fixes #17170 (#17171)

compiler/ccgcalls.nim

@@ -289,6 +289,7 @@ proc genArg(p: BProc, n: PNode, param: PSym; call: PNode, needsTmp = false): Rop
   else:
     initLocExprSingleUse(p, n, a)
     result = rdLoc(withTmpIfNeeded(p, a, needsTmp))
+  #assert result != nil
 
 proc genArgNoParam(p: BProc, n: PNode, needsTmp = false): Rope =
   var a: TLoc

compiler/closureiters.nim

@@ -427,6 +427,7 @@ proc addExprAssgn(ctx: Ctx, output, input: PNode, sym: PSym) =
 
 proc convertExprBodyToAsgn(ctx: Ctx, exprBody: PNode, res: PSym): PNode =
   result = newNodeI(nkStmtList, exprBody.info)
+  if exprBody.typ != nil:
     ctx.addExprAssgn(result, exprBody, res)
 
 proc newNotCall(g: ModuleGraph; e: PNode): PNode =

tests/arc/torcmisc.nim

@@ -0,0 +1,34 @@
+discard """
+  output: '''success'''
+  cmd: "nim c --gc:orc -d:release $file"
+"""
+
+# bug #17170
+
+when true:
+  import asyncdispatch
+
+  type
+    Flags = ref object
+      returnedEof, reading: bool
+
+  proc dummy(): Future[string] {.async.} =
+    result = "foobar"
+
+  proc hello(s: Flags) {.async.} =
+    let buf =
+      try:
+        await dummy()
+      except CatchableError as exc:
+        # When an exception happens here, the Bufferstream is effectively
+        # broken and no more reads will be valid - for now, return EOF if it's
+        # called again, though this is not completely true - EOF represents an
+        # "orderly" shutdown and that's not what happened here..
+        s.returnedEof = true
+        raise exc
+      finally:
+        s.reading = false
+
+  waitFor hello(Flags())
+  echo "success"
+