Fix codegen for mInSet magic and empty sets (#8140)

Fixes #8041

compiler/ccgexprs.nim

@@ -1587,8 +1587,9 @@ proc genInOp(p: BProc, e: PNode, d: var TLoc) =
                e.sons[2]
     initLocExpr(p, ea, a)
     initLoc(b, locExpr, e, OnUnknown)
-    b.r = rope("(")
     var length = sonsLen(e.sons[1])
+    if length > 0:
+      b.r = rope("(")
       for i in countup(0, length - 1):
         let it = e.sons[1].sons[i]
         if it.kind == nkRange:
@@ -1601,6 +1602,9 @@ proc genInOp(p: BProc, e: PNode, d: var TLoc) =
           addf(b.r, "$1 == $2", [rdCharLoc(a), rdCharLoc(x)])
         if i < length - 1: add(b.r, " || ")
       add(b.r, ")")
+    else:
+      # handle the case of an empty set
+      b.r = rope("0")
     putIntoDest(p, d, e, b.r)
   else:
     assert(e.sons[1].typ != nil)

tests/iter/t8041.nim

@@ -0,0 +1,7 @@
+iterator xy[T](a: T, b: set[T]): T =
+  if a in b:
+    yield a
+
+for a in xy(1'i8, {}):
+  for b in xy(a, {}):
+    echo a