Fix: Prevent ICE on final switch forward referencing its enum (#21001)

* Fix: Prevent ICE on final switch forward referencing its enum

* more simpler approach

* moved the enum member analysis

* Move enum number analysis to sementic2

* WhiteSpace Remove

* Remove Redundant Code in enumsem

compiler/src/dmd/enumsem.d

@@ -240,6 +240,10 @@ void enumSemantic(Scope* sc, EnumDeclaration ed)
 
     //if (ed.defaultval) printf("ed.defaultval: %s %s\n", ed.defaultval.toChars(), ed.defaultval.type.toChars());
     //printf("members = %s\n", members.toChars());
+
+    // Set semantic2done here to indicate all members have been processed
+    // This prevents using the enum in a final switch while being defined
+    ed.semanticRun = PASS.semantic2done;
 }
 
 Expression getDefaultValue(EnumDeclaration ed, Loc loc)

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();
+
+            // Circular references
+            // Check if enum semantic analysis is not yet complete
+            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;
+    } ()
+}