diff --git a/stdlib/public/SwiftShims/RuntimeShims.h b/stdlib/public/SwiftShims/RuntimeShims.h
index b5ffb04c81958..c133adae411a2 100644
--- a/stdlib/public/SwiftShims/RuntimeShims.h
+++ b/stdlib/public/SwiftShims/RuntimeShims.h
@@ -20,6 +20,7 @@
 
 #include "SwiftStddef.h"
 #include "SwiftStdint.h"
+#include "SwiftStdbool.h"
 #include "Visibility.h"
 
 #ifdef __cplusplus
@@ -61,6 +62,12 @@ int _swift_stdlib_putc_stderr(int C);
 SWIFT_RUNTIME_STDLIB_API
 __swift_size_t _swift_stdlib_getHardwareConcurrency(void);
 
+#ifdef __swift__
+/// Called by ReflectionMirror in stdlib through C-calling-convention
+SWIFT_RUNTIME_STDLIB_API
+__swift_bool swift_isClassType(const void *type);
+#endif
+
 /// Manually allocated memory is at least 16-byte aligned in Swift.
 ///
 /// When swift_slowAlloc is called with "default" alignment (alignMask ==
diff --git a/stdlib/public/core/ReflectionMirror.swift b/stdlib/public/core/ReflectionMirror.swift
index 2f68b9078aa21..8631695d99e79 100644
--- a/stdlib/public/core/ReflectionMirror.swift
+++ b/stdlib/public/core/ReflectionMirror.swift
@@ -12,8 +12,11 @@
 
 import SwiftShims
 
-@_silgen_name("swift_isClassType")
-internal func _isClassType(_: Any.Type) -> Bool
+internal func _isClassType(_ type: Any.Type) -> Bool {
+  // a thick metatype is represented with a pointer metadata structure,
+  // so this unsafeBitCast is a safe operation here.
+  return swift_isClassType(unsafeBitCast(type, to: UnsafeRawPointer.self))
+}
 
 @_silgen_name("swift_getMetadataKind")
 internal func _metadataKind(_: Any.Type) -> UInt