fix #15463 (#15831)

(cherry picked from commit a8af664e8b)

compiler/lowerings.nim

@@ -136,6 +136,7 @@ proc lowerSwap*(g: ModuleGraph; n: PNode; owner: PSym): PNode =
   var temp = newSym(skVar, getIdent(g.cache, genPrefix), owner, n.info, owner.options)
   temp.typ = n[1].typ
   incl(temp.flags, sfFromGeneric)
+  incl(temp.flags, sfGenSym)
 
   var v = newNodeI(nkVarSection, n.info)
   let tempAsNode = newSymNode(temp)

compiler/vmgen.nim

@@ -1122,7 +1122,7 @@ proc genMagic(c: PCtx; n: PNode; dest: var TDest; m: TMagic) =
     c.freeTemp(d)
   of mSwap:
     unused(c, n, dest)
-    c.gen(lowerSwap(c.graph, n, if c.prc == nil: c.module else: c.prc.sym))
+    c.gen(lowerSwap(c.graph, n, if c.prc == nil or c.prc.sym == nil: c.module else: c.prc.sym))
   of mIsNil: genUnaryABC(c, n, dest, opcIsNil)
   of mParseBiggestFloat:
     if dest < 0: dest = c.getTemp(n.typ)

tests/vm/tswap.nim

@@ -3,7 +3,9 @@ nimout: '''
 x.data = @[10]
 y = @[11]
 x.data = @[11]
-y = @[10]'''
+y = @[10]
+@[3, 2, 1]
+'''
 """
 
 # bug #2946
@@ -22,3 +24,11 @@ proc testSwap(): int {.compiletime.} =
   result = 99
 
 const something = testSwap()
+
+# bug #15463
+block:
+  static:
+    var s = @[1, 2, 3]
+    swap(s[0], s[2])
+
+    echo s