1 files changed, 26 insertions, 0 deletions
diff --git a/src/test/ui/traits/infer-from-object-issue-26952.rs b/src/test/ui/traits/infer-from-object-issue-26952.rs
new file mode 100644
index 000000000..ed258dbb2
--- /dev/null
+++ b/src/test/ui/traits/infer-from-object-issue-26952.rs
@@ -0,0 +1,26 @@
+// run-pass
+#![allow(dead_code)]
+#![allow(unused_variables)]
+// Test that when we match a trait reference like `Foo<A>: Foo<_#0t>`,
+// we unify with `_#0t` with `A`. In this code, if we failed to do
+// that, then you get an unconstrained type-variable in `call`.
+//
+// Also serves as a regression test for issue #26952, though the test
+// was derived from another reported regression with the same cause.
+
+use std::marker::PhantomData;
+
+trait Trait<A> { fn foo(&self); }
+
+struct Type<A> { a: PhantomData<A> }
+
+fn as_trait<A>(t: &Type<A>) -> &dyn Trait<A> { loop {  } }
+
+fn want<A,T:Trait<A>+?Sized>(t: &T) { }
+
+fn call<A>(p: Type<A>) {
+    let q = as_trait(&p);
+    want(q); // parameter A to `want` *would* be unconstrained
+}
+
+fn main() { }