Sema: Error for potentially unavailable conformances in Swift 6.

Diagnostics about use of potentially unavailable conformances had to be
downgraded to warnings in Swift 5 in order to preserve source compatibility.
These diagnostics should be errors by default in Swift 6.

Resolves rdar://88210812

Author: tshortli

include/swift/AST/DiagnosticsSema.def

@@ -6714,10 +6714,6 @@ ERROR(conformance_availability_only_version_newer, none,
       "conformance of %0 to %1 is only available in %2 %3 or newer",
       (Type, Type, StringRef, llvm::VersionTuple))
 
-WARNING(conformance_availability_only_version_newer_warn, none,
-        "conformance of %0 to %1 is only available in %2 %3 or newer",
-        (Type, Type, StringRef, llvm::VersionTuple))
-
 //------------------------------------------------------------------------------
 // MARK: @discardableResult
 //------------------------------------------------------------------------------

lib/Sema/TypeCheckAvailability.cpp

@@ -2182,17 +2182,14 @@ void TypeChecker::diagnosePotentialUnavailability(
   {
     auto type = rootConf->getType();
     auto proto = rootConf->getProtocol()->getDeclaredInterfaceType();
-
-    auto diagID = (ctx.LangOpts.EnableConformanceAvailabilityErrors
-                   ? diag::conformance_availability_only_version_newer
-                   : diag::conformance_availability_only_version_newer_warn);
-    auto behavior = behaviorLimitForExplicitUnavailability(rootConf, dc);
-    auto err =
-      ctx.Diags.diagnose(
-               loc, diagID,
-               type, proto, prettyPlatformString(targetPlatform(ctx.LangOpts)),
-               reason.getRequiredOSVersionRange().getLowerEndpoint());
-    err.limitBehavior(behavior);
+    auto err = ctx.Diags.diagnose(
+        loc, diag::conformance_availability_only_version_newer, type, proto,
+        prettyPlatformString(targetPlatform(ctx.LangOpts)),
+        reason.getRequiredOSVersionRange().getLowerEndpoint());
+
+    err.warnUntilSwiftVersionIf(
+        !ctx.LangOpts.EnableConformanceAvailabilityErrors, 6);
+    err.limitBehavior(behaviorLimitForExplicitUnavailability(rootConf, dc));
 
     // Direct a fixit to the error if an existing guard is nearly-correct
     if (fixAvailabilityByNarrowingNearbyVersionCheck(loc, dc,

test/Sema/conformance_availability.swift

@@ -1,6 +1,8 @@
 // RUN: %target-typecheck-verify-swift -swift-version 5 -enable-conformance-availability-errors
+// RUN: %target-typecheck-verify-swift -swift-version 6
 
 // REQUIRES: OS=macosx
+// REQUIRES: asserts
 
 public protocol Horse {}
 func takesHorse<T : Horse>(_: T) {}

test/Sema/conformance_availability_warn.swift

@@ -25,22 +25,22 @@ extension HasAvailableConformance1 : Horse {}
 // example but with this flag.
 
 func passAvailableConformance1(x: HasAvailableConformance1) { // expected-note 6{{add @available attribute to enclosing global function}}
-  takesHorse(x) // expected-warning {{conformance of 'HasAvailableConformance1' to 'Horse' is only available in macOS 100 or newer}}
+  takesHorse(x) // expected-warning {{conformance of 'HasAvailableConformance1' to 'Horse' is only available in macOS 100 or newer; this is an error in Swift 6}}
   // expected-note@-1 {{add 'if #available' version check}}
 
-  takesHorseExistential(x) // expected-warning {{conformance of 'HasAvailableConformance1' to 'Horse' is only available in macOS 100 or newer}}
+  takesHorseExistential(x) // expected-warning {{conformance of 'HasAvailableConformance1' to 'Horse' is only available in macOS 100 or newer; this is an error in Swift 6}}
   // expected-note@-1 {{add 'if #available' version check}}
 
-  x.giddyUp() // expected-warning {{conformance of 'HasAvailableConformance1' to 'Horse' is only available in macOS 100 or newer}}
+  x.giddyUp() // expected-warning {{conformance of 'HasAvailableConformance1' to 'Horse' is only available in macOS 100 or newer; this is an error in Swift 6}}
   // expected-note@-1 {{add 'if #available' version check}}
 
-  _ = x.isGalloping // expected-warning {{conformance of 'HasAvailableConformance1' to 'Horse' is only available in macOS 100 or newer}}
+  _ = x.isGalloping // expected-warning {{conformance of 'HasAvailableConformance1' to 'Horse' is only available in macOS 100 or newer; this is an error in Swift 6}}
   // expected-note@-1 {{add 'if #available' version check}}
 
-  _ = x[keyPath: \.isGalloping] // expected-warning {{conformance of 'HasAvailableConformance1' to 'Horse' is only available in macOS 100 or newer}}
+  _ = x[keyPath: \.isGalloping] // expected-warning {{conformance of 'HasAvailableConformance1' to 'Horse' is only available in macOS 100 or newer; this is an error in Swift 6}}
   // expected-note@-1 {{add 'if #available' version check}}
 
-  _ = UsesHorse<HasAvailableConformance1>.self // expected-warning {{conformance of 'HasAvailableConformance1' to 'Horse' is only available in macOS 100 or newer}}
+  _ = UsesHorse<HasAvailableConformance1>.self // expected-warning {{conformance of 'HasAvailableConformance1' to 'Horse' is only available in macOS 100 or newer; this is an error in Swift 6}}
   // expected-note@-1 {{add 'if #available' version check}}
 }