Add regression test for issue 405

    error[E0599]: the method `as_display` exists for reference `&<T as FromStr>::Err`, but its trait bounds were not satisfied
       --> tests/test_generics.rs:178:13
        |
    178 |     #[error("couldn't parse entry: {0}")]
        |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ method cannot be called on `&<T as FromStr>::Err` due to unsatisfied trait bounds
        |
        = note: the following trait bounds were not satisfied:
                `<T as FromStr>::Err: std::fmt::Display`
                which is required by `&<T as FromStr>::Err: AsDisplay<'_>`

Author: dtolnay

tests/test_generics.rs

@@ -1,6 +1,7 @@
 #![allow(clippy::needless_late_init, clippy::uninlined_format_args)]
 
 use core::fmt::{self, Debug, Display};
+use core::str::FromStr;
 use thiserror::Error;
 
 pub struct NoFormat;
@@ -160,6 +161,24 @@ pub struct StructFromGeneric<E> {
 #[error(transparent)]
 pub struct StructTransparentGeneric<E>(pub E);
 
+// Should expand to:
+//
+//     impl<T: FromStr> Display for AssociatedTypeError<T>
+//     where
+//         T::Err: Display;
+//
+//     impl<T: FromStr> Error for AssociatedTypeError<T>
+//     where
+//         Self: Debug + Display;
+//
+#[derive(Error, Debug)]
+pub enum AssociatedTypeError<T: FromStr> {
+    #[error("couldn't parse matrix")]
+    Other,
+    #[error("couldn't parse entry: {0}")]
+    EntryParseError(T::Err),
+}
+
 // Regression test for https://github.com/dtolnay/thiserror/issues/345
 #[test]
 fn test_no_bound_on_named_fmt() {