add tests for rust-lang#114061

Author: lcnr

tests/ui/coherence/aliases/ambig-assoc-type-with-bound-vars-1.rs

@@ -0,0 +1,25 @@
+// Regression test for #114061. We previously did
+// not consider `for<'a> <T as WithAssoc<'a>>::Assoc: IsUnit`
+// to be unknowable, even though the projection is
+// ambiguous.
+trait IsUnit {}
+impl IsUnit for () {}
+
+
+pub trait WithAssoc<'a> {
+    type Assoc;
+}
+
+// The two impls of `Trait` overlap
+pub trait Trait {}
+impl<T> Trait for T
+where
+    T: 'static,
+    for<'a> T: WithAssoc<'a>,
+    for<'a> <T as WithAssoc<'a>>::Assoc: IsUnit,
+{
+}
+impl<T> Trait for Box<T> {}
+//~^ ERROR conflicting implementations of trait `Trait`
+
+fn main() {}

tests/ui/coherence/aliases/ambig-assoc-type-with-bound-vars-1.stderr

@@ -0,0 +1,18 @@
+error[E0119]: conflicting implementations of trait `Trait` for type `Box<_>`
+  --> $DIR/ambig-assoc-type-with-bound-vars-1.rs:22:1
+   |
+LL | / impl<T> Trait for T
+LL | | where
+LL | |     T: 'static,
+LL | |     for<'a> T: WithAssoc<'a>,
+LL | |     for<'a> <T as WithAssoc<'a>>::Assoc: IsUnit,
+   | |________________________________________________- first implementation here
+...
+LL |   impl<T> Trait for Box<T> {}
+   |   ^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Box<_>`
+   |
+   = note: downstream crates may implement trait `WithAssoc<'a>` for type `std::boxed::Box<_>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.

tests/ui/coherence/aliases/ambig-assoc-type-with-bound-vars-2.rs

@@ -0,0 +1,23 @@
+// Another regression test for #114061.
+
+pub trait WhereBound {}
+impl WhereBound for () {}
+
+pub trait WithAssoc<'a> {
+    type Assoc;
+}
+
+// The two impls of `Trait` overlap
+pub trait Trait {}
+impl<T> Trait for T
+where
+    T: 'static,
+    for<'a> T: WithAssoc<'a>,
+    // This bound was previously treated as knowable
+    for<'a> Box<<T as WithAssoc<'a>>::Assoc>: WhereBound,
+{
+}
+impl<T> Trait for Box<T> {}
+//~^ ERROR conflicting implementations of trait `Trait`
+
+fn main() {}

tests/ui/coherence/aliases/ambig-assoc-type-with-bound-vars-2.stderr

@@ -0,0 +1,20 @@
+error[E0119]: conflicting implementations of trait `Trait` for type `Box<_>`
+  --> $DIR/ambig-assoc-type-with-bound-vars-2.rs:20:1
+   |
+LL | / impl<T> Trait for T
+LL | | where
+LL | |     T: 'static,
+LL | |     for<'a> T: WithAssoc<'a>,
+LL | |     // This bound was previously treated as knowable
+LL | |     for<'a> Box<<T as WithAssoc<'a>>::Assoc>: WhereBound,
+   | |_________________________________________________________- first implementation here
+...
+LL |   impl<T> Trait for Box<T> {}
+   |   ^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Box<_>`
+   |
+   = note: downstream crates may implement trait `WithAssoc<'a>` for type `std::boxed::Box<_>`
+   = note: downstream crates may implement trait `WhereBound` for type `std::boxed::Box<<std::boxed::Box<_> as WithAssoc<'a>>::Assoc>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.