add test to close #7209 (#22110)

* add test to close #7209

was fixed by #22029

* fix echo => doAssert

tests/statictypes/tstaticgenericparam.nim

@@ -9,3 +9,16 @@ block: # issue #19365
   doAssert ss == @["123: int"]
   f("abc")
   doAssert ss == @["123: int", "abc: string"]
+
+block: # issue #7209
+  type Modulo[A; M: static[A]] = distinct A
+
+  proc `$`[A; M: static[A]](x: Modulo[A, M]): string =
+    $(A(x)) & " mod " & $(M)
+
+  proc modulo[A](a: A, M: static[A]): Modulo[A, M] = Modulo[A, M](a %% M)
+
+  proc `+`[A; M: static[A]](x, y: Modulo[A, M]): Modulo[A, M] =
+    (A(x) + A(y)).modulo(M)
+
+  doAssert $(3.modulo(7) + 5.modulo(7)) == "1 mod 7"