Add regression test for #104582

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

src/test/ui/typeck/issue-104582.rs

@@ -0,0 +1,5 @@
+fn main(){
+    let my_var: String(String?);
+    //~^ ERROR: invalid `?` in type
+    //~| ERROR: parenthesized type parameters may only be used with a `Fn` trait
+}

src/test/ui/typeck/issue-104582.stderr

@@ -0,0 +1,25 @@
+error: invalid `?` in type
+  --> $DIR/issue-104582.rs:2:30
+   |
+LL |     let my_var: String(String?);
+   |                              ^ `?` is only allowed on expressions, not types
+   |
+help: if you meant to express that the type might not contain a value, use the `Option` wrapper type
+   |
+LL |     let my_var: String(Option<String>);
+   |                        +++++++      ~
+
+error[E0214]: parenthesized type parameters may only be used with a `Fn` trait
+  --> $DIR/issue-104582.rs:2:17
+   |
+LL |     let my_var: String(String?);
+   |                 ^^^^^^^^^^^^^^^ only `Fn` traits may use parentheses
+   |
+help: use angle brackets instead
+   |
+LL |     let my_var: String<String?>;
+   |                       ~       ~
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0214`.