diff --git a/lib/AST/ConformanceLookupTable.cpp b/lib/AST/ConformanceLookupTable.cpp
index 7381bcf3cb607..ca4aa834dea50 100644
--- a/lib/AST/ConformanceLookupTable.cpp
+++ b/lib/AST/ConformanceLookupTable.cpp
@@ -599,6 +599,11 @@ ConformanceLookupTable::Ordering ConformanceLookupTable::compareConformances(
     }
   }
 
+  auto isUnavailable = [](DeclContext *dc) -> bool {
+    auto *ext = dyn_cast<ExtensionDecl>(dc);
+    return ext && ext->isUnavailable();
+  };
+
   // If only one of the conformances is unconditionally available on the
   // current deployment target, pick that one.
   //
@@ -610,7 +615,9 @@ ConformanceLookupTable::Ordering ConformanceLookupTable::compareConformances(
       rhs->getDeclContext()->isAlwaysAvailableConformanceContext()) {
     // Diagnose conflicting marker protocol conformances that differ in
     // un-availability.
-    diagnoseSuperseded = lhs->getProtocol()->isMarkerProtocol();
+    diagnoseSuperseded = (lhs->getProtocol()->isMarkerProtocol() &&
+                          isUnavailable(lhs->getDeclContext()) !=
+                          isUnavailable(rhs->getDeclContext()));
 
     return (lhs->getDeclContext()->isAlwaysAvailableConformanceContext()
             ? Ordering::Before
diff --git a/test/Sema/Inputs/conformance_availability_implied_other.swift b/test/Sema/Inputs/conformance_availability_implied_other.swift
new file mode 100644
index 0000000000000..7e045ec5cbea0
--- /dev/null
+++ b/test/Sema/Inputs/conformance_availability_implied_other.swift
@@ -0,0 +1,8 @@
+
+@available(macOS 200, *)
+extension Conformer1: Derived2 {}
+
+@available(macOS 100, *)
+extension Conformer2: Derived1 {}
+// expected-error@-1 {{conformance of 'Conformer2' to 'Base' is only available in macOS 200 or newer}}
+
diff --git a/test/Sema/conformance_availability_implied.swift b/test/Sema/conformance_availability_implied.swift
new file mode 100644
index 0000000000000..00c8f0a4019ad
--- /dev/null
+++ b/test/Sema/conformance_availability_implied.swift
@@ -0,0 +1,59 @@
+// RUN: %target-typecheck-verify-swift -swift-version 6
+// REQUIRES: OS=macosx
+
+// Protocols:
+
+protocol Base {
+  func f() // expected-note {{protocol requirement here}}
+}
+
+func takesBase<T: Base>(_: T.Type) {}
+
+protocol Derived1: Base {}
+protocol Derived2: Base {}
+
+@_marker protocol MarkerBase {}
+
+func takesMarkerBase<T: MarkerBase>(_: T.Type) {}
+
+protocol MarkerDerived1: MarkerBase {}
+protocol MarkerDerived2: MarkerBase {}
+
+// Verify that the implied conformance is macOS 100:
+struct Conformer1 {}
+
+@available(macOS 100, *)
+extension Conformer1: Derived1 {
+  func f() {}  // okay!
+}
+
+@available(macOS 200, *)
+extension Conformer1: Derived2 {}
+
+takesBase(Conformer1.self)
+// expected-error@-1 {{conformance of 'Conformer1' to 'Base' is only available in macOS 100 or newer}}
+// expected-note@-2 {{add 'if #available' version check}}
+
+// No warning about redundant MarkerBase conformance (rdar://142873265):
+@available(macOS 100, *)
+extension Conformer1: MarkerDerived1 {}
+
+@available(macOS 200, *)
+extension Conformer1: MarkerDerived2 {}
+
+takesMarkerBase(Conformer1.self)
+// expected-error@-1 {{conformance of 'Conformer1' to 'MarkerBase' is only available in macOS 100 or newer}}
+// expected-note@-2 {{add 'if #available' version check}}
+
+// Bad availability on the Base.f() witness:
+struct Conformer2 {}
+
+@available(macOS 100, *)
+extension Conformer2: Derived1 {
+// expected-error@-1 {{protocol 'Base' requires 'f()' to be available in macOS 100 and newer}}
+}
+
+@available(macOS 200, *)
+extension Conformer2: Derived2 {
+  func f() {} // expected-note {{'f()' declared here}}
+}
diff --git a/test/Sema/conformance_availability_implied_multifile.swift b/test/Sema/conformance_availability_implied_multifile.swift
new file mode 100644
index 0000000000000..fccb6a9f94834
--- /dev/null
+++ b/test/Sema/conformance_availability_implied_multifile.swift
@@ -0,0 +1,48 @@
+// RUN: %target-swift-frontend -typecheck -verify %s %S/Inputs/conformance_availability_implied_other.swift -swift-version 6
+// RUN: %target-swift-frontend -typecheck -verify %S/Inputs/conformance_availability_implied_other.swift %s -swift-version 6
+// REQUIRES: OS=macosx
+
+protocol Base {
+  func f()
+}
+
+func takesBase<T: Base>(_: T.Type) {}
+
+protocol Derived1: Base {}
+protocol Derived2: Base {}
+
+// Verify that the implied conformance is macOS 100:
+struct Conformer1 {}
+
+@available(macOS 100, *)
+extension Conformer1: Derived1 {
+  func f() {}  // okay!
+}
+
+// Note that Conformer1: Derived2 is in the other file
+
+func check1() {
+// expected-note@-1 {{add '@available' attribute to enclosing global function}}
+  takesBase(Conformer1.self)
+  // expected-error@-1 {{conformance of 'Conformer1' to 'Base' is only available in macOS 100 or newer}}
+  // expected-note@-2 {{add 'if #available' version check}}
+}
+
+// Verify that the implied conformance is macOS 200:
+// FIXME: This appears to be unsound!
+struct Conformer2 {}
+
+@available(macOS 200, *)
+extension Conformer2: Derived2 {
+  func f() {}
+}
+
+// Note that Conformer2: Derived1 is in the other file
+
+func check2() {
+// expected-note@-1 {{add '@available' attribute to enclosing global function}}
+  takesBase(Conformer2.self)
+  // expected-error@-1 {{conformance of 'Conformer2' to 'Base' is only available in macOS 200 or newer}}
+  // expected-note@-2 {{add 'if #available' version check}}
+}
+