diff --git a/lib/compress.js b/lib/compress.js
index 1f311f0668f..8358e70c5a2 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -364,7 +364,7 @@ Compressor.prototype.compress = function(node) {
                 expression: make_node(AST_Function, self, {
                     argnames: [],
                     body: self.body,
-                }).init_vars(self),
+                }).init_vars(self, self),
                 args: [],
             });
         }
@@ -8963,7 +8963,7 @@ Compressor.prototype.compress = function(node) {
                     argnames: [],
                     body: [],
                     value: make_value(),
-                }).init_vars(self.parent_scope),
+                }).init_vars(self.parent_scope, self),
                 args: [],
             }));
             return make_sequence(self, exprs);
diff --git a/lib/scope.js b/lib/scope.js
index c2108c50f16..4f80ad9dd74 100644
--- a/lib/scope.js
+++ b/lib/scope.js
@@ -439,35 +439,43 @@ AST_Toplevel.DEFMETHOD("def_global", function(node) {
     }
 });
 
-function init_block_vars(scope, parent) {
-    scope.enclosed = [];                            // variables from this or outer scope(s) that are referenced from this or inner scopes
-    scope.parent_scope = parent;                    // the parent scope (null if this is the top level)
-    scope.functions = new Dictionary();             // map name to AST_SymbolDefun (functions defined in this scope)
-    scope.variables = new Dictionary();             // map name to AST_SymbolVar (variables defined in this scope; includes functions)
-    if (parent) scope.make_def = parent.make_def;   // top-level tracking of SymbolDef instances
+function init_block_vars(scope, parent, orig) {
+    // variables from this or outer scope(s) that are referenced from this or inner scopes
+    scope.enclosed = orig ? orig.enclosed.slice() : [];
+    // map name to AST_SymbolDefun (functions defined in this scope)
+    scope.functions = orig ? orig.functions.clone() : new Dictionary();
+    // map name to AST_SymbolVar (variables defined in this scope; includes functions)
+    scope.variables = orig ? orig.variables.clone() : new Dictionary();
+    if (!parent) return;
+    // top-level tracking of SymbolDef instances
+    scope.make_def = parent.make_def;
+    // the parent scope (null if this is the top level)
+    scope.parent_scope = parent;
 }
 
-function init_scope_vars(scope, parent) {
-    init_block_vars(scope, parent);
-    scope.uses_eval = false;                        // will be set to true if this or nested scope uses the global `eval`
-    scope.uses_with = false;                        // will be set to true if this or some nested scope uses the `with` statement
+function init_scope_vars(scope, parent, orig) {
+    init_block_vars(scope, parent, orig);
+    // will be set to true if this or nested scope uses the global `eval`
+    scope.uses_eval = false;
+    // will be set to true if this or some nested scope uses the `with` statement
+    scope.uses_with = false;
 }
 
-AST_BlockScope.DEFMETHOD("init_vars", function(parent_scope) {
-    init_block_vars(this, parent_scope);
+AST_BlockScope.DEFMETHOD("init_vars", function(parent, orig) {
+    init_block_vars(this, parent, orig);
 });
-AST_Scope.DEFMETHOD("init_vars", function(parent_scope) {
-    init_scope_vars(this, parent_scope);
+AST_Scope.DEFMETHOD("init_vars", function(parent, orig) {
+    init_scope_vars(this, parent, orig);
 });
-AST_Arrow.DEFMETHOD("init_vars", function(parent_scope) {
-    init_scope_vars(this, parent_scope);
+AST_Arrow.DEFMETHOD("init_vars", function(parent, orig) {
+    init_scope_vars(this, parent, orig);
     return this;
 });
-AST_AsyncArrow.DEFMETHOD("init_vars", function(parent_scope) {
-    init_scope_vars(this, parent_scope);
+AST_AsyncArrow.DEFMETHOD("init_vars", function(parent, orig) {
+    init_scope_vars(this, parent, orig);
 });
-AST_Lambda.DEFMETHOD("init_vars", function(parent_scope) {
-    init_scope_vars(this, parent_scope);
+AST_Lambda.DEFMETHOD("init_vars", function(parent, orig) {
+    init_scope_vars(this, parent, orig);
     this.uses_arguments = false;
     this.def_variable(new AST_SymbolFunarg({
         name: "arguments",
diff --git a/test/compress/classes.js b/test/compress/classes.js
index 8b6c8b59ae3..bb56f71aa2b 100644
--- a/test/compress/classes.js
+++ b/test/compress/classes.js
@@ -3901,3 +3901,29 @@ issue_5682_class_key_computed: {
     expect_stdout: "PASS"
     node_version: ">=4"
 }
+
+issue_5724: {
+    options = {
+        arrows: true,
+        inline: true,
+        keep_fargs: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        "use strict";
+        class A {
+            static P = function(a) {
+                console.log(a, a);
+            }(a);
+        }
+    }
+    expect: {
+        "use strict";
+        (function(a) {
+            console.log(a, a);
+        })(a);
+    }
+    expect_stdout: ReferenceError("a is not defined")
+    node_version: ">=12"
+}