[NFC] Test unsupported typed throws use case

Fixing this would be necessary to adopt `@abi` for `Swift.Result.init(__untyped_throws_catching:)`.

Author: beccadax

test/attr/attr_abi.swift

@@ -974,6 +974,20 @@ func testNormalProtocolsGeneric<A: CustomStringConvertible, B>( // expected-note
   _: A, _: B
 ) {}
 
+enum ErsatzResult<Success, Failure: Error> {}
+extension ErsatzResult where Failure == Swift.Error {
+  // Similar to Swift.Result.init(__untyped_throws_catching:)
+  // FIXME: The where clause makes this ABI-compatible, but we can't tell that.
+  @abi(
+    init(
+      catching body: () throws -> Success // expected-error {{type '() throws -> Success' in '@abi' should match '() throws(Failure) -> Success'}}
+    )
+  )
+  init(
+    __untyped_throws_catching body: () throws(Failure) -> Success // expected-note {{should match type here}}
+  ) {}
+}
+
 //
 // Static/Instance and interactions with `final`
 //