Init Solution

compiler/src/dmd/statementsem.d

@@ -1959,6 +1959,16 @@ Statement statementSemanticVisit(Statement s, Scope* sc)
                 ed = ds.isEnumDeclaration(); // typedef'ed enum
             if (!ed && te && ((ds = te.toDsymbol(sc)) !is null))
                 ed = ds.isEnumDeclaration();
+
+            // We need to check semantic2 status as enum member values are fully processed in semantic2
+            if (ed && (ed.semanticRun < PASS.semanticdone ||
+                     (ed.semanticRun < PASS.semantic2done && ed.members)))
+            {
+                error(ss.loc, "cannot use `final switch` on enum `%s` while it is being defined", ed.toChars());
+                sc.pop();
+                return setError();
+            }
+
             if (ed && ss.cases.length < ed.members.length)
             {
                 int missingMembers = 0;

compiler/test/fail_compilation/test20867.d

@@ -0,0 +1,20 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/test20867.d(14): Error: cannot use `final switch` on enum `E` while it is being defined
+---
+*/
+// Test case to verify fix for ICE when using final switch on an enum that's being defined
+
+enum E
+{
+    a = 3,
+    b = () {
+        E e;
+        final switch (e)  // This should error out instead of segfaulting
+        {
+            case E.a: break;
+        }
+        return 4;
+    } ()
+}