Add regression test for #87258

src/test/ui/generic-associated-types/issue-87258_a.rs

@@ -0,0 +1,24 @@
+#![feature(type_alias_impl_trait)]
+#![feature(generic_associated_types)]
+
+// See https://github.com/rust-lang/rust/issues/87258#issuecomment-883293367
+
+trait Trait1 {}
+
+struct Struct<'b>(&'b ());
+
+impl<'d> Trait1 for Struct<'d> {}
+
+pub trait Trait2 {
+    type FooFuture<'a>: Trait1;
+    fn foo<'a>() -> Self::FooFuture<'a>;
+}
+
+impl<'c, S: Trait2> Trait2 for &'c mut S {
+    type FooFuture<'a> = impl Trait1;
+    fn foo<'a>() -> Self::FooFuture<'a> { //~ ERROR
+        Struct(unimplemented!())
+    }
+}
+
+fn main() {}

src/test/ui/generic-associated-types/issue-87258_a.stderr

@@ -0,0 +1,11 @@
+error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
+  --> $DIR/issue-87258_a.rs:19:21
+   |
+LL |     fn foo<'a>() -> Self::FooFuture<'a> {
+   |                     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: hidden type `Struct<'_>` captures lifetime '_#7r
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0700`.

src/test/ui/generic-associated-types/issue-87258_b.rs

@@ -0,0 +1,26 @@
+#![feature(type_alias_impl_trait)]
+#![feature(generic_associated_types)]
+
+// See https://github.com/rust-lang/rust/issues/87258#issuecomment-883293367
+
+trait Trait1 {}
+
+struct Struct<'b>(&'b ());
+
+impl<'d> Trait1 for Struct<'d> {}
+
+pub trait Trait2 {
+    type FooFuture<'a>: Trait1;
+    fn foo<'a>() -> Self::FooFuture<'a>;
+}
+
+type Helper<'xenon, 'yttrium, KABOOM: Trait2> = impl Trait1;
+
+impl<'c, S: Trait2> Trait2 for &'c mut S {
+    type FooFuture<'a> = Helper<'c, 'a, S>;
+    fn foo<'a>() -> Self::FooFuture<'a> { //~ ERROR
+        Struct(unimplemented!())
+    }
+}
+
+fn main() {}

src/test/ui/generic-associated-types/issue-87258_b.stderr

@@ -0,0 +1,11 @@
+error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
+  --> $DIR/issue-87258_b.rs:21:21
+   |
+LL |     fn foo<'a>() -> Self::FooFuture<'a> {
+   |                     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: hidden type `Struct<'_>` captures lifetime '_#7r
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0700`.