fix corner case in unused (#5920)

fixes #5918

Author: alexlamsl

lib/compress.js

@@ -7253,6 +7253,10 @@ Compressor.prototype.compress = function(node) {
                     var def = node.definition();
                     var_defs[def.id] = (var_defs[def.id] || 0) + 1;
                     assignments.add(def.id, node);
+                    var fixed = node.fixed_value(true);
+                    if (fixed && fixed.tail_node().operator == "=") {
+                        verify_safe_usage(def, node, value_modified[def.id]);
+                    }
                     return true;
                 }
                 if (node instanceof AST_SymbolImport) {

test/compress/drop-unused.js

@@ -3870,3 +3870,32 @@ issue_5908_2: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5918: {
+    options = {
+        pure_getters: "strict",
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        (function(b) {
+            b.p = 42;
+        })(a = function() {
+            arguments;
+        });
+        for (var i in a)
+            console.log("PASS");
+    }
+    expect: {
+        var a;
+        (function(b) {
+            b.p = 42;
+        })(a = function() {
+            arguments;
+        });
+        for (var i in a)
+            console.log("PASS");
+    }
+    expect_stdout: "PASS"
+}