2 files changed, 56 insertions, 0 deletions
diff --git a/tests/ui/traits/new-solver/coherence/issue-102048.rs b/tests/ui/traits/new-solver/coherence/issue-102048.rs
new file mode 100644
index 000000000..11636bfeb
--- /dev/null
+++ b/tests/ui/traits/new-solver/coherence/issue-102048.rs
@@ -0,0 +1,44 @@
+// This must fail coherence.
+//
+// Getting this to pass was fairly difficult, so here's an explanation
+// of what's happening:
+//
+// Normalizing projections currently tries to replace them with inference variables
+// while emitting a nested `Projection` obligation. This cannot be done if the projection
+// has bound variables which is the case here.
+//
+// So the projections stay until after normalization. When unifying two projections we
+// currently treat them as if they are injective, so we **incorrectly** unify their
+// substs. This means that coherence for the two impls ends up unifying `?T` and `?U`
+// as it tries to unify `<?T as WithAssoc1<'a>>::Assoc` with `<?U as WithAssoc1<'a>>::Assoc`.
+//
+// `impl1` therefore has the projection `<?T as WithAssoc2<'a>>::Assoc` and we have the
+// assumption `?T: for<'a> WithAssoc2<'a, Assoc = i32>` in the `param_env`, so we normalize
+// that to `i32`. We then try to unify `i32` from `impl1` with `u32` from `impl2` which fails,
+// causing coherence to consider these two impls distinct.
+
+// compile-flags: -Ztrait-solver=next
+pub trait Trait<T> {}
+
+pub trait WithAssoc1<'a> {
+    type Assoc;
+}
+pub trait WithAssoc2<'a> {
+    type Assoc;
+}
+
+// impl 1
+impl<T, U> Trait<for<'a> fn(<T as WithAssoc1<'a>>::Assoc, <U as WithAssoc2<'a>>::Assoc)> for (T, U)
+where
+    T: for<'a> WithAssoc1<'a> + for<'a> WithAssoc2<'a, Assoc = i32>,
+    U: for<'a> WithAssoc2<'a>,
+{
+}
+
+// impl 2
+impl<T, U> Trait<for<'a> fn(<U as WithAssoc1<'a>>::Assoc, u32)> for (T, U) where
+    U: for<'a> WithAssoc1<'a> //~^ ERROR conflicting implementations of trait
+{
+}
+
+fn main() {}
diff --git a/tests/ui/traits/new-solver/coherence/issue-102048.stderr b/tests/ui/traits/new-solver/coherence/issue-102048.stderr
new file mode 100644
index 000000000..17a43838f
--- /dev/null
+++ b/tests/ui/traits/new-solver/coherence/issue-102048.stderr
@@ -0,0 +1,12 @@
+error[E0119]: conflicting implementations of trait `Trait<for<'a> fn(<_ as WithAssoc1<'a>>::Assoc, <_ as WithAssoc2<'a>>::Assoc)>` for type `(_, _)`
+  --> $DIR/issue-102048.rs:39:1
+   |
+LL | impl<T, U> Trait<for<'a> fn(<T as WithAssoc1<'a>>::Assoc, <U as WithAssoc2<'a>>::Assoc)> for (T, U)
+   | --------------------------------------------------------------------------------------------------- first implementation here
+...
+LL | impl<T, U> Trait<for<'a> fn(<U as WithAssoc1<'a>>::Assoc, u32)> for (T, U) where
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(_, _)`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.