Add temporary ui test of 'multiple applicable items' error

tests/ui/multiple-provide.rs

@@ -0,0 +1,19 @@
+#![feature(error_generic_member_access, provide_any)]
+
+use thiserror::Error;
+use std::any::Provider;
+use std::error::Error;
+
+// FIXME: this should work. https://github.com/dtolnay/thiserror/issues/185
+#[derive(Error, Debug)]
+#[error("...")]
+struct MyError {
+    #[source]
+    #[backtrace]
+    x: std::io::Error,
+}
+
+fn main() {
+    let _: dyn Error;
+    let _: dyn Provider;
+}

tests/ui/multiple-provide.stderr

@@ -0,0 +1,16 @@
+error[E0034]: multiple applicable items in scope
+  --> tests/ui/multiple-provide.rs:13:5
+   |
+13 |     x: std::io::Error,
+   |     ^ multiple `provide` found
+   |
+   = note: candidate #1 is defined in an impl of the trait `Provider` for the type `E`
+   = note: candidate #2 is defined in an impl of the trait `std::error::Error` for the type `std::io::Error`
+help: disambiguate the associated function for candidate #1
+   |
+13 |     Provider::provide(&x, Error): std::io::Error,
+   |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+help: disambiguate the associated function for candidate #2
+   |
+13 |     std::error::Error::provide(&x, Error): std::io::Error,
+   |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~