Prior to Swift 6, diagnostic unavailable Sendable conformances via warnings.

Author: DougGregor

lib/Sema/TypeCheckAvailability.cpp

@@ -2446,6 +2446,7 @@ bool swift::diagnoseExplicitUnavailability(SourceLoc loc,
   auto proto = rootConf->getProtocol()->getDeclaredInterfaceType();
 
   StringRef platform;
+  auto behavior = DiagnosticBehavior::Unspecified;
   switch (attr->getPlatformAgnosticAvailability()) {
   case PlatformAgnosticAvailabilityKind::Deprecated:
     llvm_unreachable("shouldn't see deprecations in explicit unavailability");
@@ -2456,6 +2457,12 @@ bool swift::diagnoseExplicitUnavailability(SourceLoc loc,
       // This was platform-specific; indicate the platform.
       platform = attr->prettyPlatformString();
       break;
+    } else if (rootConf->getProtocol()->isSpecificProtocol(
+                   KnownProtocolKind::Sendable) &&
+               !ctx.LangOpts.isSwiftVersionAtLeast(6)) {
+      // Downgrade unavailable Sendable conformances to warnings prior to
+      // Swift 6.
+      behavior = DiagnosticBehavior::Warning;
     }
     LLVM_FALLTHROUGH;
 
@@ -2474,7 +2481,8 @@ bool swift::diagnoseExplicitUnavailability(SourceLoc loc,
   EncodedDiagnosticMessage EncodedMessage(attr->Message);
   diags.diagnose(loc, diag::conformance_availability_unavailable,
                  type, proto,
-                 platform.empty(), platform, EncodedMessage.Message);
+                 platform.empty(), platform, EncodedMessage.Message)
+      .limitBehavior(behavior);
 
   switch (attr->getVersionAvailability(ctx)) {
   case AvailableVersionComparison::Available:

test/Concurrency/require-explicit-sendable.swift

@@ -45,6 +45,7 @@ public struct S5 { } // no diagnostic: S5 is not Sendable
 
 @available(*, unavailable)
 extension S5: Sendable { }
+// expected-note@-1{{conformance of 'S5' to 'Sendable' has been explicitly marked unavailable here}}
 
 // Public type with a conditional conformance, so don't complain
 public struct S6<T, U> {
@@ -54,3 +55,18 @@ public struct S6<T, U> {
 
 extension S6: Sendable where T: Sendable, U: Sendable { }
 
+func acceptSendable<T: Sendable>(_: T) {
+}
+
+struct S7 {
+}
+
+@available(*, unavailable)
+extension S7: Sendable { }
+// expected-note@-1{{conformance of 'S7' to 'Sendable' has been explicitly marked unavailable here}}
+
+
+func testMe(s5: S5, s7: S7) {
+  acceptSendable(s5) // expected-warning{{conformance of 'S5' to 'Sendable' is unavailable}}
+  acceptSendable(s7) // expected-warning{{conformance of 'S7' to 'Sendable' is unavailable}}
+}