[Concurrency] Downgrade preconcurrency errors about unavailable conformances

to `Sendable`.

Author: hborla

lib/Sema/TypeCheckAvailability.cpp

@@ -53,7 +53,8 @@ concreteSyntaxDeclForAvailableAttribute(const Decl *AbstractSyntaxDecl);
 static bool diagnoseExplicitUnavailability(
     SourceLoc loc, const RootProtocolConformance *rootConf,
     const ExtensionDecl *ext, const ExportContext &where,
-    bool warnIfConformanceUnavailablePreSwift6 = false);
+    bool warnIfConformanceUnavailablePreSwift6 = false,
+    bool preconcurrency = false);
 
 /// Emit a diagnostic for references to declarations that have been
 /// marked as unavailable, either through "unavailable" or "obsoleted:".
@@ -2986,7 +2987,8 @@ getExplicitUnavailabilityDiagnosticInfo(const Decl *decl,
 bool diagnoseExplicitUnavailability(
     SourceLoc loc, const RootProtocolConformance *rootConf,
     const ExtensionDecl *ext, const ExportContext &where,
-    bool warnIfConformanceUnavailablePreSwift6) {
+    bool warnIfConformanceUnavailablePreSwift6,
+    bool preconcurrency) {
   // Invertible protocols are never unavailable.
   if (rootConf->getProtocol()->getInvertibleProtocolKind())
     return false;
@@ -3012,7 +3014,7 @@ bool diagnoseExplicitUnavailability(
   diags
       .diagnose(loc, diag::conformance_availability_unavailable, type, proto,
                 platform.empty(), platform, EncodedMessage.Message)
-      .limitBehaviorUntilSwiftVersion(behavior, 6)
+      .limitBehaviorWithPreconcurrency(behavior, preconcurrency)
       .warnUntilSwiftVersionIf(warnIfConformanceUnavailablePreSwift6, 6);
 
   switch (diagnosticInfo->getStatus()) {
@@ -4641,7 +4643,8 @@ swift::diagnoseConformanceAvailability(SourceLoc loc,
       // FIXME: diagnoseExplicitUnavailability() should take unmet requirement
       if (diagnoseExplicitUnavailability(
               loc, rootConf, ext, where,
-              warnIfConformanceUnavailablePreSwift6)) {
+              warnIfConformanceUnavailablePreSwift6,
+              preconcurrency)) {
         maybeEmitAssociatedTypeNote();
         return true;
       }

test/Concurrency/predates_concurrency_swift6.swift

@@ -200,31 +200,52 @@ 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}}
   }
 }