Add regression test for issue 2408.

Fixes #2408.

test/libponyc-run/regression-2408/main.pony

@@ -0,0 +1,24 @@
+interface SafeOps[T]
+  fun addc(t: T): (T, Bool)
+  fun subc(t: T): (T, Bool)
+
+class GenericSum[T: (Int & Integer[T] val & SafeOps[T])]
+  fun _plus_safe(x: T, y: T): (T, Bool) =>
+    x.addc(y)
+
+  fun sum(x: T, y: T): T ? =>
+    match _plus_safe(x, y)
+    // In the issue 2408 ponyc < 0.26 was reporting this pattern to never match
+    | (let res: T, false) => res
+    | (_, true) => error
+    else
+      // this else is only needed because ponyc does not recognize
+      // the match above as exhaustive
+      error
+    end
+
+actor Main
+  new create(env: Env) =>
+    try
+      GenericSum[U8].sum(1, 2)?
+    end