fix #13739 (#13742)

compiler/transf.nim

@@ -560,8 +560,8 @@ proc putArgInto(arg: PNode, formal: PType): TPutArgInto =
     of nkBracket:
       return paFastAsgnTakeTypeFromArg
     else:
-      return paDirectMapping    # XXX really correct?
+      # XXX incorrect, causes #13417 when `arg` has side effects.
-                                # what if ``arg`` has side-effects?
+      return paDirectMapping
   case arg.kind
   of nkEmpty..nkNilLit:
     result = paDirectMapping
@@ -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

@@ -18,6 +18,15 @@ end
 1
 2
 7
+9002
+9004
+9006
+9008
+9010
+9012
+9014
+9016
+9018
 '''
 """
 
@@ -213,3 +222,21 @@ block t2023_objiter:
 
   var o = init()
   echo(o.iter())
+
+
+block:
+  # issue #13739
+  iterator myIter(arg: openarray[int]): int =
+    var tmp = 0
+    let len = arg.len
+    while tmp < len:
+      yield arg[tmp] * 2
+      inc tmp
+
+  proc someProc() =
+    var data = [4501,4502,4503,4504,4505,4506,4507,4508,4509]
+    # StmtListExpr should not get special treatment.
+    for x in myIter((discard;data)):
+      echo x
+
+  someProc()