diff --git a/lldb/test/Shell/SwiftREPL/Inputs/A.swift b/lldb/test/Shell/SwiftREPL/Inputs/A.swift
index d0054413e07bb..887eb06b3efb5 100644
--- a/lldb/test/Shell/SwiftREPL/Inputs/A.swift
+++ b/lldb/test/Shell/SwiftREPL/Inputs/A.swift
@@ -24,3 +24,7 @@ public struct FromInterface {}
 public struct OtherType {}
 
 public let testValue = FromInterface()
+
+extension StringProtocol {
+  public func foo() -> Bool { true }
+}
diff --git a/lldb/test/Shell/SwiftREPL/LookupAfterImport.test b/lldb/test/Shell/SwiftREPL/LookupAfterImport.test
new file mode 100644
index 0000000000000..d067501e5f155
--- /dev/null
+++ b/lldb/test/Shell/SwiftREPL/LookupAfterImport.test
@@ -0,0 +1,15 @@
+// Make sure we properly load in new extension members after an import.
+// rdar://64040436
+
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: %target-swiftc -emit-library %S/Inputs/A.swift -module-name A -emit-module-path %t/A.swiftmodule -o %t/libA%target-shared-library-suffix
+// RUN: %lldb --repl="-I%t -L%t -lA" < %s 2>&1 | FileCheck %s
+
+"".foo()
+// CHECK: error: value of type 'String' has no member 'foo'
+
+import A
+
+"".foo()
+// CHECK: $R0: Bool = true