Fix: Prevent ICE on final switch forward referencing its enum

Author: abulgit

compiler/src/dmd/statementsem.d

@@ -1959,8 +1959,44 @@ Statement statementSemanticVisit(Statement s, Scope* sc)
                 ed = ds.isEnumDeclaration(); // typedef'ed enum
             if (!ed && te && ((ds = te.toDsymbol(sc)) !is null))
                 ed = ds.isEnumDeclaration();
+
+            // Circular references
+            if (ed)
+            {
+                // Check if inside the initializer of one of the enum's members
+                for (Scope* scx = sc; scx; scx = scx.enclosing)
+                {
+                    if (scx.scopesym && scx.scopesym.isEnumDeclaration() && scx.scopesym == ed)
+                    {
+                        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)
             {
+                // Add a check for incomplete enum declaration to prevent segfault
+                // when the enum is being defined while it's referenced in a final switch
+                bool isEnumIncomplete = false;
+                foreach (es; *ed.members)
+                {
+                    EnumMember em = es.isEnumMember();
+                    if (em && em.value is null)
+                    {
+                        isEnumIncomplete = true;
+                        break;
+                    }
+                }
+
+                if (isEnumIncomplete)
+                {
+                    error(ss.loc, "cannot use `final switch` on enum `%s` while it is being defined", ed.toChars());
+                    sc.pop();
+                    return setError();
+                }
+
                 int missingMembers = 0;
                 const maxShown = global.params.v.errorSupplementCount();
             Lmembers:

compiler/test/fail_compilation/fix20867.d

@@ -0,0 +1,70 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/fix20867.d(16): Error: cannot use `final switch` on enum `E` while it is being defined
+fail_compilation/fix20867.d(31): Error: cannot use `final switch` on enum `E2` while it is being defined
+fail_compilation/fix20867.d(63): Error: cannot use `final switch` on enum `E4` while it is being defined
+---
+*/
+
+// Test case 1: The exact scenario Issue(From Github Issue(#20867))
+enum E
+{
+    a = 3,
+    b = () {
+        E e;
+        final switch (e)  // This should error out instead of segfaulting
+        {
+            case E.a: break;
+        }
+        return 4;
+    } ()
+}
+
+// Test case 2: Variation with multiple members
+enum E2
+{
+    x = 10,
+    y = 20,
+    z = () {
+        E2 e;
+        final switch (e)  // Should also error out safely
+        {
+            case E2.x: return 30;
+            case E2.y: return 40;
+        }
+    } ()
+}
+
+// Test case 3: Regular use of final switch (this should still compile)
+enum E3
+{
+    p = 1,
+    q = 2
+}
+
+void testE3()
+{
+    E3 e = E3.p;
+    final switch (e)
+    {
+        case E3.p: break;
+        case E3.q: break;
+    }
+}
+
+// Test case 4: Nested circular reference
+enum E4
+{
+    r = 5,
+    s = () {
+        int foo() {
+            E4 e;
+            final switch (e)  // Should error out
+            {
+                case E4.r: return 6;
+            }
+        }
+        return foo();
+    } ()
+}