add regression test

rust-version

@@ -1 +1 @@
-404c8471aba60c2d837fa728e7c729a0f52d5830
+c5ecc157043ba413568b09292001a4a74b541a4e

tests/run-pass/issue-91636.rs

@@ -0,0 +1,20 @@
+type BuiltIn = for<'a> fn(&str);
+
+struct Function {
+    inner: BuiltIn,
+}
+
+impl Function {
+    fn new(subr: BuiltIn) -> Self {
+        Self { inner: subr }
+    }
+}
+
+fn dummy(_: &str) {}
+
+fn main() {
+    let func1 = Function::new(dummy);
+    let func2 = Function::new(dummy);
+    let inner: fn(&'static _) -> _ = func1.inner;
+    assert!(inner == func2.inner);
+}