[Sema] Non-exhaustive switch statements are always an error in Swift 6.

include/swift/AST/DiagnosticsSema.def

@@ -7074,10 +7074,9 @@ WARNING(redundant_particular_literal_case,none,
 NOTE(redundant_particular_literal_case_here,none,
      "first occurrence of identical literal pattern is here", ())
 
-WARNING(non_exhaustive_switch_warn,none, "switch must be exhaustive", ())
+ERROR(non_exhaustive_switch_unknown_only,none,
-WARNING(non_exhaustive_switch_unknown_only,none,
+      "switch covers known cases, but %0 may have additional unknown values"
-        "switch covers known cases, but %0 may have additional unknown values"
+      "%select{|, possibly added in future versions}1", (Type, bool))
-        "%select{|, possibly added in future versions}1", (Type, bool))
 
 ERROR(override_nsobject_hashvalue_error,none,
       "'NSObject.hashValue' is not overridable; "

lib/Sema/TypeCheckSwitchStmt.cpp

@@ -1128,13 +1128,13 @@ namespace {
       // Decide whether we want an error or a warning.
       std::optional<decltype(diag::non_exhaustive_switch)> mainDiagType =
           diag::non_exhaustive_switch;
+      bool downgrade = false;
       if (unknownCase) {
         switch (defaultReason) {
         case RequiresDefault::EmptySwitchBody:
           llvm_unreachable("there's an @unknown case; the body can't be empty");
         case RequiresDefault::No:
-          if (!uncovered.isEmpty())
+          downgrade = !uncovered.isEmpty();
-            mainDiagType = diag::non_exhaustive_switch_warn;
           break;
         case RequiresDefault::UncoveredSwitch:
         case RequiresDefault::SpaceTooLarge: {
@@ -1170,7 +1170,8 @@ namespace {
               theEnum->getParentModule()->isSystemModule();
         }
         DE.diagnose(startLoc, diag::non_exhaustive_switch_unknown_only,
-                    subjectType, shouldIncludeFutureVersionComment);
+                    subjectType, shouldIncludeFutureVersionComment)
+          .warnUntilSwiftVersion(6);
         mainDiagType = std::nullopt;
       }
         break;
@@ -1187,7 +1188,8 @@ namespace {
         return;
       case RequiresDefault::UncoveredSwitch: {
         OS << tok::kw_default << ":\n" << placeholder << "\n";
-        DE.diagnose(startLoc, mainDiagType.value());
+        DE.diagnose(startLoc, mainDiagType.value())
+          .warnUntilSwiftVersionIf(downgrade, 6);
         DE.diagnose(startLoc, diag::missing_several_cases, /*default*/true)
           .fixItInsert(insertLoc, buffer.str());
       }
@@ -1205,8 +1207,10 @@ namespace {
       if (uncovered.isEmpty()) return;
 
       // Check if we still have to emit the main diagnostic.
-      if (mainDiagType.has_value())
+      if (mainDiagType.has_value()) {
-        DE.diagnose(startLoc, mainDiagType.value());
+        DE.diagnose(startLoc, mainDiagType.value())
+          .warnUntilSwiftVersionIf(downgrade, 6);
+      }
 
       // Add notes to explain what's missing.
       auto processUncoveredSpaces =

test/ClangImporter/enum-exhaustivity-system.swift

@@ -1,9 +1,13 @@
-// RUN: %target-swift-frontend -typecheck %s -Xcc -isystem -Xcc %S/Inputs/custom-modules -verify -enable-nonfrozen-enum-exhaustivity-diagnostics
+// RUN: %target-swift-frontend -typecheck %s -Xcc -isystem -Xcc %S/Inputs/custom-modules -verify -swift-version 5 -verify-additional-prefix swift5-
+// RUN: %target-swift-frontend -typecheck %s -Xcc -isystem -Xcc %S/Inputs/custom-modules -verify -swift-version 6 -verify-additional-prefix swift6-
 
 import EnumExhaustivity
 
 func test(_ value: RegularEnum, _ exhaustiveValue: ExhaustiveEnum) {
-  switch value { // expected-warning {{switch covers known cases, but 'RegularEnum' may have additional unknown values, possibly added in future versions}} expected-note {{handle unknown values using "@unknown default"}}
+  switch value {
+  // expected-swift5-warning@-1 {{switch covers known cases, but 'RegularEnum' may have additional unknown values, possibly added in future versions}}
+  // expected-swift6-error@-2 {{switch covers known cases, but 'RegularEnum' may have additional unknown values, possibly added in future versions}}
+  // expected-note@-3 {{handle unknown values using "@unknown default"}}
   case .A: break
   case .B: break
   }
@@ -29,7 +33,11 @@ func testAttributes(
   case .A, .B: break
   }
 
-  switch retetb { // expected-warning {{switch covers known cases, but 'RegularEnumTurnedExhaustiveThenBackViaAPINotes' may have additional unknown values, possibly added in future versions}} expected-note {{handle unknown values using "@unknown default"}}
+  switch retetb {
+  // expected-swift5-warning@-1 {{switch covers known cases, but 'RegularEnumTurnedExhaustiveThenBackViaAPINotes' may have additional unknown values, possibly added in future versions}}
+  // expected-swift6-error@-2 {{switch covers known cases, but 'RegularEnumTurnedExhaustiveThenBackViaAPINotes' may have additional unknown values, possibly added in future versions}}
+  // expected-note@-3 {{handle unknown values using "@unknown default"}}
+
   case .A, .B: break
   }
 

test/Compatibility/exhaustive_switch_swift_6.swift

@@ -0,0 +1,34 @@
+// RUN: %target-typecheck-verify-swift -swift-version 6 -enable-library-evolution
+
+enum OverlyLargeSpaceEnum {
+  case case0
+  case case1
+  case case2
+  case case3
+  case case4
+  case case5
+  case case6
+  case case7
+  case case8
+  case case9
+  case case10
+  case case11
+}
+
+func testSwitch() -> Bool {
+  switch (OverlyLargeSpaceEnum.case1, OverlyLargeSpaceEnum.case2) { // expected-error {{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
+  }
+}