fix #21109 (#21127)

compiler/semexprs.nim

@@ -1947,6 +1947,8 @@ proc semYieldVarResult(c: PContext, n: PNode, restype: PType) =
             tupleConstr[i] = takeImplicitAddr(c, tupleConstr[i], e.kind == tyLent)
         else:
           localError(c.config, n[0].info, errXExpected, "tuple constructor")
+      elif e.kind == tyEmpty:
+        localError(c.config, n[0].info, errTypeExpected)
   else:
     when false:
       # XXX investigate what we really need here.

tests/misc/t21109.nim

@@ -0,0 +1,13 @@
+discard """
+  action: reject
+  errormsg: "type expected"
+  file: "iterators.nim"
+"""
+
+
+template b(j: untyped) = j
+template m() = discard
+
+b:
+  for t, f in @[]:
+    m()