Don't fatal when calling expect_one_of when recovering arg in parse_seq

compiler/rustc_parse/src/parser/mod.rs

@@ -898,6 +898,7 @@ fn parse_seq_to_before_tokens<T>(
                             }
 
                             // Attempt to keep parsing if it was an omitted separator.
+                            self.last_unexpected_token_span = None;
                             match f(self) {
                                 Ok(t) => {
                                     // Parsed successfully, therefore most probably the code only

tests/ui/impl-trait/precise-capturing/unexpected-token.rs

@@ -0,0 +1,9 @@
+// We used to fatal error without any useful diagnostic when we had an unexpected
+// token due to a strange interaction between the sequence parsing code and the
+// param/lifetime parsing code.
+
+fn hello() -> impl use<'a {}> Sized {}
+//~^ ERROR expected one of `,` or `>`, found `{`
+//~| ERROR expected item, found `>`
+
+fn main() {}

tests/ui/impl-trait/precise-capturing/unexpected-token.stderr

@@ -0,0 +1,16 @@
+error: expected one of `,` or `>`, found `{`
+  --> $DIR/unexpected-token.rs:5:27
+   |
+LL | fn hello() -> impl use<'a {}> Sized {}
+   |                           ^ expected one of `,` or `>`
+
+error: expected item, found `>`
+  --> $DIR/unexpected-token.rs:5:29
+   |
+LL | fn hello() -> impl use<'a {}> Sized {}
+   |                             ^ expected item
+   |
+   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
+
+error: aborting due to 2 previous errors
+