iterators now return tyIter(T);

tyIter(T) represents an "iteration yielding values of type T"

I'm planning to use that in the context of the `is` operator
supporting predicates such as `C.items is iterator` and also
in the upcoming support for higher-order inline iterators.

compiler/cgen.nim

@@ -1192,7 +1192,7 @@ proc nullify[T](arr: var T) =
   for i in low(arr)..high(arr):
     arr[i] = nil
 
-proc resetModule*(m: var BModule) =
+proc resetModule*(m: BModule) =
   # between two compilations in CAAS mode, we can throw
   # away all the data that was written to disk
   initLinkedList(m.headerFiles)

compiler/cgendata.nim

@@ -146,7 +146,7 @@ proc newProc*(prc: PSym, module: BModule): BProc =
   result.nestedTryStmts = @[]
   result.finallySafePoints = @[]
 
-iterator cgenModules*: var BModule =
+iterator cgenModules*: BModule =
   for i in 0..high(gModules):
     # ultimately, we are iterating over the file ids here.
     # some "files" won't have an associated cgen module (like stdin)

compiler/semexprs.nim

@@ -1266,7 +1266,7 @@ proc semYield(c: PContext, n: PNode): PNode =
     n.sons[0] = semExprWithType(c, n.sons[0]) # check for type compatibility:
     var restype = c.p.owner.typ.sons[0]
     if restype != nil:
-      n.sons[0] = fitNode(c, restype, n.sons[0])
+      n.sons[0] = fitNode(c, restype.base, n.sons[0])
       if n.sons[0].typ == nil: internalError(n.info, "semYield")
       semYieldVarResult(c, n, restype)
     else:
@@ -1884,7 +1884,7 @@ proc semExpr(c: PContext, n: PNode, flags: TExprFlags = {}): PNode =
     message(n.info, warnDeprecated, "bind")
     result = semExpr(c, n.sons[0], flags)
   of nkTypeOfExpr, nkTupleTy, nkRefTy..nkEnumTy, nkStaticTy:
-    var typ = semTypeNode(c, n, nil).skipTypes({tyTypeDesc})
+    var typ = semTypeNode(c, n, nil).skipTypes({tyTypeDesc, tyIter})
     result.typ = makeTypeDesc(c, typ)
     #result = symNodeFromType(c, typ, n.info)
   of nkCall, nkInfix, nkPrefix, nkPostfix, nkCommand, nkCallStrLit: 

compiler/semstmts.nim

@@ -616,7 +616,8 @@ proc symForVar(c: PContext, n: PNode): PSym =
 proc semForVars(c: PContext, n: PNode): PNode =
   result = n
   var length = sonsLen(n)
-  var iter = skipTypes(n.sons[length-2].typ, {tyGenericInst})
+  let iterBase = n.sons[length-2].typ.skipTypes({tyIter})
+  var iter = skipTypes(iterBase, {tyGenericInst})
   # length == 3 means that there is one for loop variable
   # and thus no tuple unpacking:
   if iter.kind != tyTuple or length == 3: 
@@ -626,7 +627,7 @@ proc semForVars(c: PContext, n: PNode): PNode =
       # BUGFIX: don't use `iter` here as that would strip away
       # the ``tyGenericInst``! See ``tests/compile/tgeneric.nim``
       # for an example:
-      v.typ = n.sons[length-2].typ
+      v.typ = iterBase
       n.sons[0] = newSymNode(v)
       if sfGenSym notin v.flags: addForVarDecl(c, v)
     else:

compiler/semtypes.nim

@@ -845,8 +845,10 @@ proc semProcTypeNode(c: PContext, n, genericParams: PNode,
                                    n.sons[0].info)
         if lifted != nil: r = lifted
         r.flags.incl tfRetType
-      result.sons[0] = skipIntLit(r)
-      res.typ = result.sons[0]
+      r = skipIntLit(r)
+      if kind == skIterator: r = newTypeWithSons(c, tyIter, @[r])
+      result.sons[0] = r
+      res.typ = r
 
 proc semStmtListType(c: PContext, n: PNode, prev: PType): PType =
   checkMinSonsLen(n, 1)
@@ -959,7 +961,7 @@ proc semTypeNode(c: PContext, n: PNode, prev: PType): PType =
   of nkTypeOfExpr:
     # for ``type(countup(1,3))``, see ``tests/ttoseq``.
     checkSonsLen(n, 1)
-    result = semExprWithType(c, n.sons[0], {efInTypeof}).typ
+    result = semExprWithType(c, n.sons[0], {efInTypeof}).typ.skipTypes({tyIter})
   of nkPar: 
     if sonsLen(n) == 1: result = semTypeNode(c, n.sons[0], prev)
     else:

compiler/sigmatch.nim

@@ -921,14 +921,18 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
         result = typeRel(c, prev.base, a.base)
       else:
         result = isNone
-  
+ 
+  of tyIter:
+    if a.kind == f.kind: result = typeRel(c, f.base, a.base)
+    else: result = isNone
+
   of tyStmt:
     result = isGeneric
   
   of tyProxy:
     result = isEqual
   
-  else: internalError("typeRel: " & $f.kind)
+  else: internalAssert false
   
 proc cmpTypes*(c: PContext, f, a: PType): TTypeRelation = 
   var m: TCandidate