diff --git a/base/array.jl b/base/array.jl
index bd9d3b8733541..15fab9d95c566 100644
--- a/base/array.jl
+++ b/base/array.jl
@@ -643,23 +643,38 @@ julia> collect(Float64, 1:2:5)
 """
 collect(::Type{T}, itr) where {T} = _collect(T, itr, IteratorSize(itr))
 
-_collect(::Type{T}, itr, isz::HasLength) where {T} = copyto!(Vector{T}(undef, Int(length(itr)::Integer)), itr)
-_collect(::Type{T}, itr, isz::HasShape) where {T}  = copyto!(similar(Array{T}, axes(itr)), itr)
+_collect(::Type{T}, itr, isz::Union{HasLength,HasShape}) where {T} =
+    copyto!(_array_for(T, isz, _similar_shape(itr, isz)), itr)
 function _collect(::Type{T}, itr, isz::SizeUnknown) where T
     a = Vector{T}()
     for x in itr
-        push!(a,x)
+        push!(a, x)
     end
     return a
 end
 
 # make a collection similar to `c` and appropriate for collecting `itr`
-_similar_for(c::AbstractArray, ::Type{T}, itr, ::SizeUnknown) where {T} = similar(c, T, 0)
-_similar_for(c::AbstractArray, ::Type{T}, itr, ::HasLength) where {T} =
-    similar(c, T, Int(length(itr)::Integer))
-_similar_for(c::AbstractArray, ::Type{T}, itr, ::HasShape) where {T} =
-    similar(c, T, axes(itr))
-_similar_for(c, ::Type{T}, itr, isz) where {T} = similar(c, T)
+_similar_for(c, ::Type{T}, itr, isz, shp) where {T} = similar(c, T)
+
+_similar_shape(itr, ::SizeUnknown) = nothing
+_similar_shape(itr, ::HasLength) = length(itr)::Integer
+_similar_shape(itr, ::HasShape) = axes(itr)
+
+_similar_for(c::AbstractArray, ::Type{T}, itr, ::SizeUnknown, ::Nothing) where {T} =
+    similar(c, T, 0)
+_similar_for(c::AbstractArray, ::Type{T}, itr, ::HasLength, len::Integer) where {T} =
+    similar(c, T, len)
+_similar_for(c::AbstractArray, ::Type{T}, itr, ::HasShape, axs) where {T} =
+    similar(c, T, axs)
+
+# make a collection appropriate for collecting `itr::Generator`
+_array_for(::Type{T}, ::SizeUnknown, ::Nothing) where {T} = Vector{T}(undef, 0)
+_array_for(::Type{T}, ::HasLength, len::Integer) where {T} = Vector{T}(undef, Int(len))
+_array_for(::Type{T}, ::HasShape{N}, axs) where {T,N} = similar(Array{T,N}, axs)
+
+# used by syntax lowering for simple typed comprehensions
+_array_for(::Type{T}, itr, isz) where {T} = _array_for(T, isz, _similar_shape(itr, isz))
+
 
 """
     collect(collection)
@@ -698,10 +713,10 @@ collect(A::AbstractArray) = _collect_indices(axes(A), A)
 collect_similar(cont, itr) = _collect(cont, itr, IteratorEltype(itr), IteratorSize(itr))
 
 _collect(cont, itr, ::HasEltype, isz::Union{HasLength,HasShape}) =
-    copyto!(_similar_for(cont, eltype(itr), itr, isz), itr)
+    copyto!(_similar_for(cont, eltype(itr), itr, isz, _similar_shape(itr, isz)), itr)
 
 function _collect(cont, itr, ::HasEltype, isz::SizeUnknown)
-    a = _similar_for(cont, eltype(itr), itr, isz)
+    a = _similar_for(cont, eltype(itr), itr, isz, nothing)
     for x in itr
         push!(a,x)
     end
@@ -759,24 +774,19 @@ else
     end
 end
 
