diff --git a/compiler/ast.nim b/compiler/ast.nim
index 556df74080b0..bc28cff845ca 100644
--- a/compiler/ast.nim
+++ b/compiler/ast.nim
@@ -549,18 +549,28 @@ proc addAllowNil*(father, son: PNode) {.inline.} =
   father.sons.add(son)
 
 proc add*(father, son: PType) =
+  assert father.kind != tyProc or father.sonsImpl.len == 0
   assert son != nil
   father.sonsImpl.add son
 
 proc addAllowNil*(father, son: PType) {.inline.} =
+  assert father.kind != tyProc or father.sonsImpl.len == 0
   father.sonsImpl.add son
 
 template `[]`*(n: PType, i: int): PType =
   if n.state == Partial: loadType(n)
-  n.sonsImpl[i]
+  if n.kind == tyProc and i > 0:
+    assert n.nImpl[i] != nil and n.nImpl[i].sym != nil
+    n.nImpl[i].sym.typ
+  else:
+    n.sonsImpl[i]
 template `[]=`*(n: PType, i: int; x: PType) =
   if n.state == Partial: loadType(n)
-  n.sonsImpl[i] = x
+  if n.kind == tyProc and i > 0:
+    assert n.nImpl[i] != nil and n.nImpl[i].sym != nil
+    n.nImpl[i].sym.typ = x
+  else:
+    n.sonsImpl[i] = x
 
 template `[]`*(n: PType, i: BackwardsIndex): PType =
   if n.state == Partial: loadType(n)
@@ -806,7 +816,10 @@ proc replaceSon*(n: PNode; i: int; newson: PNode) {.inline.} =
 
 proc last*(n: PType): PType {.inline.} =
   if n.state == Partial: loadType(n)
-  n.sonsImpl[^1]
+  if n.kind == tyProc and n.nImpl.len > 1:
+    n.nImpl[^1].sym.typ
+  else:
+    n.sonsImpl[^1]
 
 proc elementType*(n: PType): PType {.inline.} =
   if n.state == Partial: loadType(n)
@@ -842,7 +855,10 @@ proc setIndexType*(n, idx: PType) {.inline.} =
 
 proc firstParamType*(n: PType): PType {.inline.} =
   if n.state == Partial: loadType(n)
-  n.sonsImpl[1]
+  if n.kind == tyProc:
+    n.nImpl[1].sym.typ
+  else:
+    n.sonsImpl[1]
 
 proc firstGenericParam*(n: PType): PType {.inline.} =
   if n.state == Partial: loadType(n)
@@ -914,10 +930,13 @@ proc `$`*(s: PSym): string =
     result = "<nil>"
 
 proc len*(n: PType): int {.inline.} =
-  result = n.sonsImpl.len
+  if n.kind == tyProc:
+    result = if n.nImpl == nil: 0 else: n.nImpl.len
+  else:
+    result = n.sonsImpl.len
 
 proc sameTupleLengths*(a, b: PType): bool {.inline.} =
-  result = a.sonsImpl.len == b.sonsImpl.len
+  result = a.len == b.len
 
 iterator tupleTypePairs*(a, b: PType): (int, PType, PType) =
   for i in 0 ..< a.len:
@@ -1012,15 +1031,20 @@ proc newType*(kind: TTypeKind; idgen: IdGenerator; owner: PSym; son: sink PType
                  alignImpl: defaultAlignment, itemId: id,
                  uniqueId: id, sonsImpl: @[])
   if son != nil:
+    assert kind != tyProc
     result.sonsImpl.add son
   when false:
     if result.itemId.module == 55 and result.itemId.item == 2:
       echo "KNID ", kind
       writeStackTrace()
 
-proc setSons*(dest: PType; sons: sink seq[PType]) {.inline.} = dest.sonsImpl = sons
-proc setSon*(dest: PType; son: sink PType) {.inline.} = dest.sonsImpl = @[son]
+proc setSons*(dest: PType; sons: sink seq[PType]) {.inline.} =
+  assert dest.kind != tyProc or sons.len <= 1
+  dest.sonsImpl = sons
+proc setSon*(dest: PType; son: sink PType) {.inline.} =
+  dest.sonsImpl = @[son]
 proc setSonsLen*(dest: PType; len: int) {.inline.} =
+  assert dest.kind != tyProc or len <= 1
   setLen(dest.sonsImpl, len)
 
 proc mergeLoc(a: var TLoc, b: TLoc) =
@@ -1034,6 +1058,7 @@ proc newSons*(father: PNode, length: int) =
   setLen(father.sons, length)
 
