fix #13417 (#13712)

* fix #13417

* add test

compiler/transf.nim

@@ -554,14 +554,14 @@ proc putArgInto(arg: PNode, formal: PType): TPutArgInto =
   # inline context.
   if formal.kind == tyTypeDesc: return paDirectMapping
   if skipTypes(formal, abstractInst).kind in {tyOpenArray, tyVarargs}:
-    case arg.kind
+    case arg.skipHidden.kind
-    of nkStmtListExpr:
-      return paComplexOpenarray
     of nkBracket:
       return paFastAsgnTakeTypeFromArg
+    of nkSym:
+      return paDirectMapping
     else:
-      return paDirectMapping    # XXX really correct?
+      return paComplexOpenarray
-                                # what if ``arg`` has side-effects?
+
   case arg.kind
   of nkEmpty..nkNilLit:
     result = paDirectMapping
@@ -644,7 +644,7 @@ proc transformFor(c: PTransf, n: PNode): PNode =
   # generate access statements for the parameters (unless they are constant)
   pushTransCon(c, newC)
   for i in 1..<call.len:
-    var arg = transform(c, call[i])
+    let arg = transform(c, call[i])
     let ff = skipTypes(iter.typ, abstractInst)
     # can happen for 'nim check':
     if i >= ff.n.len: return result
@@ -671,9 +671,8 @@ proc transformFor(c: PTransf, n: PNode): PNode =
       idNodeTablePut(newC.mapping, formal, arg)
       # XXX BUG still not correct if the arg has a side effect!
     of paComplexOpenarray:
-      let typ = newType(tySequence, formal.owner)
+      # arrays will deep copy here (pretty bad).
-      addSonSkipIntLit(typ, formal.typ[0])
+      var temp = newTemp(c, arg.typ, formal.info)
-      var temp = newTemp(c, typ, formal.info)
       addVar(v, temp)
       stmtList.add(newAsgnStmt(c, nkFastAsgn, temp, arg))
       idNodeTablePut(newC.mapping, formal, temp)

tests/iter/titer_issues.nim

@@ -213,3 +213,24 @@ block t2023_objiter:
 
   var o = init()
   echo(o.iter())
+
+block:
+  # bug #13417
+
+  var effectCounterP1 = 0
+
+  proc p1(): seq[int] =
+     inc effectCounterP1
+     @[1,2]
+
+  iterator ip1(v: openArray[int]): auto =
+     for x in v:
+        yield x
+
+  var effectCounterLoop = 0
+
+  for x in ip1(p1()):
+    inc effectCounterLoop
+
+  doAssert effectCounterP1 == 1
+  doAssert effectCounterLoop == 2