Tslint: strict-type-predicates reports 'Expression is always true.' which is not true

[Martin-Wegner]

Bug Report

• __TSLint version__: 5.2.0
• __TypeScript version__: 2.3.2
• __Running TSLint via__: ng lint --type-check from Angular CLI

TypeScript code being linted

['a', 'b', 'c'].map((s: string) => {
  if (s === 'b') {
    return undefined;
  }
  return 1;
}).filter((n: number) => n !== undefined);

['a', 'b', 'c'].map((s: string) => {
  if (s === 'b') {
    return undefined;
  }
  return 1;
}).filter((n: number | undefined) => n !== undefined);

['a', 'b', 'c'].map((s: string) => {
  if (s === 'b') {
    return undefined;
  }
  return 1;
}).filter(n => n !== undefined);

with tslint.json configuration:

{
  "rules": {
    "strict-type-predicates": true
  }
}

Actual behavior

ERROR: ...: Expression is always true.

Expected behavior

No error.

[ajafff]

Remove the type annotation :number which is wrong anyway. That should
make it work

Am 18.05.2017 17:19 schrieb "Martin-Wegner" [e-mail]:

Bug Report

• TSLint version: 5.2.0
• TypeScript version: 2.3.2
• Running TSLint via: ng lint --type-check from Angular CLI

TypeScript code being linted

['a', 'b', 'c'].map((s: string) => {
if (s === 'b') {
return undefined;
}
return 1;
}).filter((n: number) => n !== undefined);

with tslint.json configuration:

{
"rules": {
"strict-type-predicates": true
}
}

Actual behavior

ERROR: ...: Expression is always true.

Expected behavior

No error.

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[Martin-Wegner]

Why is number wrong? The mapping function maps string to number... When I use any the same error occurs.

Without type annotation the same error occurs.

[ajafff]

Your function maps to number | undefined. Either use that as type annotation of the parameter or just omit it and let typescript infer the type.

The bug with any is fixed by a recent PR

[Martin-Wegner]

I omitted it and the same error occurs...

[Martin-Wegner]

Using number | undefined also not work :(

[ajafff]

That indeed looks like a bug. Do you have strictNullChecks enabled in your tsconfig.json?

[Martin-Wegner]

No strictNullChecks is not enabled.

Btw: any now works for me, strange...

[ajafff]

This rule does not work as expected without strictNullChecks, because the current implementation assumes that it's enabled.

I doubt it would be useful to add special handling for strictNullChecks = false, because the purpose of this rule is to check if you are comparing to null or undefined on non-nullable values. Everything else is already checked by the compiler.
The only check that could be performed without strictNullChecks would be somethink like this:

declare var num: number;
typeof num === "function"; // this will never be true
typeof num === "symbol"; // this will never be true
typeof num === "string"; // this will never be true
...
typeof num === "object"; // could be true when num === null
typeof num === "undefined"; // could be true when num === undefined

[ajafff]

Of course we could do a better job documenting that behavior. Maybe even warn at runtime if this rule is used without strictNullChecks (and not apply this rule?).