Sema: Allow resilient modules to add enum elements without breaking clients.

In the implementation of #73472,
warnings about uncovered cases were upgraded to errors in switches covering
enums with an `@unknown default` case. This change was a mistake; it should not
be source breaking for a resilient library to add new cases to a non-frozen
enum.

Resolves rdar://138488976.

Author: tshortli

lib/Sema/TypeCheckSwitchStmt.cpp

@@ -1188,7 +1188,7 @@ namespace {
       case RequiresDefault::UncoveredSwitch: {
         OS << tok::kw_default << ":\n" << placeholder << "\n";
         DE.diagnose(startLoc, mainDiagType.value())
-          .warnUntilSwiftVersionIf(downgrade, 6);
+          .limitBehaviorIf(downgrade, DiagnosticBehavior::Warning);
         DE.diagnose(startLoc, diag::missing_several_cases, /*default*/true)
           .fixItInsert(insertLoc, buffer.str());
       }
@@ -1208,7 +1208,7 @@ namespace {
       // Check if we still have to emit the main diagnostic.
       if (mainDiagType.has_value()) {
         DE.diagnose(startLoc, mainDiagType.value())
-          .warnUntilSwiftVersionIf(downgrade, 6);
+          .limitBehaviorIf(downgrade, DiagnosticBehavior::Warning);
       }
 
       // Add notes to explain what's missing.

test/Compatibility/exhaustive_switch_swift_6.swift

@@ -29,6 +29,37 @@ func testSwitch() -> Bool {
   case (.case8, _): return true
   case (.case9, _): return true
   case (.case10, _): return true
+  }
+
+  switch (OverlyLargeSpaceEnum.case1, OverlyLargeSpaceEnum.case2) { // expected-warning {{switch must be exhaustive}}
+  // expected-note@-1 {{add missing case: '(.case11, _)'}}
+  case (.case0, _): return true
+  case (.case1, _): return true
+  case (.case2, _): return true
+  case (.case3, _): return true
+  case (.case4, _): return true
+  case (.case5, _): return true
+  case (.case6, _): return true
+  case (.case7, _): return true
+  case (.case8, _): return true
+  case (.case9, _): return true
+  case (.case10, _): return true
   @unknown default: return false
   }
+
+  // No diagnostic
+  switch (OverlyLargeSpaceEnum.case1, OverlyLargeSpaceEnum.case2) {
+  case (.case0, _): return true
+  case (.case1, _): return true
+  case (.case2, _): return true
+  case (.case3, _): return true
+  case (.case4, _): return true
+  case (.case5, _): return true
+  case (.case6, _): return true
+  case (.case7, _): return true
+  case (.case8, _): return true
+  case (.case9, _): return true
+  case (.case10, _): return true
+  case (.case11, _): return true
+  }
 }

test/FixCode/fixits-switch-nonfrozen.swift

@@ -219,14 +219,14 @@ public func testNonExhaustive(_ value: NonExhaustive, _ payload: NonExhaustivePa
   }
 
   switch value {
-  // expected-error@-1 {{switch must be exhaustive}}
+  // expected-warning@-1 {{switch must be exhaustive}}
   // expected-note@-2 {{add missing case: '.b'}} {{+4:3-3=case .b:\n<#code#>\n}}
   case .a: break
   @unknown case _: break
   }
 
   switch value {
-  // expected-error@-1 {{switch must be exhaustive}}
+  // expected-warning@-1 {{switch must be exhaustive}}
   // expected-note@-2 {{add missing case: '.a'}} {{+5:3-3=case .a:\n<#code#>\n}}
   // expected-note@-3 {{add missing case: '.b'}} {{+5:3-3=case .b:\n<#code#>\n}}
   // expected-note@-4 {{add missing cases}} {{+5:3-3=case .a:\n<#code#>\ncase .b:\n<#code#>\n}}
@@ -350,7 +350,7 @@ public func testNonExhaustive(_ value: NonExhaustive, _ payload: NonExhaustivePa
   }
 
   switch payload {
-  // expected-error@-1 {{switch must be exhaustive}}
+  // expected-warning@-1 {{switch must be exhaustive}}
   // expected-note@-2 {{add missing case: '.b(_)'}} {{+4:3-3=case .b(_):\n<#code#>\n}}
   case .a: break
   @unknown case _: break
@@ -366,7 +366,7 @@ public func testNonExhaustive(_ value: NonExhaustive, _ payload: NonExhaustivePa
   }
 
   switch payload {
-  // expected-error@-1 {{switch must be exhaustive}}
+  // expected-warning@-1 {{switch must be exhaustive}}
   // expected-note@-2 {{add missing case: '.b(true)'}} {{+5:3-3=case .b(true):\n<#code#>\n}}
   case .a: break
   case .b(false): break