diff --git a/lib/Sema/TypeCheckAvailability.cpp b/lib/Sema/TypeCheckAvailability.cpp
index 2ae5c6333c10a..21f10b23f3c8d 100644
--- a/lib/Sema/TypeCheckAvailability.cpp
+++ b/lib/Sema/TypeCheckAvailability.cpp
@@ -2910,7 +2910,7 @@ bool swift::diagnoseExplicitUnavailability(SourceLoc loc,
   diags.diagnose(loc, diag::conformance_availability_unavailable,
                  type, proto,
                  platform.empty(), platform, EncodedMessage.Message)
-      .limitBehavior(behavior)
+      .limitBehaviorUntilSwiftVersion(behavior, 6)
       .warnUntilSwiftVersionIf(warnIfConformanceUnavailablePreSwift6, 6);
 
   switch (attr->getVersionAvailability(ctx)) {
@@ -3468,6 +3468,11 @@ class ExprAvailabilityWalker : public ASTWalker {
       }
     }
 
+    if (auto UTO = dyn_cast<UnderlyingToOpaqueExpr>(E)) {
+      diagnoseSubstitutionMapAvailability(
+          UTO->getLoc(), UTO->substitutions, Where);
+    }
+
     if (auto ME = dyn_cast<MacroExpansionExpr>(E)) {
       diagnoseDeclRefAvailability(
           ME->getMacroRef(), ME->getMacroNameLoc().getSourceRange());
diff --git a/test/Concurrency/sendable_checking.swift b/test/Concurrency/sendable_checking.swift
index aa4dcfc2ba515..5865d6de55806 100644
--- a/test/Concurrency/sendable_checking.swift
+++ b/test/Concurrency/sendable_checking.swift
@@ -427,3 +427,15 @@ extension ImplicitSendableViaMain {
 struct TestImplicitSendable: Sendable {
   var x: ImplicitSendableViaMain
 }
+
+struct UnavailableSendable {}
+
+@available(*, unavailable)
+extension UnavailableSendable: Sendable {}
+// expected-note@-1 {{conformance of 'UnavailableSendable' to 'Sendable' has been explicitly marked unavailable here}}
+
+@available(SwiftStdlib 5.1, *)
+func checkOpaqueType() -> some Sendable {
+  UnavailableSendable()
+  // expected-warning@-1 {{conformance of 'UnavailableSendable' to 'Sendable' is unavailable; this is an error in the Swift 6 language mode}}
+}
diff --git a/test/Concurrency/sendable_checking_swift6.swift b/test/Concurrency/sendable_checking_swift6.swift
new file mode 100644
index 0000000000000..0ef9dc1bdf80c
--- /dev/null
+++ b/test/Concurrency/sendable_checking_swift6.swift
@@ -0,0 +1,16 @@
+// RUN: %target-swift-frontend -emit-sil -swift-version 6 %s -o /dev/null -verify
+
+// REQUIRES: concurrency
+
+
+struct UnavailableSendable {}
+
+@available(*, unavailable)
+extension UnavailableSendable: Sendable {}
+// expected-note@-1 {{conformance of 'UnavailableSendable' to 'Sendable' has been explicitly marked unavailable here}}
+
+@available(SwiftStdlib 5.1, *)
+func checkOpaqueType() -> some Sendable {
+  UnavailableSendable()
+  // expected-error@-1 {{conformance of 'UnavailableSendable' to 'Sendable' is unavailable}}
+}
diff --git a/test/Sema/availability.swift b/test/Sema/availability.swift
index 4ad9596476041..89f370f758a21 100644
--- a/test/Sema/availability.swift
+++ b/test/Sema/availability.swift
@@ -169,3 +169,14 @@ struct DeferBody {
   }
 }
 
+struct NotP {}
+
+protocol P {}
+
+@available(*, unavailable)
+extension NotP: P {} // expected-note {{conformance of 'NotP' to 'P' has been explicitly marked unavailable here}}
+
+@available(SwiftStdlib 5.1, *)
+func requireP() -> some P {
+  NotP() // expected-error {{conformance of 'NotP' to 'P' is unavailable}}
+}