-_array_for(::Type{T}, itr, isz::HasLength) where {T} = _array_for(T, itr, isz, length(itr))
-_array_for(::Type{T}, itr, isz::HasShape{N}) where {T,N} = _array_for(T, itr, isz, axes(itr))
-_array_for(::Type{T}, itr, ::HasLength, len) where {T} = Vector{T}(undef, len)
-_array_for(::Type{T}, itr, ::HasShape{N}, axs) where {T,N} = similar(Array{T,N}, axs)
-
 function collect(itr::Generator)
     isz = IteratorSize(itr.iter)
     et = @default_eltype(itr)
     if isa(isz, SizeUnknown)
         return grow_to!(Vector{et}(), itr)
     else
-        shape = isz isa HasLength ? length(itr) : axes(itr)
+        shp = _similar_shape(itr, isz)
         y = iterate(itr)
         if y === nothing
-            return _array_for(et, itr.iter, isz)
+            return _array_for(et, isz, shp)
         end
         v1, st = y
-        dest = _array_for(typeof(v1), itr.iter, isz, shape)
+        dest = _array_for(typeof(v1), isz, shp)
         # The typeassert gives inference a helping hand on the element type and dimensionality
         # (work-around for #28382)
         et′ = et <: Type ? Type : et
@@ -786,15 +796,22 @@ function collect(itr::Generator)
 end
 
 _collect(c, itr, ::EltypeUnknown, isz::SizeUnknown) =
-    grow_to!(_similar_for(c, @default_eltype(itr), itr, isz), itr)
+    grow_to!(_similar_for(c, @default_eltype(itr), itr, isz, nothing), itr)
 
 function _collect(c, itr, ::EltypeUnknown, isz::Union{HasLength,HasShape})
+    et = @default_eltype(itr)
+    shp = _similar_shape(itr, isz)
     y = iterate(itr)
     if y === nothing
-        return _similar_for(c, @default_eltype(itr), itr, isz)
+        return _similar_for(c, et, itr, isz, shp)
     end
     v1, st = y
-    collect_to_with_first!(_similar_for(c, typeof(v1), itr, isz), v1, itr, st)
+    dest = _similar_for(c, typeof(v1), itr, isz, shp)
+    # The typeassert gives inference a helping hand on the element type and dimensionality
+    # (work-around for #28382)
+    et′ = et <: Type ? Type : et
+    RT = dest isa AbstractArray ? AbstractArray{<:et′, ndims(dest)} : Any
+    collect_to_with_first!(dest, v1, itr, st)::RT
 end
 
 function collect_to_with_first!(dest::AbstractArray, v1, itr, st)
diff --git a/base/compiler/ssair/inlining.jl b/base/compiler/ssair/inlining.jl
index f67406dad9308..7c229a3b2081a 100644
--- a/base/compiler/ssair/inlining.jl
+++ b/base/compiler/ssair/inlining.jl
@@ -109,7 +109,7 @@ function inline_into_block!(state::CFGInliningState, block::Int)
         new_range = state.first_bb+1:block
         l = length(state.new_cfg_blocks)
         state.bb_rename[new_range] = (l+1:l+length(new_range))
-        append!(state.new_cfg_blocks, map(copy, state.cfg.blocks[new_range]))
+        append!(state.new_cfg_blocks, (copy(block) for block in state.cfg.blocks[new_range]))
         push!(state.merged_orig_blocks, last(new_range))
     end
     state.first_bb = block
diff --git a/base/compiler/ssair/passes.jl b/base/compiler/ssair/passes.jl
index 2d097b3d46b6f..23517ea3ed860 100644
--- a/base/compiler/ssair/passes.jl
+++ b/base/compiler/ssair/passes.jl
@@ -27,31 +27,6 @@ function try_compute_fieldidx_args(typ::DataType, args::Vector{Any})
     return try_compute_fieldidx(typ, field)
 end
 
