t7868: typechecking patters with numerics(weak subptyping?)

[DarkDimius]

[info] compiling /Users/dark/workspace/dotty/tests/partest-generated/run/t7868.scala
[info] options: -pagewidth 160 -Yno-deep-subtypes -Yno-double-bindings -Ycheck:tailrec,resolveSuper,mixin,restoreScopes -d ./tests/partest-generated/run/t7868-run.obj
[info] /Users/dark/workspace/dotty/tests/partest-generated/run/t7868.scala:11: error: Pattern type Int is neither a subtype nor a supertype of selector type Short
[info]     case A(_) =>
[info]           ^