diff --git a/compiler/ast.nim b/compiler/ast.nim
index e35a0b2031c8..2073a5630862 100644
--- a/compiler/ast.nim
+++ b/compiler/ast.nim
@@ -795,6 +795,15 @@ type
 
   TPairSeq* = seq[TPair]
 
+  TIdPair*[T] = object
+    key*: ItemId
+    val*: T
+
+  TIdPairSeq*[T] = seq[TIdPair[T]]
+  TIdTable*[T] = object
+    counter*: int
+    data*: TIdPairSeq[T]
+
   TNodePair* = object
     h*: Hash                 # because it is expensive to compute!
     key*: PNode
@@ -939,9 +948,11 @@ proc getPIdent*(a: PNode): PIdent {.inline.} =
 const
   moduleShift = when defined(cpu32): 20 else: 24
 
-template id*(a: PType | PSym): int =
+template toId*(a: ItemId): int =
   let x = a
-  (x.itemId.module.int shl moduleShift) + x.itemId.item.int
+  (x.module.int shl moduleShift) + x.item.int
+
+template id*(a: PType | PSym): int = toId(a.itemId)
 
 type
   IdGenerator* = ref object # unfortunately, we really need the 'shared mutable' aspect here.
@@ -1268,6 +1279,11 @@ proc copyStrTable*(dest: var TStrTable, src: TStrTable) =
   setLen(dest.data, src.data.len)
   for i in 0..high(src.data): dest.data[i] = src.data[i]
 
+proc copyIdTable*[T](dest: var TIdTable[T], src: TIdTable[T]) =
+  dest.counter = src.counter
+  newSeq(dest.data, src.data.len)
+  for i in 0..high(src.data): dest.data[i] = src.data[i]
+
 proc copyObjectSet*(dest: var TObjectSet, src: TObjectSet) =
   dest.counter = src.counter
   setLen(dest.data, src.data.len)
@@ -1606,6 +1622,16 @@ proc initStrTable*(): TStrTable =
   result = TStrTable(counter: 0)
   newSeq(result.data, StartSize)
 
+proc initIdTable*[T](): TIdTable[T] =
+  result = TIdTable[T](counter: 0)
+  newSeq(result.data, StartSize)
+
+proc resetIdTable*[T](x: var TIdTable[T]) =
+  x.counter = 0
+  # clear and set to old initial size:
+  setLen(x.data, 0)
+  setLen(x.data, StartSize)
+
 proc initObjectSet*(): TObjectSet =
   result = TObjectSet(counter: 0)
   newSeq(result.data, StartSize)
@@ -2134,14 +2160,8 @@ proc isTrue*(n: PNode): bool =
     n.kind == nkIntLit and n.intVal != 0
 
 type
-  TypeMapping* = Table[ItemId, PType]
-  SymMapping* = Table[ItemId, PSym]
-
-template idTableGet*(tab: typed; key: PSym | PType): untyped = tab.getOrDefault(key.itemId)
-template idTablePut*(tab: typed; key, val: PSym | PType) = tab[key.itemId] = val
-
-template initSymMapping*(): Table[ItemId, PSym] = initTable[ItemId, PSym]()
-template initTypeMapping*(): Table[ItemId, PType] = initTable[ItemId, PType]()
+  TypeMapping* = TIdTable[PType]
+  SymMapping* = TIdTable[PSym]
 
-template resetIdTable*(tab: Table[ItemId, PSym]) = tab.clear()
-template resetIdTable*(tab: Table[ItemId, PType]) = tab.clear()
+template initSymMapping*(): SymMapping = initIdTable[PSym]()
+template initTypeMapping*(): TypeMapping = initIdTable[PType]()
diff --git a/compiler/astalgo.nim b/compiler/astalgo.nim
index 7a9892f78ad2..14dc7c59944a 100644
--- a/compiler/astalgo.nim
+++ b/compiler/astalgo.nim
@@ -713,6 +713,70 @@ iterator items*(tab: TStrTable): PSym =
     yield s
     s = nextIter(it, tab)
 
