fix corner case in collapse_vars (#4917)

fixes #4916

lib/compress.js

@@ -1490,7 +1490,7 @@ merge(Compressor.prototype, {
                 lhs = lhs.fixed_value();
             }
             if (!lhs) return true;
-            if (lhs.is_constant()) return true;
+            if (lhs.tail_node().is_constant()) return true;
             return is_lhs_read_only(lhs, compressor);
         }
         if (lhs instanceof AST_SymbolRef) {

test/compress/default-values.js

@@ -1703,3 +1703,29 @@ issue_4854: {
     expect_stdout: "undefined"
     node_version: ">=6"
 }
+
+issue_4916: {
+    options = {
+        collapse_vars: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+    }
+    input: {
+        var log = console.log;
+        (function(b = "foo") {
+            b.value = "FAIL";
+            b;
+            log(b.value);
+        })();
+    }
+    expect: {
+        var log = console.log;
+        (function(b = "foo") {
+            b.value = "FAIL";
+            b;
+            log(b.value);
+        })();
+    }
+    expect_stdout: "undefined"
+    node_version: ">=6"
+}