Destructing nested arrays/tuples does not infer type

[ryall]

TypeScript Version: Nightly

Search Terms:
"typescript array destructing types not working"

Expected behavior:
fieldName to be of type string
emptyValue to be of type any
expectedError to be of type string

Actual behavior:
fieldName is of type string | boolean
emptyValue is of type string | boolean
expectedError is of type string | boolean

Related Issues:
#32465

Code

// Fails
for (const [fieldName, emptyValue, expectedError] of [
    ['email', '', 'must enter an email address'],
    ['password', '', 'must enter a password'],
    ['tos', false, 'must accept the ToS'],
]) {
  fieldName.charAt(0);
}

// Works
const [fieldName, emptyValue, expectedError] = ['email', '', 'must enter an email address'];

fieldName.charAt(0);

Output

"use strict";
// Fails
for (const [fieldName, emptyValue, expectedError] of [
    ['email', '', 'must enter an email address'],
    ['password', '', 'must enter a password'],
    ['tos', false, 'must accept the ToS'],
]) {
    fieldName.charAt(0);
}
// Works
const [fieldName, emptyValue, expectedError] = ['email', '', 'must enter an email address'];
fieldName.charAt(0);

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "useDefineForClassFields": false,
    "alwaysStrict": true,
    "allowUnreachableCode": false,
    "allowUnusedLabels": false,
    "downlevelIteration": false,
    "noEmitHelpers": false,
    "noLib": false,
    "noStrictGenericChecks": false,
    "noUnusedLocals": false,
    "noUnusedParameters": false,
    "esModuleInterop": true,
    "preserveConstEnums": false,
    "removeComments": false,
    "skipLibCheck": false,
    "checkJs": false,
    "allowJs": false,
    "declaration": true,
    "experimentalDecorators": false,
    "emitDecoratorMetadata": false,
    "target": "ES2017",
    "module": "ESNext"
  }
}

Playground Link: Provided

[nmain]

In the playground link you provided, typescript is already giving you exactly the right error message at exactly the right place that tells you exactly why this won't work.

The left-hand side of a 'for...of' statement cannot use a type annotation.(2483)

Relevant discussion: #3500 (comment)

Edit: Looks like you've edited your code so this response isn't relevant anymore.

[ryall]

Yeah apologies, I have updated the code to demonstrate the problem more clearly.

[ryall]

I can fix the issue (for now) by explicitly defining the type of the tuple, with:

for (const [fieldName, emptyValue, expectedError] of [
    ['email', '', 'must enter an email address'],
    ['password', '', 'must enter a password'],
    ['tos', false, 'must accept the ToS'],
] as [string, any, string][]) {
  fieldName.charAt(0);
}

[MartinJohns]

I can fix the issue (for now) by explicitly defining the type of the tuple, with:

Without the type assertion you don't have a tuple, you just have an array. You can also use as const, it's intended for this.

Changing this behavior (infer tuple instead of array by default) would be a major breaking change.

[ryall]

OK thanks, makes sense.