no-unused-variable interferes with other type checked rules

[CSchulz]

Bug Report

• TSLint version: 5.2.0 / 5.4.3
• TypeScript version: 2.3.1
• Running TSLint via: (pick one) CLI / Node.js API / VSCode / grunt-tslint / Atom / Visual Studio / etc

TypeScript code being linted

https://github.com/CSchulz/tslint-type-check-rule-collision

  protected createDefaultLogMessage(msg: LogMessage): string {
    const format: LogFormat = msg.logGroupRule.logFormat;
    let result: string = '';
    if (format.showTimeStamp) {
      result += MessageFormatUtils.renderDate(msg.date, format.dateFormat) + ' ';
    }

with tslint.json configuration:

"restrict-plus-operands": true,
"no-unused-variable": true

Actual behavior

app/app.component.ts[30, 17]: Operands of '+' operation must either be both strings or both numbers
app/app.component.ts[33, 15]: Operands of '+' operation must either be both strings or both numbers
app/app.component.ts[38, 15]: Operands of '+' operation must either be both strings or both numbers
app/app.component.ts[40, 17]: Operands of '+' operation must either be both strings or both numbers

Expected behavior

Nothing should violate.

If you disable the rule no-unused-variable it works fine.

[andy-hanson]

no-unused-variable may be exposing some bugs within TypeScript; see #2649.
(You could always just set "no-unused-locals" and "no-unused-parameters" in your config instead of using the lint rule.)

[CSchulz]

My reproducer is still broken with tslint 5.4.

A workaround is the usage of type assertion:

  protected createDefaultLogMessage(msg: LogMessage): string {
    const format: LogFormat = msg.logGroupRule.logFormat;
    let result: string = '';
    if (format.showTimeStamp) {
      result += <string>MessageFormatUtils.renderDate(msg.date, format.dateFormat) + ' ';
    }

[jscharett]

Any plans on fixing this?

[JoshuaKGoldberg]

I can no longer get this to reproduce with TypeScript 3.1.3 and TSLint 5.11. Hilarious bug, likely aged away.

Closing for housekeeping purposes, but please do post more source code & versions if it pops up again!