 proc newSons*(father: PType, length: int) =
+  assert father.kind != tyProc or length <= 1
   setLen(father.sonsImpl, length)
 
 proc truncateInferredTypeCandidates*(t: PType) {.inline.} =
@@ -1058,8 +1083,16 @@ proc assignType*(dest, src: PType) =
       mergeLoc(dest.sym.locImpl, src.sym.loc)
     else:
       dest.symImpl = src.sym
-  newSons(dest, src.len)
-  for i in 0..<src.len: dest[i] = src[i]
+  if src.kind == tyProc:
+    # `tyProc` uses only `sonsImpl[0]` to store return type.
+    # parameter symbols and types are stored in `nImpl`.
+    assert src.sonsImpl.len <= 1
+    if src.len > 0:
+      setLen(dest.sonsImpl, 1)
+      dest.sonsImpl[0] = src.sonsImpl[0]
+  else:
+    newSons(dest, src.len)
+    for i in 0..<src.len: dest[i] = src[i]
 
 proc copyType*(t: PType, idgen: IdGenerator, owner: PSym): PType =
   result = newType(t.kind, idgen, owner)
@@ -1169,7 +1202,8 @@ proc propagateToOwner*(owner, elem: PType; propagateHasAsgn = true) =
 
 proc rawAddSon*(father, son: PType; propagateHasAsgn = true) =
   ensureMutable father
-  father.sonsImpl.add(son)
+  if father.kind != tyProc or father.sonsImpl.len == 0:
+    father.sonsImpl.add(son)
   if not son.isNil: propagateToOwner(father, son, propagateHasAsgn)
 
 proc addSonNilAllowed*(father, son: PNode) =
@@ -1575,7 +1609,7 @@ proc newProcType*(info: TLineInfo; idgen: IdGenerator; owner: PSym): PType =
   result.n.add newNodeI(nkEffectList, info)
 
 proc addParam*(procType: PType; param: PSym) =
-  param.position = procType.sons.len-1
+  param.position = procType.n.len - 1
   procType.n.add newSymNode(param)
   rawAddSon(procType, param.typ)
 
diff --git a/compiler/ic/ic.nim b/compiler/ic/ic.nim
index 81877f07944e..3249482f6054 100644
--- a/compiler/ic/ic.nim
+++ b/compiler/ic/ic.nim
@@ -370,8 +370,14 @@ proc storeType(t: PType; c: var PackedEncoder; m: var PackedModule): PackedItemI
       paddingAtEnd: t.paddingAtEnd)
     storeNode(p, t, n)
     p.typeInst = t.typeInst.storeType(c, m)
-    for kid in kids t:
-      p.types.add kid.storeType(c, m)
+    if t.kind == tyProc and t.len > 0:
+      # if kind == tyProc, parameter types are stored in t.n
+      # and you can access them with `kits` iterator.
+      # return type is stored in t.sons[0].
+      p.types.add t[0].storeType(c, m)
+    else:
+      for kid in kids t:
+        p.types.add kid.storeType(c, m)
     c.addMissing t.sym
     p.sym = t.sym.safeItemId(c, m)
     c.addMissing t.owner
diff --git a/compiler/seminst.nim b/compiler/seminst.nim
index b34c7ef58eff..a34467636a3b 100644
--- a/compiler/seminst.nim
+++ b/compiler/seminst.nim
@@ -244,7 +244,8 @@ proc instantiateProcType(c: PContext, pt: LayeredIdTable,
   var result = instCopyType(cl, prc.typ)
   let originalParams = result.n
   result.n = originalParams.shallowCopy
-  for i, resulti in paramTypes(result):
+  for i in 1 ..< originalParams.len:
+    let resulti = originalParams[i].sym.typ
     # twrong_field_caching requires these 'resetIdTable' calls:
     if i > FirstParamAt:
       resetIdTable(cl.symMap)
@@ -258,23 +259,23 @@ proc instantiateProcType(c: PContext, pt: LayeredIdTable,
     let needsTypeDescSkipping = resulti.kind == tyTypeDesc and tfUnresolved in resulti.flags
     if resulti.kind == tyFromExpr:
       resulti.incl tfNonConstExpr
-    result[i] = replaceTypeVarsT(cl, resulti)
+    var paramType = replaceTypeVarsT(cl, resulti)
     if needsStaticSkipping:
-      result[i] = result[i].skipTypes({tyStatic})
+      paramType = paramType.skipTypes({tyStatic})
     if needsTypeDescSkipping:
-      result[i] = result[i].skipTypes({tyTypeDesc})
-      typeToFit = result[i]
+      paramType = paramType.skipTypes({tyTypeDesc})
+      typeToFit = paramType
 
