[Concurrency] Downgrade preconcurrency errors about unavailable conformances

to `Sendable`.

(cherry picked from commit 7331c5e)

Author: hborla

lib/Sema/TypeCheckAvailability.cpp

@@ -2848,7 +2848,8 @@ bool swift::diagnoseExplicitUnavailability(SourceLoc loc,
                                            const RootProtocolConformance *rootConf,
                                            const ExtensionDecl *ext,
                                            const ExportContext &where,
-                                           bool warnIfConformanceUnavailablePreSwift6) {
+                                           bool warnIfConformanceUnavailablePreSwift6,
+                                           bool preconcurrency) {
   auto *attr = AvailableAttr::isUnavailable(ext);
   if (!attr)
     return false;
@@ -2909,7 +2910,7 @@ bool swift::diagnoseExplicitUnavailability(SourceLoc loc,
   diags.diagnose(loc, diag::conformance_availability_unavailable,
                  type, proto,
                  platform.empty(), platform, EncodedMessage.Message)
-      .limitBehaviorUntilSwiftVersion(behavior, 6)
+      .limitBehaviorWithPreconcurrency(behavior, preconcurrency)
       .warnUntilSwiftVersionIf(warnIfConformanceUnavailablePreSwift6, 6);
 
   switch (attr->getVersionAvailability(ctx)) {
@@ -4499,7 +4500,8 @@ swift::diagnoseConformanceAvailability(SourceLoc loc,
     }
 
     if (diagnoseExplicitUnavailability(loc, rootConf, ext, where,
-                                       warnIfConformanceUnavailablePreSwift6)) {
+                                       warnIfConformanceUnavailablePreSwift6,
+                                       preconcurrency)) {
       maybeEmitAssociatedTypeNote();
       return true;
     }

lib/Sema/TypeCheckAvailability.h

@@ -276,7 +276,8 @@ bool diagnoseExplicitUnavailability(
     const RootProtocolConformance *rootConf,
     const ExtensionDecl *ext,
     const ExportContext &where,
-    bool warnIfConformanceUnavailablePreSwift6 = false);
+    bool warnIfConformanceUnavailablePreSwift6 = false,
+    bool preconcurrency = false);
 
 /// Diagnose uses of the runtime support of the given type, such as
 /// type metadata and dynamic casting.

test/Concurrency/predates_concurrency_swift6.swift

@@ -182,31 +182,52 @@ struct RequireSendable<T: Sendable> {}
 
 class NotSendable {} // expected-note 4 {{class 'NotSendable' does not conform to the 'Sendable' protocol}}
 
+class UnavailableSendable {}
+
+@available(*, unavailable)
+extension UnavailableSendable: @unchecked Sendable {}
+// expected-note@-1 4 {{conformance of 'UnavailableSendable' to 'Sendable' has been explicitly marked unavailable here}}
+
 typealias T = RequireSendable<NotSendable>
 // expected-warning@-1 {{type 'NotSendable' does not conform to the 'Sendable' protocol}}
 
-func testRequirementDowngrade(ns: NotSendable) {
+typealias T2 = RequireSendable<UnavailableSendable>
+// expected-warning@-1 {{conformance of 'UnavailableSendable' to 'Sendable' is unavailable}}
+
+func testRequirementDowngrade(ns: NotSendable, us: UnavailableSendable) {
   requireSendable(ns)
   // expected-warning@-1 {{type 'NotSendable' does not conform to the 'Sendable' protocol}}
+
+  requireSendable(us)
+  // expected-warning@-1 {{conformance of 'UnavailableSendable' to 'Sendable' is unavailable}}
 }
 
 
 protocol P2 {}
 
 extension NotSendable: P2 {}
 
+extension UnavailableSendable: P2 {}
+
 @preconcurrency
 func requireSendableExistential(_: any P2 & Sendable) {}
 
 func requireSendableExistentialAlways(_: any P2 & Sendable) {}
 
-func testErasureDowngrade(ns: NotSendable) {
+func testErasureDowngrade(ns: NotSendable, us: UnavailableSendable) {
   requireSendableExistential(ns)
   // expected-warning@-1 {{type 'NotSendable' does not conform to the 'Sendable' protocol}}
 
+  requireSendableExistential(us)
+  // expected-warning@-1 {{conformance of 'UnavailableSendable' to 'Sendable' is unavailable}}
+
   withSendableClosure {
     let ns = NotSendable()
     requireSendableExistentialAlways(ns)
     // expected-error@-1 {{type 'NotSendable' does not conform to the 'Sendable' protocol}}
+
+    let us = UnavailableSendable()
+    requireSendableExistentialAlways(us)
+    // expected-error@-1 {{conformance of 'UnavailableSendable' to 'Sendable' is unavailable}}
   }
 }