add tests for pyella regression

tests/ui/coherence/super-traits/super-trait-knowable-1.rs

@@ -0,0 +1,14 @@
+// Added in #124532. While `(): Super` is knowable, `(): Sub<?t>` is not.
+//
+// We therefore elaborate super trait bounds in the implicit negative
+// overlap check.
+
+trait Super {}
+trait Sub<T>: Super {}
+
+trait Overlap<T> {}
+impl<T, U: Sub<T>> Overlap<T> for U {}
+impl<T> Overlap<T> for () {}
+//~^ ERROR conflicting implementations of trait `Overlap<_>` for type `()`
+
+fn main() {}

tests/ui/coherence/super-traits/super-trait-knowable-1.stderr

@@ -0,0 +1,13 @@
+error[E0119]: conflicting implementations of trait `Overlap<_>` for type `()`
+  --> $DIR/super-trait-knowable-1.rs:11:1
+   |
+LL | impl<T, U: Sub<T>> Overlap<T> for U {}
+   | ----------------------------------- first implementation here
+LL | impl<T> Overlap<T> for () {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `()`
+   |
+   = note: downstream crates may implement trait `Sub<_>` for type `()`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.

tests/ui/coherence/super-traits/super-trait-knowable-2.rs

@@ -0,0 +1,29 @@
+// A regression test for pyella-0.1.5 which broke when
+// enabling the new solver in coherence.
+//
+// `Tensor: TensorValue` is knowable while `Tensor: TensorOp<?t2>`
+// may be implemented downstream. We previously didn't check the
+// super trait bound in coherence, causing these impls to overlap.
+//
+// However, we did fail to normalize `<Tensor as TensorValue::Unmasked`
+// which caused the old solver to emit a `Tensor: TensorValue` goal in
+// `fn normalize_to_error` which then failed, causing this test to pass.
+
+pub trait TensorValue {
+    type Unmasked;
+}
+
+trait TensorCompare<T> {}
+pub trait TensorOp<T>: TensorValue {}
+
+pub struct Tensor;
+impl<T2> TensorCompare<T2> for Tensor {}
+impl<T1, T2> TensorCompare<T2> for T1
+//~^ ERROR conflicting implementations
+where
+    T1: TensorOp<T2>,
+    T1::Unmasked: Sized,
+{}
+
+
+fn main() {}

tests/ui/coherence/super-traits/super-trait-knowable-2.stderr

@@ -0,0 +1,17 @@
+error[E0119]: conflicting implementations of trait `TensorCompare<_>` for type `Tensor`
+  --> $DIR/super-trait-knowable-2.rs:21:1
+   |
+LL |   impl<T2> TensorCompare<T2> for Tensor {}
+   |   ------------------------------------- first implementation here
+LL | / impl<T1, T2> TensorCompare<T2> for T1
+LL | |
+LL | | where
+LL | |     T1: TensorOp<T2>,
+LL | |     T1::Unmasked: Sized,
+   | |________________________^ conflicting implementation for `Tensor`
+   |
+   = note: downstream crates may implement trait `TensorOp<_>` for type `Tensor`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.

tests/ui/coherence/super-traits/super-trait-knowable-nested.current.stderr

@@ -0,0 +1,13 @@
+error[E0119]: conflicting implementations of trait `Overlap<_>` for type `()`
+  --> $DIR/super-trait-knowable-nested.rs:19:1
+   |
+LL | impl<T, U: Bound<T>> Overlap<T> for U {}
+   | ------------------------------------- first implementation here
+LL | impl<T> Overlap<T> for () {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `()`
+   |
+   = note: downstream crates may implement trait `Bound<_>` for type `()`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.

tests/ui/coherence/super-traits/super-trait-knowable-nested.next.stderr

@@ -0,0 +1,13 @@
+error[E0119]: conflicting implementations of trait `Overlap<_>` for type `()`
+  --> $DIR/super-trait-knowable-nested.rs:19:1
+   |
+LL | impl<T, U: Bound<T>> Overlap<T> for U {}
+   | ------------------------------------- first implementation here
+LL | impl<T> Overlap<T> for () {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `()`
+   |
+   = note: downstream crates may implement trait `Bound<_>` for type `()`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.

tests/ui/coherence/super-traits/super-trait-knowable-nested.rs

@@ -0,0 +1,22 @@
+// Unlike in `super-trait-knowable-1.rs`, the knowable
+// super trait bound is in a nested goal and we currently
+// only elaborate in the root. This can, and should, be#
+// changed in the future.
+
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+trait Super {}
+trait Sub<T>: Super {}
+
+trait Bound<T> {}
+
+impl<T: Sub<T>, U> Bound<U> for T {}
+
+trait Overlap<T> {}
+impl<T, U: Bound<T>> Overlap<T> for U {}
+impl<T> Overlap<T> for () {}
+//~^ ERROR conflicting implementations of trait `Overlap<_>` for type `()`
+
+fn main() {}