[NFC] Add minimal test case for rdar://problem/58578342.

Author: varungandhi-apple

test/Sema/exhaustive_switch.swift

@@ -1407,6 +1407,17 @@ enum SR11212Tests {
     }
   }
 
+  // rdar://problem/58578342
+  func sr11212_content_generic_pattern_untupled3(b: Box<((Int, Int), Int)>) -> (Int, Int, Int) {
+    switch b {
+    // expected-note@-1 {{add missing case: '.box((_, _))'}}
+    // expected-error@-2 {{switch must be exhaustive}}
+    // FIXME: This should not be an error, analogous to the test cases above.
+    case let .box((x, y), z): return (x, y, z)
+    // expected-warning@-1 {{the enum case has a single tuple as an associated value, but there are several patterns here, implicitly tupling the patterns and trying to match that instead}}
+    }
+  }
+
   func sr11212_content_generic_pattern_ambiguous1(b: Box<(Int, Int)>) -> (Int, Int) {
     switch b {
     case .box(let b_): return b_