect(cont, itr, ::HasEltype, isz::SizeUnknown)
 end
 
 if isdefined(Core, :Inference)
-    function _default_eltype(itrt::ANY)
-        rt = Core.Inference.return_type(first, Tuple{itrt})
-        return isleaftype(rt) ? rt : Union{}
-    end
+    _default_eltype(itrt::ANY) = Core.Inference.return_type(first, Tuple{itrt})
 else
-    _default_eltype(itr::ANY) = Union{}
+    _default_eltype(itr::ANY) = Any
 end
-_default_eltype{I,T}(::Type{Generator{I,Type{T}}}) = T
 
 _array_for(T, itr, ::HasLength) = Array{T,1}(Int(length(itr)::Integer))
 _array_for(T, itr, ::HasShape) = similar(Array{T}, indices(itr))
@@ -320,7 +316,12 @@ function collect_to!{T}(dest::AbstractArray{T}, itr, offs, st)
     return dest
 end
 
-function grow_to!(dest, itr, st = start(itr))
+function grow_to!(dest, itr)
+    out = grow_to!(similar(dest,Union{}), itr, start(itr))
+    return isempty(out) ? dest : out
+end
+
+function grow_to!(dest, itr, st)
     T = eltype(dest)
     while !done(itr, st)
         el, st = next(itr, st)

base/dict.jl

@@ -390,11 +390,18 @@ end
 
 dict_with_eltype{K,V}(kv, ::Type{Tuple{K,V}}) = Dict{K,V}(kv)
 dict_with_eltype{K,V}(kv, ::Type{Pair{K,V}}) = Dict{K,V}(kv)
-dict_with_eltype(kv, t) = grow_to!(Dict{Union{},Union{}}(), kv)
+dict_with_eltype{K,V}(::Type{Pair{K,V}}) = Dict{K,V}()
+dict_with_eltype(::Type) = Dict()
+dict_with_eltype(kv, t) = grow_to!(dict_with_eltype(_default_eltype(typeof(kv))), kv)
 
 # this is a special case due to (1) allowing both Pairs and Tuples as elements,
 # and (2) Pair being invariant. a bit annoying.
-function grow_to!{K,V}(dest::Associative{K,V}, itr, st = start(itr))
+function grow_to!(dest::Associative, itr)
+    out = grow_to!(similar(dest, Pair{Union{},Union{}}), itr, start(itr))
+    return isempty(out) ? dest : out
+end
+
+function grow_to!{K,V}(dest::Associative{K,V}, itr, st)
     while !done(itr, st)
         (k,v), st = next(itr, st)
         if isa(k,K) && isa(v,V)