fixes #12171 (#12173)

tests/pragmas/t8741.nim

@@ -1,6 +1,10 @@
 discard """
-  errormsg: "cannot attach a custom pragma to 'a'"
-  line: 9
+  cmd: "nim check --hint[processing]:off $file"
+  errormsg: "3 is not two"
+  nimout: '''t8741.nim(13, 9) Error: cannot attach a custom pragma to 'a'
+t8741.nim(29, 15) template/generic instantiation of `onlyTwo` from here
+t8741.nim(25, 12) Error: 3 is not two
+'''
 """
 
 for a {.gensym, inject.} in @[1,2,3]:
@@ -8,3 +12,18 @@ for a {.gensym, inject.} in @[1,2,3]:
 
 for a {.foobar.} in @[1,2,3]:
   discard
+
+type Foo[N: static[int]] = distinct int
+
+proc isTwo(n: int): bool =
+  n == 2
+
+proc onlyTwo[N: static[int]](a: Foo[N]): int =
+  when isTwo(N):
+    int(a)
+  else:
+    {.error: $(N) & " is not two".}
+
+when isMainModule:
+  let foo: Foo[3] = Foo[3](5)
+  echo onlyTwo(foo)