Test trait member dispatch

Tests/EndToEndTests/TestCases/TraitMemberDispatch.hylo

@@ -0,0 +1,30 @@
+//- compileAndRun expecting: success
+
+// This program tests the dispatch behavior of calls to trait members. `P` declares a single
+// requirement `f` that is given a default implementation. Two conformances to `P` are defined:
+// `Int: P` "overrides" the default implementation of `f` and `Bool: P` uses it. `call_f(on:)`
+// accepts a generic parameter bound by `P` and calls `f`. Because it is generic, the compiler
+// can't determine how to dispatch `f` before depolymoprhization.
+
+trait P {
+  fun f() -> Int
+}
+
+extension P {
+  public fun f() -> Int { 0 }
+}
+
+conformance Bool: P {}
+
+conformance Int: P {
+  public fun f() -> Int { self.copy() }
+}
+
+fun call_f<T: P>(on x: T) -> Int {
+  x.f()
+}
+
+public fun main() {
+  precondition(call_f(on: 1234).f() == 1234)
+  precondition(call_f(on: true).f() == 0)
+}