strict-boolean-expression false negative

[OliverJAsh]

Bug Report

• TSLint version: 5.2.0
• TypeScript version: 2.3.2
• Running TSLint via: (pick one) CLI

TypeScript code being linted

// fp-ts is the npm module, version 0.2.7
import { array } from 'fp-ts';

const hasTweetsAfterRangeEnd = (tweets: {}[]) => (
    array.last(tweets)
        .map(() => true)
        .getOrElse(() => false)
);

export const isLastPage = () => (tweets: {}[]) => {
    return !hasTweetsAfterRangeEnd(tweets);
};

with tslint.json configuration:

{
    "rules": {
        "no-unused-variable": true,
        "strict-boolean-expressions": true
    }
}

and tsconfig.json:

{
    "compilerOptions": {
        "module": "commonjs",
        "target": "es6",
        "strictNullChecks": true
    },
    "files": [
        "./src/publication.ts"
    ]
}

Actual behavior

❯ tslint --project tsconfig.json --type-check ./src/publication.ts

ERROR: src/publication.ts[10, 13]: This type is not allowed in the operand for the '!' operator because it is always truthy. Only booleans are allowed.

Expected behavior

No errors.

Notes

If I remove no-unused-variable, the error disappears.

[OliverJAsh]

Also, if I make the return type of hasTweetsAfterRangeEnd explicit as boolean, the error disappears.