     # ...otherwise, we use the instantiated type in `fitNode`
     if (typeToFit.kind != tyTypeDesc or typeToFit.base.kind != tyNone) and
        (typeToFit.kind != tyStatic):
-      typeToFit = result[i]
+      typeToFit = paramType
 
     internalAssert c.config, originalParams[i].kind == nkSym
     let oldParam = originalParams[i].sym
     let param = copySym(oldParam, c.idgen)
     setOwner(param, prc)
-    param.typ = result[i]
+    param.typ = paramType
 
     # The default value is instantiated and fitted against the final
     # concrete param type. We avoid calling `replaceTypeVarsN` on the
@@ -305,12 +306,12 @@ proc instantiateProcType(c: PContext, pt: LayeredIdTable,
         param.ast.typ = def.typ
       else:
         param.ast = fitNodePostMatch(c, typeToFit, converted)
-      param.typ = result[i]
+      param.typ = paramType
 
     result.n[i] = newSymNode(param)
-    if isRecursiveStructuralType(result[i]):
+    if isRecursiveStructuralType(paramType):
       localError(c.config, originalParams[i].sym.info, "illegal recursion in type '" & typeToString(result[i]) & "'")
-    propagateToOwner(result, result[i])
+    propagateToOwner(result, paramType)
     addDecl(c, param)
 
   resetIdTable(cl.symMap)
diff --git a/compiler/semtypinst.nim b/compiler/semtypinst.nim
index 031683d04ab5..5b6620810591 100644
--- a/compiler/semtypinst.nim
+++ b/compiler/semtypinst.nim
@@ -542,14 +542,12 @@ proc eraseVoidParams*(t: PType) =
 
   for i in FirstParamAt..<t.signatureLen:
     # don't touch any memory unless necessary
-    if t[i].kind == tyVoid:
+    if t.n[i].kind == nkRecList or t[i].kind == tyVoid:
       var pos = i
       for j in i+1..<t.signatureLen:
         if t[j].kind != tyVoid:
-          t[pos] = t[j]
           t.n[pos] = t.n[j]
           inc pos
-      newSons t, pos
       setLen t.n.sons, pos
       break
 
@@ -743,7 +741,8 @@ proc replaceTypeVarsTAux(cl: var TReplTypeVars, t: PType, isInstValue = false):
             let r2 = r.skipTypes({tyAlias, tySink, tyOwned})
             if r2.kind in {tyPtr, tyRef}:
               r = skipTypes(r2, {tyPtr, tyRef})
-          result[i] = r
+          if result.kind != tyProc or i == 0:
+            result[i] = r
           if result.kind != tyArray or i != 0:
             propagateToOwner(result, r)
       # bug #4677: Do not instantiate effect lists
diff --git a/compiler/sigmatch.nim b/compiler/sigmatch.nim
index 5b38f99b6d4d..145d9ed1036a 100644
--- a/compiler/sigmatch.nim
+++ b/compiler/sigmatch.nim
@@ -233,13 +233,11 @@ proc copyingEraseVoidParams(m: TCandidate, t: var PType) =
       if not copied:
         # keep first i children
         t = copyType(original, m.c.idgen, t.owner)
-        t.setSonsLen(i)
         t.n = copyNode(original.n)
         t.n.sons = original.n.sons
         t.n.sons.setLen(i)
         copied = true
     elif copied:
-      t.add(f)
       t.n.add(original.n[i])
 
 proc initCandidate*(ctx: PContext, callee: PSym,
diff --git a/compiler/sinkparameter_inference.nim b/compiler/sinkparameter_inference.nim
index 1e025d1ac97a..3c6e4cf09d66 100644
--- a/compiler/sinkparameter_inference.nim
+++ b/compiler/sinkparameter_inference.nim
@@ -38,7 +38,7 @@ proc checkForSink*(config: ConfigRef; idgen: IdGenerator; owner: PSym; arg: PNod
         sinkType.add argType
 
         arg.sym.typ = sinkType
-        owner.typ[arg.sym.position+1] = sinkType
+        assert owner.typ.n[arg.sym.position+1].sym == arg.sym
 
         #message(config, arg.info, warnUser,
         #  ("turned '$1' to a sink parameter") % [$arg])