fix #15704 wrong VM register was freed

Author: timotheecour

compiler/vmgen.nim

@@ -1515,7 +1515,7 @@ proc genAsgn(c: PCtx; le, ri: PNode; requiresCopy: bool) =
     let idx = genField(c, le[1])
     let tmp = c.genx(ri)
     c.preventFalseAlias(le, opcWrObj, dest, idx, tmp)
-    c.freeTemp(idx)
+    # c.freeTemp(idx) # BUGFIX: idx is an immediate (field position), not a register
     c.freeTemp(tmp)
     c.freeTemp(dest)
   of nkDerefExpr, nkHiddenDeref:

tests/vm/tvmmisc.nim

@@ -281,3 +281,45 @@ block: # bug #8007
   # OK with seq & object variants
   const d = @[Cost(kind: Fixed, cost: 999), Cost(kind: Dynamic, handler: foo)]
   doAssert $d == "@[(kind: Fixed, cost: 999), (kind: Dynamic, handler: ...)]"
+
+
+block: # bug #15704
+  proc bar2(head: string): string = "asdf"
+  proc gook(u1: int) = discard
+
+  type PathEntry = object
+    kind: int
+    path: string
+
+  iterator globOpt(): int =
+    var u1: int
+
+    gook(u1)
+    gook(u1)
+    gook(u1)
+    gook(u1)
+    gook(u1)
+    gook(u1)
+    gook(u1)
+    gook(u1)
+    gook(u1)
+    gook(u1)
+    gook(u1)
+    gook(u1)
+    gook(u1)
+    gook(u1)
+
+    var entry = PathEntry()
+    entry.path = bar2("")
+    echo "here2"
+
+  proc processAux(a: float) = discard
+
+  template bar(iter: untyped): untyped =
+    var ret: float
+    for x in iter: break
+    ret
+
+  proc main() =
+    processAux(bar(globOpt()))
+  static: main()