/// Returns `true` if `E` is the type wrapped by this error object.

    /// Returns true if `E` is the type held by this error object.
    ///
    /// For errors with context, this method returns true if `E` matches the
    /// type of the context `C` **or** the type of the error on which the
    /// context has been attached. For details about the interaction between
    /// context and downcasting, [see here].
    ///
    /// [see here]: trait.Context.html#effect-on-downcasting

///
/// <br>

///
/// <br>
///
/// # Effect on downcasting
///
/// After attaching context of type `C` onto an error of type `E`, the resulting
/// `anyhow::Error` may be downcast to `C` **or** to `E`.
///
/// That is, in codebases that rely on downcasting, Anyhow's context supports
/// both of the following use cases:
///
///   - **Attaching context whose type is insignificant onto errors whose type
///     is used in downcasts.**
///
///     In other error libraries whose context is not designed this way, it can
///     be risky to introduce context to existing code because new context might
///     break existing working downcasts. In Anyhow, any downcast that worked
///     before adding context will continue to work after you add a context, so
///     you should freely add human-readable context to errors wherever it would
///     be helpful.
///
///     ```
///     # use anyhow::bail;
///     # use thiserror::Error;
///     #
///     # #[derive(Error, Debug)]
///     # #[error("???")]
///     # struct SuspiciousError;
///     #
///     # fn helper() -> Result<()> {
///     #     bail!(SuspiciousError);
///     # }
///     #
///     use anyhow::{Context, Result};
///
///     fn do_it() -> Result<()> {
///         helper().context("failed to complete the work")?;
///         # const IGNORE: &str = stringify! {
///         ...
///         # };
///         # unreachable!()
///     }
///
///     fn main() {
///         let err = do_it().unwrap_err();
///         if let Some(e) = err.downcast_ref::<SuspiciousError>() {
///             // If helper() returned SuspiciousError, this downcast will
///             // correctly succeed even with the context in between.
///             # return;
///         }
///         # panic!("expected downcast to succeed");
///     }
///     ```
///
///   - **Attaching context whose type is used in downcasts onto errors whose
///     type is insignificant.**
///
///     Some codebases prefer to use machine-readable context to categorize
///     lower level errors in a way that will be actionable to higher levels of
///     the application.
///
///     ```
///     # use anyhow::bail;
///     # use thiserror::Error;
///     #
///     # #[derive(Error, Debug)]
///     # #[error("???")]
///     # struct HelperFailed;
///     #
///     # fn helper() -> Result<()> {
///     #     bail!("no such file or directory");
///     # }
///     #
///     use anyhow::{Context, Result};
///
///     fn do_it() -> Result<()> {
///         helper().context(HelperFailed)?;
///         # const IGNORE: &str = stringify! {
///         ...
///         # };
///         # unreachable!()
///     }
///
///     fn main() {
///         let err = do_it().unwrap_err();
///         if let Some(e) = err.downcast_ref::<HelperFailed>() {
///             // If helper failed, this downcast will succeed because
///             // HelperFailed is the context that has been attached to
///             // that error.
///             # return;
///         }
///         # panic!("expected downcast to succeed");
///     }
///     ```