-function lift_defuse(cfg::CFG, ssa::SSADefUse)
-    # We remove from `uses` any block where all uses are dominated
-    # by a def. This prevents insertion of dead phi nodes at the top
-    # of such a block if that block happens to be in a loop
-    ordered = Tuple{Int, Int, Bool}[(x, block_for_inst(cfg, x), true) for x in ssa.uses]
-    for x in ssa.defs
-        push!(ordered, (x, block_for_inst(cfg, x), false))
-    end
-    ordered = sort(ordered, by=x->x[1])
-    bb_defs = Int[]
-    bb_uses = Int[]
-    last_bb = last_def_bb = 0
-    for (_, bb, is_use) in ordered
-        if bb != last_bb && is_use
-            push!(bb_uses, bb)
-        end
-        last_bb = bb
-        if last_def_bb != bb && !is_use
-            push!(bb_defs, bb)
-            last_def_bb = bb
-        end
-    end
-    SSADefUse(bb_uses, bb_defs, Int[])
-end
-
 function find_curblock(domtree::DomTree, allblocks::Vector{Int}, curblock::Int)
     # TODO: This can be much faster by looking at current level and only
     # searching for those blocks in a sorted order
@@ -1209,12 +1184,12 @@ function cfg_simplify!(ir::IRCode)
         # Compute (renamed) successors and predecessors given (renamed) block
         function compute_succs(i)
             orig_bb = follow_merged_succ(result_bbs[i])
-            return map(i -> bb_rename_succ[i], bbs[orig_bb].succs)
+            return Int[bb_rename_succ[i] for i in bbs[orig_bb].succs]
         end
         function compute_preds(i)
             orig_bb = result_bbs[i]
             preds = bbs[orig_bb].preds
-            return map(pred -> bb_rename_pred[pred], preds)
+            return Int[bb_rename_pred[pred] for pred in preds]
         end
 
         BasicBlock[
diff --git a/base/compiler/ssair/show.jl b/base/compiler/ssair/show.jl
index 4174267ec5a5e..013278ed001d5 100644
--- a/base/compiler/ssair/show.jl
+++ b/base/compiler/ssair/show.jl
@@ -79,14 +79,15 @@ show_unquoted(io::IO, val::Argument, indent::Int, prec::Int) = show_unquoted(io,
 
 show_unquoted(io::IO, stmt::PhiNode, indent::Int, ::Int) = show_unquoted_phinode(io, stmt, indent, "%")
 function show_unquoted_phinode(io::IO, stmt::PhiNode, indent::Int, prefix::String)
-    args = map(1:length(stmt.edges)) do i
+    args = String[let
         e = stmt.edges[i]
         v = !isassigned(stmt.values, i) ? "#undef" :
             sprint() do io′
                 show_unquoted(io′, stmt.values[i], indent)
             end
-        return "$prefix$e => $v"
-    end
+        "$prefix$e => $v"
+        end for i in 1:length(stmt.edges)
+    ]
     print(io, "φ ", '(')
     join(io, args, ", ")
     print(io, ')')
diff --git a/base/compiler/ssair/slot2ssa.jl b/base/compiler/ssair/slot2ssa.jl
index 2e3d1da1c168f..489c98c6549cc 100644
--- a/base/compiler/ssair/slot2ssa.jl
+++ b/base/compiler/ssair/slot2ssa.jl
@@ -33,16 +33,6 @@ function scan_entry!(result::Vector{SlotInfo}, idx::Int, @nospecialize(stmt))
 end
 
 
-function lift_defuse(cfg::CFG, defuse)
-    map(defuse) do slot
-        SlotInfo(
-            Int[block_for_inst(cfg, x) for x in slot.defs],
-            Int[block_for_inst(cfg, x) for x in slot.uses],
-            slot.any_newvar
-        )
-    end
-end
-
 function scan_slot_def_use(nargs::Int, ci::CodeInfo, code::Vector{Any})
     nslots = length(ci.slotflags)
     result = SlotInfo[SlotInfo() for i = 1:nslots]
@@ -524,7 +514,7 @@ function domsort_ssa!(ir::IRCode, domtree::DomTree)
     return new_ir
 end
 
-function compute_live_ins(cfg::CFG, defuse)
+function compute_live_ins(cfg::CFG, defuse #=::Union{SlotInfo,SSADefUse}=#)
     # We remove from `uses` any block where all uses are dominated
     # by a def. This prevents insertion of dead phi nodes at the top
     # of such a block if that block happens to be in a loop
@@ -586,8 +576,8 @@ function recompute_type(node::Union{PhiNode, PhiCNode}, ci::CodeInfo, ir::IRCode
     return new_typ
 end
 
