Add a regression test for issue-66286

src/test/ui/proc-macro/auxiliary/issue-66286.rs

@@ -0,0 +1,14 @@
+// force-host
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+
+#[proc_macro_attribute]
+pub fn vec_ice(_attr: TokenStream, input: TokenStream) -> TokenStream {
+    // This redundant convert is necessary to reproduce ICE.
+    input.into_iter().collect()
+}

src/test/ui/proc-macro/issue-66286.rs

@@ -0,0 +1,13 @@
+// aux-build:issue-66286.rs
+
+// Regression test for #66286.
+
+extern crate issue_66286;
+
+#[issue_66286::vec_ice]
+pub extern fn foo(_: Vec(u32)) -> u32 {
+    //~^ ERROR: parenthesized type parameters may only be used with a `Fn` trait
+    0
+}
+
+fn main() {}

src/test/ui/proc-macro/issue-66286.stderr

@@ -0,0 +1,12 @@
+error[E0214]: parenthesized type parameters may only be used with a `Fn` trait
+  --> $DIR/issue-66286.rs:8:22
+   |
+LL | pub extern fn foo(_: Vec(u32)) -> u32 {
+   |                      ^^^^^^^^
+   |                      |
+   |                      only `Fn` traits may use parentheses
+   |                      help: use angle brackets instead: `Vec<u32>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0214`.