+proc isNil(x: ItemId): bool {.inline.} =
+  x.module == 0 and x.item == 0
+
+proc hasEmptySlot[T](data: TIdPairSeq[T]): bool =
+  for h in 0..high(data):
+    if isNil(data[h].key):
+      return true
+  result = false
+
+proc idTableRawGet[T](t: TIdTable[T], key: int): int =
+  var h: Hash
+  h = key and high(t.data)    # start with real hash value
+  while not isNil(t.data[h].key):
+    if toId(t.data[h].key) == key:
+      return h
+    h = nextTry(h, high(t.data))
+  result = - 1
+
+proc getOrDefault*[T](t: TIdTable[T], key: ItemId): T =
+  var index = idTableRawGet(t, toId(key))
+  if index >= 0: result = t.data[index].val
+  else: result = default(T)
+
+template idTableGet*[T](t: TIdTable[T], key: PType | PSym): T =
+  getOrDefault(t, key.itemId)
+
+proc idTableRawInsert[T](data: var TIdPairSeq[T], key: ItemId, val: T) =
+  var h: Hash
+  let keyId = toId(key)
+  h = keyId and high(data)
+  while not isNil(data[h].key):
+    assert(toId(data[h].key) != keyId)
+    h = nextTry(h, high(data))
+  assert(isNil(data[h].key))
+  data[h].key = key
+  data[h].val = val
+
+proc `[]=`*[T](t: var TIdTable[T], key: ItemId, val: T) =
+  var
+    index: int
+    n: TIdPairSeq[T]
+  index = idTableRawGet(t, toId(key))
+  if index >= 0:
+    assert(not isNil(t.data[index].key))
+    t.data[index].val = val
+  else:
+    if mustRehash(t.data.len, t.counter):
+      newSeq(n, t.data.len * GrowthFactor)
+      for i in 0..high(t.data):
+        if not isNil(t.data[i].key):
+          idTableRawInsert(n, t.data[i].key, t.data[i].val)
+      assert(hasEmptySlot(n))
+      swap(t.data, n)
+    idTableRawInsert(t.data, key, val)
+    inc(t.counter)
+
+template idTablePut*[T](t: var TIdTable[T], key: PType | PSym, val: T) =
+  t[key.itemId] = val
+
+iterator idTablePairs*[T](t: TIdTable[T]): tuple[key: ItemId, val: T] =
+  for i in 0..high(t.data):
+    if not isNil(t.data[i].key):
+      yield (t.data[i].key, t.data[i].val)
+
 proc initIITable(x: var TIITable) =
   x.counter = 0
   newSeq(x.data, StartSize)
diff --git a/compiler/layeredtable.nim b/compiler/layeredtable.nim
index 61a86cff84ee..248ec4bcf2ef 100644
--- a/compiler/layeredtable.nim
+++ b/compiler/layeredtable.nim
@@ -1,5 +1,5 @@
 import std/[tables]
-import ast
+import ast, astalgo
 
 type
   LayeredIdTableObj* {.acyclic.} = object
@@ -28,14 +28,15 @@ proc shallowCopy*(pt: LayeredIdTable): LayeredIdTable {.inline.} =
   ## copies only the type bindings of the current layer, but not any parent layers,
   ## useful for write-only bindings
   result = LayeredIdTable(topLayer: pt.topLayer, nextLayer: pt.nextLayer, previousLen: pt.previousLen)
+  #copyIdTable(result.topLayer, pt.topLayer)
 
 proc currentLen*(pt: LayeredIdTable): int =
   ## the sum of the cached total binding count of the parents and
   ## the current binding count, just used to track if bindings were added
-  pt.previousLen + pt.topLayer.len
+  pt.previousLen + pt.topLayer.counter
 
