Support safe index access for noImplicitAny

It is hard to use noImplicitAny without enabling suppressImplicitAnyIndexErrors on the current TypeScript.

Search Terms

noImplicitAny suppressImplicitAnyIndexErrors

Suggestion

Provide safe indexes without explicit type declarations to avoid unsafe index access errors.

Use Cases

• for-in
• Object.keys

Examples

const o = {};
for (const key in o) {
    o[key]; // Should be no error.
}
for (const key of Object.keys(o)) {
    o[key]; // Should be no error.
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code (for-in only)
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Start by reviewing the noImplicitAny and suppressImplicitAnyIndexErrors behavior described in the issue, along with the for-in and Object.keys examples. Done means the shown indexed accesses are accepted safely without explicit type declarations, while preserving the stated runtime and JavaScript compatibility constraints.