fix corner case in collapse_vars (#4041)

fixes #4040

lib/compress.js

@@ -1549,10 +1549,9 @@ merge(Compressor.prototype, {
                     return lvalues.has(node.name.name) || side_effects && may_modify(node.name);
                 }
                 var sym = is_lhs(node.left, node);
-                if (!sym) return false;
-                return lvalues.has(sym.name)
-                    || sym instanceof AST_PropAccess
-                    || read_toplevel && compressor.exposed(sym.definition());
+                if (sym instanceof AST_PropAccess) return true;
+                if (!(sym instanceof AST_SymbolRef)) return false;
+                return lvalues.has(sym.name) || read_toplevel && compressor.exposed(sym.definition());
             }
 
             function extract_args() {

test/compress/collapse_vars.js

@@ -8375,3 +8375,19 @@ issue_4038: {
     }
     expect_stdout: "PASS"
 }
+
+issue_4040: {
+    options = {
+        collapse_vars: true,
+        toplevel: true,
+    }
+    input: {
+        var a = console.log("PASS") && a.p;
+        delete NaN;
+    }
+    expect: {
+        var a = console.log("PASS") && a.p;
+        delete NaN;
+    }
+    expect_stdout: "PASS"
+}