Remove incorrect assertion in type parsing code

compiler/rustc_parse/src/parser/ty.rs

@@ -334,7 +334,6 @@ impl<'a> Parser<'a> {
         mut bounds: GenericBounds,
         plus: bool,
     ) -> PResult<'a, TyKind> {
-        assert_ne!(self.token, token::Question);
         if plus {
             self.eat_plus(); // `+`, or `+=` gets split and `+` is discarded
             bounds.append(&mut self.parse_generic_bounds(Some(self.prev_token.span))?);

src/test/ui/parser/issue-84148-1.rs

@@ -0,0 +1,4 @@
+fn f(t:for<>t?)
+//~^ ERROR: expected parameter name
+//~| ERROR: expected one of
+//~| ERROR: expected one of

src/test/ui/parser/issue-84148-1.stderr

@@ -0,0 +1,23 @@
+error: expected parameter name, found `?`
+  --> $DIR/issue-84148-1.rs:1:14
+   |
+LL | fn f(t:for<>t?)
+   |              ^ expected parameter name
+
+error: expected one of `(`, `)`, `+`, `,`, `::`, or `<`, found `?`
+  --> $DIR/issue-84148-1.rs:1:14
+   |
+LL | fn f(t:for<>t?)
+   |              ^
+   |              |
+   |              expected one of `(`, `)`, `+`, `,`, `::`, or `<`
+   |              help: missing `,`
+
+error: expected one of `->`, `;`, `where`, or `{`, found `<eof>`
+  --> $DIR/issue-84148-1.rs:1:15
+   |
+LL | fn f(t:for<>t?)
+   |               ^ expected one of `->`, `;`, `where`, or `{`
+
+error: aborting due to 3 previous errors
+

src/test/ui/parser/issue-84148-2.rs

@@ -0,0 +1,4 @@
+// error-pattern: this file contains an unclosed delimiter
+// error-pattern: expected parameter name
+// error-pattern: expected one of
+fn f(t:for<>t?

src/test/ui/parser/issue-84148-2.stderr

@@ -0,0 +1,31 @@
+error: this file contains an unclosed delimiter
+  --> $DIR/issue-84148-2.rs:4:16
+   |
+LL | fn f(t:for<>t?
+   |     -          ^
+   |     |
+   |     unclosed delimiter
+
+error: expected parameter name, found `?`
+  --> $DIR/issue-84148-2.rs:4:14
+   |
+LL | fn f(t:for<>t?
+   |              ^ expected parameter name
+
+error: expected one of `(`, `)`, `+`, `,`, `::`, or `<`, found `?`
+  --> $DIR/issue-84148-2.rs:4:14
+   |
+LL | fn f(t:for<>t?
+   |              ^
+   |              |
+   |              expected one of `(`, `)`, `+`, `,`, `::`, or `<`
+   |              help: missing `,`
+
+error: expected one of `->`, `;`, `where`, or `{`, found `<eof>`
+  --> $DIR/issue-84148-2.rs:4:16
+   |
+LL | fn f(t:for<>t?
+   |                ^ expected one of `->`, `;`, `where`, or `{`
+
+error: aborting due to 4 previous errors
+