fixes #15360 [backport:1.2] (#15378)

compiler/semstmts.nim

@@ -718,7 +718,7 @@ proc semForVars(c: PContext, n: PNode; flags: TExprFlags): PNode =
   result = n
   let iterBase = n[^2].typ
   var iter = skipTypes(iterBase, {tyGenericInst, tyAlias, tySink, tyOwned})
-  var iterAfterVarLent = iter.skipTypes({tyLent, tyVar})
+  var iterAfterVarLent = iter.skipTypes({tyGenericInst, tyAlias, tyLent, tyVar})
   # n.len == 3 means that there is one for loop variable
   # and thus no tuple unpacking:
   if iterAfterVarLent.kind != tyTuple or n.len == 3:

tests/iter/titer.nim

@@ -55,3 +55,10 @@ const
 for i in 0..len(stringArray)-1:
   echo(stringArray[i])
 
+# bug #15360
+
+type Rule[T] = (int, T)
+
+var t: seq[Rule[int]]
+for (c, t) in t:
+  discard