diff --git a/test/Interop/Cxx/class/Inputs/module.modulemap b/test/Interop/Cxx/class/Inputs/module.modulemap
index 1b740175317d4..daf6d70bf544b 100644
--- a/test/Interop/Cxx/class/Inputs/module.modulemap
+++ b/test/Interop/Cxx/class/Inputs/module.modulemap
@@ -9,3 +9,7 @@ module MemoryLayout {
 module MemberVariables {
   header "member-variables.h"
 }
+
+module ProtocolConformance {
+  header "protocol-conformance.h"
+}
diff --git a/test/Interop/Cxx/class/Inputs/protocol-conformance.h b/test/Interop/Cxx/class/Inputs/protocol-conformance.h
new file mode 100644
index 0000000000000..d607b5a3caa54
--- /dev/null
+++ b/test/Interop/Cxx/class/Inputs/protocol-conformance.h
@@ -0,0 +1,7 @@
+struct ConformsToProtocol {
+  int return42() { return 42; }
+};
+
+struct DoesNotConformToProtocol {
+  int returnFortyTwo() { return 42; }
+};
diff --git a/test/Interop/Cxx/class/protocol-conformance-silgen.swift b/test/Interop/Cxx/class/protocol-conformance-silgen.swift
new file mode 100644
index 0000000000000..951f10a0b9ca6
--- /dev/null
+++ b/test/Interop/Cxx/class/protocol-conformance-silgen.swift
@@ -0,0 +1,15 @@
+// Tests that a C++ class can conform to a Swift protocol.
+
+// RUN: %target-swift-emit-silgen -I %S/Inputs -enable-cxx-interop %s
+
+import ProtocolConformance
+
+protocol HasReturn42 {
+  mutating func return42() -> CInt
+}
+
+// FIXME:
+// https://bugs.swift.org/browse/SR-12750
+// SILGen currently hits an assertion failure in getParameterTypes() when the
+// following protocol conformance is declared.
+// extension ConformsToProtocol : HasReturn42 {}
diff --git a/test/Interop/Cxx/class/protocol-conformance-typechecker.swift b/test/Interop/Cxx/class/protocol-conformance-typechecker.swift
new file mode 100644
index 0000000000000..bafb5809b6d40
--- /dev/null
+++ b/test/Interop/Cxx/class/protocol-conformance-typechecker.swift
@@ -0,0 +1,13 @@
+// Tests that a C++ class can conform to a Swift protocol.
+
+// RUN: %target-typecheck-verify-swift -I %S/Inputs -enable-cxx-interop
+
+import ProtocolConformance
+
+protocol HasReturn42 {
+  mutating func return42() -> CInt // expected-note {{requires function 'return42()'}}
+}
+
+extension ConformsToProtocol : HasReturn42 {}
+
+extension DoesNotConformToProtocol : HasReturn42 {} // expected-error {{'DoesNotConformToProtocol' does not conform to protocol}}