fixes #21317; 1.6.4 regression; etyBaseIndex should return fat pointers [backport 1.6] (#21320)

fixes #21317; regression; etyBaseIndex should return fat pointers

(cherry picked from commit cbf3ed9d92)

compiler/jsgen.nim

@@ -2393,17 +2393,18 @@ proc genProc(oldProc: PProc, prc: PSym): Rope =
   if prc.typ[0] != nil and sfPure notin prc.flags:
     resultSym = prc.ast[resultPos].sym
     let mname = mangleName(p.module, resultSym)
-    let returnAddress = not isIndirect(resultSym) and
+    # otherwise uses "fat pointers"
+    let useRawPointer = not isIndirect(resultSym) and
       resultSym.typ.kind in {tyVar, tyPtr, tyLent, tyRef, tyOwned} and
         mapType(p, resultSym.typ) == etyBaseIndex
-    if returnAddress:
+    if useRawPointer:
       resultAsgn = p.indentLine(("var $# = null;$n") % [mname])
       resultAsgn.add p.indentLine("var $#_Idx = 0;$n" % [mname])
     else:
       let resVar = createVar(p, resultSym.typ, isIndirect(resultSym))
       resultAsgn = p.indentLine(("var $# = $#;$n") % [mname, resVar])
     gen(p, prc.ast[resultPos], a)
-    if returnAddress:
+    if mapType(p, resultSym.typ) == etyBaseIndex:
       returnStmt = "return [$#, $#];$n" % [a.address, a.res]
     else:
       returnStmt = "return $#;$n" % [a.res]

tests/system/trefs.nim

@@ -0,0 +1,15 @@
+discard """
+  targets: "c js"
+"""
+
+# bug #21317
+proc parseHook*(v: var ref int) =
+  var a: ref int
+  new(a)
+  a[] = 123
+  v = a
+
+proc fromJson2*(): ref int =
+  parseHook(result)
+
+doAssert fromJson2()[] == 123