fixes nim-lang#12956 (nim-lang#13020)

Author: Araq

compiler/closureiters.nim

@@ -838,7 +838,8 @@ proc transformReturnsInTry(ctx: var Ctx, n: PNode): PNode =
     if n[0].kind != nkEmpty:
       let asgnTmpResult = newNodeI(nkAsgn, n.info)
       asgnTmpResult.add(ctx.newTmpResultAccess())
-      asgnTmpResult.add(n[0])
+      let x = if n[0].kind in {nkAsgn, nkFastAsgn}: n[0][1] else: n[0]
+      asgnTmpResult.add(x)
       result.add(asgnTmpResult)
 
     result.add(ctx.newNullifyCurExc(n.info))

compiler/injectdestructors.nim

@@ -227,7 +227,7 @@ proc genCopy(c: var Con; dest, ri: PNode): PNode =
     # try to improve the error message here:
     if c.otherRead == nil: discard isLastRead(ri, c)
     checkForErrorPragma(c, t, ri, "=")
-  genCopyNoCheck(c, dest, ri)
+  result = genCopyNoCheck(c, dest, ri)
 
 proc genDestroy(c: Con; dest: PNode): PNode =
   let t = dest.typ.skipTypes({tyGenericInst, tyAlias, tySink})
@@ -571,6 +571,7 @@ proc p(n: PNode; c: var Con; mode: ProcessMode): PNode =
         else:
           if n[0].kind in {nkDotExpr, nkCheckedFieldExpr}:
             cycleCheck(n, c)
+          assert n[1].kind notin {nkAsgn, nkFastAsgn}
           result = moveOrCopy(n[0], n[1], c, isFirstWrite = false)
       else:
         result = copyNode(n)