add regression test for #90871

Author: TaKO8Ki

src/test/ui/closures/issue-90871.rs

@@ -0,0 +1,5 @@
+fn main() {
+    2: n([u8; || 1])
+    //~^ ERROR cannot find type `n` in this scope
+    //~| ERROR mismatched types
+}

src/test/ui/closures/issue-90871.stderr

@@ -0,0 +1,23 @@
+error[E0412]: cannot find type `n` in this scope
+  --> $DIR/issue-90871.rs:2:8
+   |
+LL |     2: n([u8; || 1])
+   |        ^ expecting a type here because of type ascription
+
+error[E0308]: mismatched types
+  --> $DIR/issue-90871.rs:2:15
+   |
+LL |     2: n([u8; || 1])
+   |               ^^^^ expected `usize`, found closure
+   |
+   = note: expected type `usize`
+           found closure `[closure@$DIR/issue-90871.rs:2:15: 2:17]`
+help: use parentheses to call this closure
+   |
+LL |     2: n([u8; (|| 1)()])
+   |               +    +++
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0308, E0412.
+For more information about an error, try `rustc --explain E0308`.