Approach 1

Author: abulgit

compiler/src/dmd/enumsem.d

@@ -542,7 +542,7 @@ void enumMemberSemantic(Scope* sc, EnumMember em)
         emax = emax.ctfeInterpret();
 
         // check that (eprev != emax)
-        Expression e = new EqualExp(EXP.equal, em.loc, eprev, emax);
+        Expression e = new EqualExp(EXP.equal, em.ed.loc, eprev, emax);
         e = e.expressionSemantic(sc);
         e = e.ctfeInterpret();
         if (global.endGagging(errors))

compiler/src/dmd/semantic2.d

@@ -696,6 +696,41 @@ private extern(C++) final class Semantic2Visitor : Visitor
     {
         td.foreachVar((s) { s.accept(this); });
     }
+
+    override void visit(EnumDeclaration ed)
+    {
+        //printf("EnumDeclaration::semantic2(%s) semanticRun = %d\n", ed.toChars(), ed.semanticRun);
+        if (ed.semanticRun >= PASS.semantic2done)
+            return;
+        assert(ed.semanticRun <= PASS.semantic2);
+        ed.semanticRun = PASS.semantic2;
+
+        if (!ed.members)
+            return;
+
+        // Process all enum members for proper value computation
+        auto scx = ed._scope ? ed._scope : sc;
+        if (!scx)
+            return;
+            
+        scx = scx.startCTFE();
+        scx.setNoFree(); // needed for getMaxMinValue()
+
+        ed.members.foreachDsymbol( (s)
+        {
+            if (EnumMember em = s.isEnumMember())
+            {
+                if (em._scope)
+                    scx = em._scope;
+                    
+                // Make sure the enum member values are fully evaluated
+                if (em.semanticRun < PASS.semanticdone)
+                    em.dsymbolSemantic(scx);
+            }
+        });
+
+        ed.semanticRun = PASS.semantic2done;
+    }
 }
 
 /**

compiler/src/dmd/statementsem.d

@@ -1959,6 +1959,12 @@ Statement statementSemanticVisit(Statement s, Scope* sc)
                 ed = ds.isEnumDeclaration(); // typedef'ed enum
             if (!ed && te && ((ds = te.toDsymbol(sc)) !is null))
                 ed = ds.isEnumDeclaration();
+            if (ed && ed.semanticRun < PASS.semantic2done)
+            {
+                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/fix20867.d

@@ -0,0 +1,20 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/fix20867.d(14): Error: cannot use `final switch` on enum `E` while it is being defined
+---
+*/
+
+// Test case from 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;
+    } ()
+}