Add test for issue-72911

Author: JohnTitor

src/test/ui/impl-trait/issue-72911.rs

@@ -0,0 +1,22 @@
+// Regression test for #72911.
+
+pub struct Lint {}
+
+impl Lint {}
+
+pub fn gather_all() -> impl Iterator<Item = Lint> {
+    //~^ ERROR: cannot resolve opaque type
+    lint_files().flat_map(|f| gather_from_file(&f))
+}
+
+fn gather_from_file(dir_entry: &foo::MissingItem) -> impl Iterator<Item = Lint> {
+    //~^ ERROR: failed to resolve
+    unimplemented!()
+}
+
+fn lint_files() -> impl Iterator<Item = foo::MissingItem> {
+    //~^ ERROR: failed to resolve
+    unimplemented!()
+}
+
+fn main() {}

src/test/ui/impl-trait/issue-72911.stderr

@@ -0,0 +1,34 @@
+error[E0433]: failed to resolve: use of undeclared type or module `foo`
+  --> $DIR/issue-72911.rs:12:33
+   |
+LL | fn gather_from_file(dir_entry: &foo::MissingItem) -> impl Iterator<Item = Lint> {
+   |                                 ^^^ use of undeclared type or module `foo`
+
+error[E0433]: failed to resolve: use of undeclared type or module `foo`
+  --> $DIR/issue-72911.rs:17:41
+   |
+LL | fn lint_files() -> impl Iterator<Item = foo::MissingItem> {
+   |                                         ^^^ use of undeclared type or module `foo`
+
+error[E0720]: cannot resolve opaque type
+  --> $DIR/issue-72911.rs:7:24
+   |
+LL | pub fn gather_all() -> impl Iterator<Item = Lint> {
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^ recursive opaque type
+LL |
+LL |     lint_files().flat_map(|f| gather_from_file(&f))
+   |     -----------------------------------------------
+   |     |
+   |     returning here with type `std::iter::FlatMap<impl std::iter::Iterator, [type error], [closure@$DIR/issue-72911.rs:9:27: 9:51]>`
+   |     returning here with type `std::iter::FlatMap<impl std::iter::Iterator, [type error], [closure@$DIR/issue-72911.rs:9:27: 9:51]>`
+...
+LL | fn gather_from_file(dir_entry: &foo::MissingItem) -> impl Iterator<Item = Lint> {
+   |                                                      -------------------------- returning this opaque type `std::iter::FlatMap<impl std::iter::Iterator, [type error], [closure@$DIR/issue-72911.rs:9:27: 9:51]>`
+...
+LL | fn lint_files() -> impl Iterator<Item = foo::MissingItem> {
+   |                    -------------------------------------- returning this opaque type `std::iter::FlatMap<impl std::iter::Iterator, [type error], [closure@$DIR/issue-72911.rs:9:27: 9:51]>`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0433, E0720.
+For more information about an error, try `rustc --explain E0433`.