Add test case for diagnostic 'because_rethrows_default_argument_throws'

The first test case added ideally shouldn't have any expected error or diagnostic.
However, due to SR-1534, there is an error emitted here. Thus, an expected error is
used here to show the compiler's behavior today.

If SR-1534 is fixed in the future, the first test case should no longer have any
expected error nor an expected note. However, the second test case should still be
left alone.

Addresses SR-14270

Author: kthchew

test/decl/func/rethrows.swift

@@ -612,3 +612,19 @@ func rdar_47550715() {
   func foo(_: A<F>? = nil) {} // Ok
   func bar(_: A<F>? = .none) {} // Ok
 }
+
+// SR-14270 - test case for diagnostic note 'because_rethrows_default_argument_throws'
+func nonThrowableDefaultRethrows(_ f: () throws -> () = {}) rethrows {
+  try f()
+}
+// NOTE: This should compile and not emit a diagnostic because ideally the compiler could statically
+// know the default argument value could never throw. See SR-1524.
+nonThrowableDefaultRethrows() // expected-error {{call can throw but is not marked with 'try'}}
+                              // expected-note@-1 {{call is to 'rethrows' function, but a defaulted argument function can throw}}
+
+func throwableDefaultRethrows(_ f: () throws -> () = { throw SomeError.Badness }) rethrows {
+  try f()
+}
+// This should always emit a diagnostic because we can statically know that default argument can throw. 
+throwableDefaultRethrows()  // expected-error {{call can throw but is not marked with 'try'}}
+                            // expected-note@-1 {{call is to 'rethrows' function, but a defaulted argument function can throw}}