diff --git a/src/dmd/optimize.d b/src/dmd/optimize.d
index 3ae30619a2..9f116fe850 100644
--- a/src/dmd/optimize.d
+++ b/src/dmd/optimize.d
@@ -697,6 +697,8 @@ Expression Expression_optimize(Expression e, int result, bool keepLvalue)
                 // See if we can remove an unnecessary cast
                 ClassDeclaration cdfrom = e.e1.type.isClassHandle();
                 ClassDeclaration cdto = e.type.isClassHandle();
+                if (cdfrom.errors || cdto.errors)
+                    return error();
                 if (cdto == ClassDeclaration.object && !cdfrom.isInterfaceDeclaration())
                     goto L1;    // can always convert a class to Object
                 // Need to determine correct offset before optimizing away the cast.
diff --git a/test/fail_compilation/test21093.d b/test/fail_compilation/test21093.d
new file mode 100644
index 0000000000..b85d0c36c3
--- /dev/null
+++ b/test/fail_compilation/test21093.d
@@ -0,0 +1,56 @@
+// https://issues.dlang.org/show_bug.cgi?id=21093
+/*
+TEST_OUTPUT:
+---
+fail_compilation/test21093.d(24): Error: function `test21093.LocalTime.hasDST` does not override any function
+fail_compilation/test21093.d(32): Error: class `test21093.LocalTime2` cannot implicitly generate a default constructor when base class `test21093.TimeZone2` is missing a default constructor
+fail_compilation/test21093.d(44): Error: function `test21093.LocalTime3.string` does not override any function
+fail_compilation/test21093.d(55): Error: cannot implicitly override base class method `test21093.TimeZone4.hasDST` with `test21093.LocalTime4.hasDST`; add `override` attribute
+---
+*/
+
+void fromUnixTime(immutable TimeZone tz = LocalTime()) { }
+void fromUnixTime(immutable TimeZone2 tz = LocalTime2()) { }
+void fromUnixTime(immutable TimeZone3 tz = LocalTime3()) { }
+void fromUnixTime(immutable TimeZone4 tz = LocalTime4()) { }
+
+class TimeZone
+{
+}
+
+class LocalTime : TimeZone
+{
+    static immutable(LocalTime) opCall() { }
+    override hasDST() { }
+}
+
+class TimeZone2
+{
+    this(string) { }
+}
+
+class LocalTime2 : TimeZone2
+{
+    static immutable(LocalTime2) opCall() { }
+}
+
+class TimeZone3
+{
+}
+
+class LocalTime3 : TimeZone3
+{
+    static immutable(LocalTime3) opCall() { }
+    override string () { }
+}
+
+class TimeZone4
+{
+    bool hasDST();
+}
+
+class LocalTime4 : TimeZone4
+{
+    static immutable(LocalTime4) opCall() { }
+    bool hasDST() { }
+}