 proc newTypeMapLayer*(pt: LayeredIdTable): LayeredIdTable =
-  result = LayeredIdTable(topLayer: initTable[ItemId, PType](), previousLen: pt.currentLen)
+  result = LayeredIdTable(topLayer: initTypeMapping(), previousLen: pt.currentLen)
   when useRef:
     result.nextLayer = pt
   else:
@@ -56,7 +57,7 @@ proc setToPreviousLayer*(pt: var LayeredIdTable) {.inline.} =
 iterator pairs*(pt: LayeredIdTable): (ItemId, PType) =
   var tm = pt
   while true:
-    for (k, v) in pairs(tm.topLayer):
+    for (k, v) in idTablePairs(tm.topLayer):
       yield (k, v)
     if tm.nextLayer == nil:
       break
diff --git a/compiler/semcall.nim b/compiler/semcall.nim
index 90376214db68..eaa723a2c712 100644
--- a/compiler/semcall.nim
+++ b/compiler/semcall.nim
@@ -907,15 +907,15 @@ proc semOverloadedCall(c: PContext, n, nOrig: PNode,
     if c.inGenericContext > 0 and c.matchedConcept == nil:
       result = semGenericStmt(c, n)
       result.typ() = makeTypeFromExpr(c, result.copyTree)
+    elif efNoUndeclared in flags:
+      result = nil
     elif efExplain notin flags:
       # repeat the overload resolution,
       # this time enabling all the diagnostic output (this should fail again)
       result = semOverloadedCall(c, n, nOrig, filter, flags + {efExplain})
-    elif efNoUndeclared notin flags:
-      result = nil
-      notFoundError(c, n, errors)
     else:
       result = nil
+      notFoundError(c, n, errors)
 
 proc explicitGenericInstError(c: PContext; n: PNode): PNode =
   localError(c.config, getCallLineInfo(n), errCannotInstantiateX % renderTree(n))
diff --git a/compiler/semdata.nim b/compiler/semdata.nim
index fa697f90cd8f..14ca22dcc597 100644
--- a/compiler/semdata.nim
+++ b/compiler/semdata.nim
@@ -17,7 +17,7 @@ when defined(nimPreviewSlimSystem):
 import
   options, ast, msgs, idents, renderer,
   magicsys, vmdef, modulegraphs, lineinfos, pathutils, layeredtable,
-  types, lowerings, trees, parampatterns
+  types, lowerings, trees, parampatterns, astalgo
 
 import ic / ic
 
@@ -42,7 +42,7 @@ type
     breakInLoop*: bool        # whether we are in a loop without block
     next*: PProcCon           # used for stacking procedure contexts
     mappingExists*: bool
-    mapping*: Table[ItemId, PSym]
+    mapping*: SymMapping
     caseContext*: seq[tuple[n: PNode, idx: int]]
     localBindStmts*: seq[PNode]
 
@@ -260,7 +260,7 @@ proc popProcCon*(c: PContext) {.inline.} = c.p = c.p.next
 
 proc put*(p: PProcCon; key, val: PSym) =
   if not p.mappingExists:
-    p.mapping = initTable[ItemId, PSym]()
+    p.mapping = initSymMapping()
     p.mappingExists = true
   #echo "put into table ", key.info
   p.mapping[key.itemId] = val
diff --git a/compiler/semtypinst.nim b/compiler/semtypinst.nim
index daee9ba4fc24..a615aeee94a8 100644
--- a/compiler/semtypinst.nim
+++ b/compiler/semtypinst.nim
@@ -68,8 +68,8 @@ type
   TReplTypeVars* = object
     c*: PContext
     typeMap*: LayeredIdTable  # map PType to PType
-    symMap*: SymMapping         # map PSym to PSym
-    localCache*: TypeMapping     # local cache for remembering already replaced
+    symMap*: SymMapping       # map PSym to PSym
+    localCache*: TypeMapping  # local cache for remembering already replaced
                               # types during instantiation of meta types
                               # (they are not stored in the global cache)
     info*: TLineInfo
diff --git a/compiler/transf.nim b/compiler/transf.nim
index 89911daf15bb..a2090af84186 100644
--- a/compiler/transf.nim
+++ b/compiler/transf.nim
@@ -40,7 +40,7 @@ import closureiters, lambdalifting
 
 type
   PTransCon = ref object # part of TContext; stackable
-    mapping: Table[ItemId, PNode]     # mapping from symbols to nodes
+    mapping: TIdTable[PNode]  # mapping from symbols to nodes
     owner: PSym               # current owner
     forStmt: PNode            # current for stmt
     forLoopBody: PNode   # transformed for loop body
@@ -78,7 +78,7 @@ proc newTransNode(kind: TNodeKind, n: PNode,
 
 proc newTransCon(owner: PSym): PTransCon =
   assert owner != nil
-  result = PTransCon(mapping: initTable[ItemId, PNode](), owner: owner)
+  result = PTransCon(mapping: initIdTable[PNode](), owner: owner)
 
 proc pushTransCon(c: PTransf, t: PTransCon) =
   t.next = c.transCon