Uniquify ReError on input mode in canonicalizer

2 files changed, 49 insertions, 0 deletions

diff --git a/tests/ui/traits/next-solver/dont-canonicalize-re-error.rs b/tests/ui/traits/next-solver/dont-canonicalize-re-error.rs
new file mode 100644
index 00000000000..57f814bc81e
--- /dev/null
+++ b/tests/ui/traits/next-solver/dont-canonicalize-re-error.rs
@@ -0,0 +1,28 @@
+//@ compile-flags: -Znext-solver
+
+trait Tr<'a> {}
+
+// Fulfillment in the new solver relies on an invariant to hold: Either
+// `has_changed` is true, or computing a goal's certainty is idempotent.
+// This isn't true for `ReError`, which we used to pass through in the
+// canonicalizer even on input mode, which can cause a goal to go from
+// ambig => pass, but we don't consider `has_changed` when the response
+// only contains region constraints (since we usually uniquify regions).
+//
+//   In this test:
+// Implicit negative coherence tries to prove `W<?0>: Constrain<'?1>`,
+// which will then match with the impl below. This constrains `'?1` to
+// `ReError`, but still bails w/ ambiguity bc we can't prove `?0: Sized`.
+// Then, when we recompute the goal `W<?0>: Constrain<'error>`, when
+// collecting ambiguities and overflows, we end up assembling a default
+// error candidate w/o ambiguity, which causes the goal to pass, and ICE.
+impl<'a, A: ?Sized> Tr<'a> for W<A> {}
+struct W<A: ?Sized>(A);
+impl<'a, A: ?Sized> Tr<'a> for A where A: Constrain<'a> {}
+//~^ ERROR conflicting implementations of trait `Tr<'_>` for type `W<_>`
+
+trait Constrain<'a> {}
+impl<A: Sized> Constrain<'missing> for W<A> {}
+//~^ ERROR use of undeclared lifetime name `'missing`
+
+fn main() {}
diff --git a/tests/ui/traits/next-solver/dont-canonicalize-re-error.stderr b/tests/ui/traits/next-solver/dont-canonicalize-re-error.stderr
new file mode 100644
index 00000000000..cf85c52fb42
--- /dev/null
+++ b/tests/ui/traits/next-solver/dont-canonicalize-re-error.stderr
@@ -0,0 +1,21 @@
+error[E0261]: use of undeclared lifetime name `'missing`
+  --> $DIR/dont-canonicalize-re-error.rs:25:26
+   |
+LL | impl<A: Sized> Constrain<'missing> for W<A> {}
+   |      -                   ^^^^^^^^ undeclared lifetime
+   |      |
+   |      help: consider introducing lifetime `'missing` here: `'missing,`
+
+error[E0119]: conflicting implementations of trait `Tr<'_>` for type `W<_>`
+  --> $DIR/dont-canonicalize-re-error.rs:21:1
+   |
+LL | impl<'a, A: ?Sized> Tr<'a> for W<A> {}
+   | ----------------------------------- first implementation here
+LL | struct W<A: ?Sized>(A);
+LL | impl<'a, A: ?Sized> Tr<'a> for A where A: Constrain<'a> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `W<_>`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0119, E0261.
+For more information about an error, try `rustc --explain E0119`.