-function construct_ssa!(ci::CodeInfo, ir::IRCode, domtree::DomTree, defuse,
-                        slottypes::Vector{Any})
+function construct_ssa!(ci::CodeInfo, ir::IRCode, domtree::DomTree,
+                        defuses::Vector{SlotInfo}, slottypes::Vector{Any})
     code = ir.stmts.inst
     cfg = ir.cfg
     left = Int[]
@@ -616,7 +606,7 @@ function construct_ssa!(ci::CodeInfo, ir::IRCode, domtree::DomTree, defuse,
     for (_, exc) in catch_entry_blocks
         phicnodes[exc] = Vector{Tuple{SlotNumber, NewSSAValue, PhiCNode}}()
     end
-    @timeit "idf" for (idx, slot) in Iterators.enumerate(defuse)
+    @timeit "idf" for (idx, slot) in Iterators.enumerate(defuses)
         # No uses => no need for phi nodes
         isempty(slot.uses) && continue
         # TODO: Restore this optimization
@@ -671,9 +661,9 @@ function construct_ssa!(ci::CodeInfo, ir::IRCode, domtree::DomTree, defuse,
     end
     # Perform SSA renaming
     initial_incoming_vals = Any[
-        if 0 in defuse[x].defs
+        if 0 in defuses[x].defs
             Argument(x)
-        elseif !defuse[x].any_newvar
+        elseif !defuses[x].any_newvar
             undef_token
         else
             SSAValue(-2)
diff --git a/base/dict.jl b/base/dict.jl
index 6918677c4f0bb..1978323e88503 100644
--- a/base/dict.jl
+++ b/base/dict.jl
@@ -826,6 +826,6 @@ length(t::ImmutableDict) = count(Returns(true), t)
 isempty(t::ImmutableDict) = !isdefined(t, :parent)
 empty(::ImmutableDict, ::Type{K}, ::Type{V}) where {K, V} = ImmutableDict{K,V}()
 
-_similar_for(c::Dict, ::Type{Pair{K,V}}, itr, isz) where {K, V} = empty(c, K, V)
-_similar_for(c::AbstractDict, ::Type{T}, itr, isz) where {T} =
+_similar_for(c::AbstractDict, ::Type{Pair{K,V}}, itr, isz, len) where {K, V} = empty(c, K, V)
+_similar_for(c::AbstractDict, ::Type{T}, itr, isz, len) where {T} =
     throw(ArgumentError("for AbstractDicts, similar requires an element type of Pair;\n  if calling map, consider a comprehension instead"))
diff --git a/base/set.jl b/base/set.jl
index 6511d1dd7e108..dd1400d11dba1 100644
--- a/base/set.jl
+++ b/base/set.jl
@@ -44,7 +44,7 @@ empty(s::AbstractSet{T}, ::Type{U}=T) where {T,U} = Set{U}()
 # by default, a Set is returned
 emptymutable(s::AbstractSet{T}, ::Type{U}=T) where {T,U} = Set{U}()
 
-_similar_for(c::AbstractSet, ::Type{T}, itr, isz) where {T} = empty(c, T)
+_similar_for(c::AbstractSet, ::Type{T}, itr, isz, len) where {T} = empty(c, T)
 
 function show(io::IO, s::Set)
     if isempty(s)
diff --git a/src/gf.c b/src/gf.c
index d6c9741f3ebf3..0b3b90540e980 100644
--- a/src/gf.c
+++ b/src/gf.c
@@ -477,6 +477,7 @@ void jl_foreach_reachable_mtable(void (*visit)(jl_methtable_t *mt, void *env), v
     }
     else {
         foreach_mtable_in_module(jl_main_module, visit, env, &visited);
+        foreach_mtable_in_module(jl_core_module, visit, env, &visited);
     }
     JL_GC_POP();
 }
@@ -493,14 +494,15 @@ static void reset_mt_caches(jl_methtable_t *mt, void *env)
 
 
 jl_function_t *jl_typeinf_func = NULL;
-size_t jl_typeinf_world = 0;
+size_t jl_typeinf_world = 1;
 
