replace implicit 'any' with 'unknown'

Search Terms

noImplicitAny, strict, any, unknown

Suggestion

When enabling "strict": true or "noImplicitAny": true in a project, could we assign unknown to un-inferred/un-typed variables, instead of causing a compiler error? This would be a non-breaking change, since all cases where this would have an effect are already compiler errors.

Use Cases

It would allow avoiding having to specify a type when it's being type-checked anyway. The current implementation forces converted-from-js projects to deal with a lot of compiler errors from the outset (many of which use various runtime type-checking methods so should be happy with implicit unknowns).

Examples

const getMessage = input => typeof input === 'string' ? input : 'hello';
const shortenMessage = message => message.substring(3);

the getMessage example would be an error currently, but would be fine in the new world and the function would have return type string.
the shortenMessage example would have a changed error. instead of Parameter 'message' implicitly has an 'any' type we'd see an error at message.substring of Object is of type 'unknown'. To me that's more intuitive in terms of what the problem actually is. message is unknown so it should have type unknown.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript / JavaScript code
• 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. new expression-level syntax)

The issue names no implementation files or tests; start by tracing the compiler behavior for strict and noImplicitAny using the two examples in the issue. Done means un-inferred variables receive unknown where proposed, getMessage remains valid, and the diagnostic for shortenMessage changes as described.