add winnowing tests

Author: lcnr

tests/ui/traits/winnowing/global-non-global-env-1.rs

@@ -0,0 +1,21 @@
+// A regression test for an edge case of candidate selection
+// in the old trait solver, see #124592 for more details.
+
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@ check-pass
+
+trait Trait<T> {}
+impl<T> Trait<T> for () {}
+
+fn impls_trait<T: Trait<U>, U>(_: T) -> U { todo!() }
+fn foo<T>() -> u32
+where
+    (): Trait<u32>,
+    (): Trait<T>,
+{
+    impls_trait(())
+}
+
+fn main() {}

tests/ui/traits/winnowing/global-non-global-env-2.current.stderr

@@ -0,0 +1,17 @@
+error[E0308]: mismatched types
+  --> $DIR/global-non-global-env-2.rs:20:5
+   |
+LL | fn foo<T>() -> u32
+   |        -       --- expected `u32` because of return type
+   |        |
+   |        found this type parameter
+...
+LL |     impls_trait(())
+   |     ^^^^^^^^^^^^^^^ expected `u32`, found type parameter `T`
+   |
+   = note:        expected type `u32`
+           found type parameter `T`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/traits/winnowing/global-non-global-env-2.rs

@@ -0,0 +1,24 @@
+// A regression test for an edge case of candidate selection
+// in the old trait solver, see #124592 for more details. Unlike
+// the first test, this one has two impl candidates.
+
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@[next] check-pass
+
+trait Trait<T> {}
+impl Trait<u32> for () {}
+impl Trait<u64> for () {}
+
+fn impls_trait<T: Trait<U>, U>(_: T) -> U { todo!() }
+fn foo<T>() -> u32
+where
+    (): Trait<u32>,
+    (): Trait<T>,
+{
+    impls_trait(())
+    //[current]~^ ERROR mismatched types
+}
+
+fn main() {}

tests/ui/traits/winnowing/global-non-global-env-3.rs

@@ -0,0 +1,23 @@
+// A regression test for an edge case of candidate selection
+// in the old trait solver, see #124592 for more details. Unlike
+// the second test, the where-bounds are in a different order.
+
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@ check-pass
+
+trait Trait<T> {}
+impl Trait<u32> for () {}
+impl Trait<u64> for () {}
+
+fn impls_trait<T: Trait<U>, U>(_: T) -> U { todo!() }
+fn foo<T>() -> u32
+where
+    (): Trait<T>,
+    (): Trait<u32>,
+{
+    impls_trait(())
+}
+
+fn main() {}

tests/ui/traits/winnowing/global-non-global-env-4.current.stderr

@@ -0,0 +1,17 @@
+error[E0308]: mismatched types
+  --> $DIR/global-non-global-env-4.rs:21:5
+   |
+LL | fn foo<T>() -> u32
+   |        -       --- expected `u32` because of return type
+   |        |
+   |        found this type parameter
+...
+LL |     impls_trait(())
+   |     ^^^^^^^^^^^^^^^ expected `u32`, found type parameter `T`
+   |
+   = note:        expected type `u32`
+           found type parameter `T`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/traits/winnowing/global-non-global-env-4.rs

@@ -0,0 +1,25 @@
+// A regression test for an edge case of candidate selection
+// in the old trait solver, see #124592 for more details. Unlike
+// the third test, this one has 3 impl candidates.
+
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@[next] check-pass
+
+trait Trait<T> {}
+impl Trait<u32> for () {}
+impl Trait<u64> for () {}
+impl Trait<u128> for () {}
+
+fn impls_trait<T: Trait<U>, U>(_: T) -> U { todo!() }
+fn foo<T>() -> u32
+where
+    (): Trait<T>,
+    (): Trait<u32>,
+{
+    impls_trait(())
+    //[current]~^ ERROR mismatched types
+}
+
+fn main() {}