iterable[T] (#17196)

* fix failing test toSeq in manual which now works
* changelog
* reject proc fn(a: iterable)
* add iterable to spec
* remove MCS/UFCS limitation that now works

compiler/semcall.nim

@@ -62,7 +62,7 @@ proc pickBestCandidate(c: PContext, headSymbol: PNode,
                        best, alt: var TCandidate,
                        errors: var CandidateErrors,
                        diagnosticsFlag: bool,
-                       errorsEnabled: bool) =
+                       errorsEnabled: bool, flags: TExprFlags) =
   var o: TOverloadIter
   var sym = initOverloadIter(o, c, headSymbol)
   var scope = o.lastOverloadScope
@@ -95,7 +95,11 @@ proc pickBestCandidate(c: PContext, headSymbol: PNode,
       matches(c, n, orig, z)
       if z.state == csMatch:
         # little hack so that iterators are preferred over everything else:
-        if sym.kind == skIterator: inc(z.exactMatches, 200)
+        if sym.kind == skIterator:
+          if not (efWantIterator notin flags and efWantIterable in flags):
+            inc(z.exactMatches, 200)
+          else:
+            dec(z.exactMatches, 200)
         case best.state
         of csEmpty, csNoMatch: best = z
         of csMatch:
@@ -356,7 +360,7 @@ proc resolveOverloads(c: PContext, n, orig: PNode,
   template pickBest(headSymbol) =
     pickBestCandidate(c, headSymbol, n, orig, initialBinding,
                       filter, result, alt, errors, efExplain in flags,
-                      errorsEnabled)
+                      errorsEnabled, flags)
   pickBest(f)
 
   let overloadsState = result.state