 JL_DLLEXPORT void jl_set_typeinf_func(jl_value_t *f)
 {
+    size_t newfunc = jl_typeinf_world == 1 && jl_typeinf_func == NULL;
     jl_typeinf_func = (jl_function_t*)f;
     jl_typeinf_world = jl_get_tls_world_age();
     ++jl_world_counter; // make type-inference the only thing in this world
-    if (jl_typeinf_world == 0) {
+    if (newfunc) {
         // give type inference a chance to see all of these
         // TODO: also reinfer if max_world != ~(size_t)0
         jl_array_t *unspec = jl_alloc_vec_any(0);
diff --git a/src/julia-syntax.scm b/src/julia-syntax.scm
index 428b0513b7e52..ef533059b3993 100644
--- a/src/julia-syntax.scm
+++ b/src/julia-syntax.scm
@@ -2734,7 +2734,7 @@
   (check-no-return expr)
   (if (has-break-or-continue? expr)
       (error "break or continue outside loop"))
-  (let ((result    (gensy))
+  (let ((result    (make-ssavalue))
         (idx       (gensy))
         (oneresult (make-ssavalue))
         (prod      (make-ssavalue))
@@ -2758,16 +2758,14 @@
     (let ((overall-itr (if (length= itrs 1) (car iv) prod)))
       `(scope-block
         (block
-         (local ,result) (local ,idx)
+         (local ,idx)
          ,.(map (lambda (v r) `(= ,v ,(caddr r))) iv itrs)
          ,.(if (length= itrs 1)
                '()
                `((= ,prod (call (top product) ,@iv))))
          (= ,isz (call (top IteratorSize) ,overall-itr))
          (= ,szunk (call (core isa) ,isz (top SizeUnknown)))
-         (if ,szunk
-             (= ,result (call (curly (core Array) ,ty 1) (core undef) 0))
-             (= ,result (call (top _array_for) ,ty ,overall-itr ,isz)))
+         (= ,result (call (top _array_for) ,ty ,overall-itr ,isz))
          (= ,idx (call (top first) (call (top LinearIndices) ,result)))
          ,(construct-loops (reverse itrs) (reverse iv))
          ,result)))))
diff --git a/test/errorshow.jl b/test/errorshow.jl
index 137f15dd6b21a..80ad770de6f1e 100644
--- a/test/errorshow.jl
+++ b/test/errorshow.jl
@@ -728,7 +728,7 @@ end
 
 # Test that implementation detail of include() is hidden from the user by default
 let bt = try
-        include("testhelpers/include_error.jl")
+        @noinline include("testhelpers/include_error.jl")
     catch
         catch_backtrace()
     end
@@ -740,7 +740,7 @@ end
 # Test backtrace printing
 module B
     module C
-        f(x; y=2.0) = error()
+        @noinline f(x; y=2.0) = error()
     end
     module D
         import ..C: f
@@ -749,7 +749,8 @@ module B
 end
 
 @testset "backtrace" begin
-    bt = try B.D.g()
+    bt = try
+        B.D.g()
     catch
         catch_backtrace()
     end
@@ -777,7 +778,8 @@ if Sys.isapple() || (Sys.islinux() && Sys.ARCH === :x86_64)
     pair_repeater_b() = pair_repeater_a()
 
     @testset "repeated stack frames" begin
-        let bt = try single_repeater()
+        let bt = try
+                single_repeater()
             catch
                 catch_backtrace()
             end
@@ -785,7 +787,8 @@ if Sys.isapple() || (Sys.islinux() && Sys.ARCH === :x86_64)
             @test occursin(r"repeats \d+ times", bt_str)
         end
 
-        let bt = try pair_repeater_a()
+        let bt = try
+                pair_repeater_a()
             catch
                 catch_backtrace()
             end
diff --git a/test/iterators.jl b/test/iterators.jl
index c7d00c4e7e2e8..86c325a85b617 100644
--- a/test/iterators.jl
+++ b/test/iterators.jl
@@ -293,11 +293,14 @@ let (a, b) = (1:3, [4 6;
 end
 
 # collect stateful iterator
-let
-    itr = (i+1 for i in Base.Stateful([1,2,3]))
+let itr
+    itr = Iterators.Stateful(Iterators.map(identity, 1:5))
+    @test collect(itr) == 1:5
+    @test collect(itr) == Int[] # Stateful do not preserve shape
+    itr = (i+1 for i in Base.Stateful([1, 2, 3]))
     @test collect(itr) == [2, 3, 4]
-    A = zeros(Int, 0, 0)
-    itr = (i-1 for i in Base.Stateful(A))
+    @test collect(itr) == Int[] # Stateful do not preserve shape
+    itr = (i-1 for i in Base.Stateful(zeros(Int, 0, 0)))
     @test collect(itr) == Int[] # Stateful do not preserve shape
 end