fix corner case in collapse_vars (#5395)

fixes #5394

lib/compress.js

@@ -2248,8 +2248,11 @@ Compressor.prototype.compress = function(node) {
                     var lvalues = get_lvalues(candidate);
                     var lhs_local = is_lhs_local(lhs);
                     var rhs_value = get_rvalue(candidate);
-                    var rvalue = !compound && rhs_value instanceof AST_Sequence ? rhs_value.tail_node() : rhs_value;
+                    var rvalue = rhs_value;
-                    if (!side_effects) side_effects = value_has_side_effects();
+                    if (!side_effects) {
+                        if (!compound && rvalue instanceof AST_Sequence) rvalue = rvalue.tail_node();
+                        side_effects = value_has_side_effects();
+                    }
                     var check_destructured = in_try || !lhs_local ? function(node) {
                         return node instanceof AST_Destructured;
                     } : return_false;

test/compress/collapse_vars.js

@@ -9925,3 +9925,25 @@ issue_5309_2: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5394: {
+    options = {
+        collapse_vars: true,
+        evaluate: true,
+    }
+    input: {
+        try {
+            throw A.p = (console.log("FAIL"), []), !1;
+        } catch (e) {
+            console.log(typeof e);
+        }
+    }
+    expect: {
+        try {
+            throw !(A.p = (console.log("FAIL"), []));
+        } catch (e) {
+            console.log(typeof e);
+        }
+    }
+    expect_stdout: "object"
+}