error[E0599]: the method `value_parser` exists for reference `&&&&&&_InferValueParserFor<Custom>`, but its trait bounds were not satisfied
 --> tests/derive_ui/value_parser_unsupported.rs:5:5
  |
5 |     foo: Custom,
  |     ^^^ method cannot be called on `&&&&&&_InferValueParserFor<Custom>` due to unsatisfied trait bounds
...
9 | struct Custom;
  | ------------- doesn't satisfy 7 bounds
  |
 ::: clap_builder/src/builder/value_parser.rs
  |
  | pub struct _InferValueParserFor<T>(std::marker::PhantomData<T>);
  | ---------------------------------- doesn't satisfy `_: _ImplsStrParse`
  |
  = note: the following trait bounds were not satisfied:
          `Custom: ValueEnum`
          which is required by `&&&&&_InferValueParserFor<Custom>: clap::builder::impl_prelude::_ImplsValueEnum`
          `Custom: ValueParserFactory`
          which is required by `&&&&&&_InferValueParserFor<Custom>: clap::builder::impl_prelude::_ImplsValueParserFactory`
          `Custom: From<OsString>`
          which is required by `&&&&_InferValueParserFor<Custom>: clap::builder::impl_prelude::_ImplsFromOsString`
          `Custom: From<&'s std::ffi::OsStr>`
          which is required by `&&&_InferValueParserFor<Custom>: clap::builder::impl_prelude::_ImplsFromOsStr`
          `Custom: From<std::string::String>`
          which is required by `&&_InferValueParserFor<Custom>: clap::builder::impl_prelude::_ImplsFromString`
          `Custom: From<&'s str>`
          which is required by `&_InferValueParserFor<Custom>: clap::builder::impl_prelude::_ImplsFromStr`
          `Custom: FromStr`
          which is required by `_InferValueParserFor<Custom>: clap::builder::impl_prelude::_ImplsStrParse`
note: the traits `From`, `FromStr`, `ValueEnum`,  and `ValueParserFactory` must be implemented
 --> clap_builder/src/builder/value_parser.rs
  |
  | pub trait ValueParserFactory {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
 ::: clap_builder/src/derive.rs
  |
  | pub trait ValueEnum: Sized + Clone {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 --> $RUST/core/src/convert/mod.rs
 --> $RUST/core/src/str/traits.rs
  = note: this error originates in the macro `clap::value_parser` (in Nightly builds, run with -Z macro-backtrace for more info)