add tests for alias bound preference

tests/ui/traits/new-solver/alias-bound-preference.rs

@@ -0,0 +1,39 @@
+// revisions: old next
+//[next] compile-flags: -Ztrait-solver=next
+// run-pass
+
+// A test for https://github.com/rust-lang/trait-system-refactor-initiative/issues/45.
+
+trait Trait {
+    type Assoc: Into<u32>;
+}
+impl<T: Into<u32>> Trait for T {
+    type Assoc = T;
+}
+fn prefer_alias_bound_projection<T: Trait>(x: T::Assoc) {
+    // There are two possible types for `x`:
+    // - `u32` by using the "alias bound" of `<T as Trait>::Assoc`
+    // - `<T as Trait>::Assoc`, i.e. `u16`, by using `impl<T> From<T> for T`
+    //
+    // We infer the type of `x` to be `u32` here as it is highly likely
+    // that this is expected by the user.
+    let x = x.into();
+    assert_eq!(std::mem::size_of_val(&x), 4);
+}
+
+fn impl_trait() -> impl Into<u32> {
+    0u16
+}
+
+fn main() {
+    // There are two possible types for `x`:
+    // - `u32` by using the "alias bound" of `impl Into<u32>`
+    // - `impl Into<u32>`, i.e. `u16`, by using `impl<T> From<T> for T`
+    //
+    // We infer the type of `x` to be `u32` here as it is highly likely
+    // that this is expected by the user.
+    let x = impl_trait().into();
+    assert_eq!(std::mem::size_of_val(&x), 4);
+
+    prefer_alias_bound_projection::<u16>(1);
+}