grammar errors should be reported as syntactic diagnostics

TypeScript Version: 3.3.0-dev.20181222

Search Terms:

Code

// @filename: test.js
function test(...args,) {} // trailing comma not allowed

class C {
  async static foo() {} // modifiers are in the wrong order
}

@decorator // invalid decorator location
async var v = 1; // invalid modifier location

let {...rest: rest} = {}; // rest property cannot have a property name

Expected behavior:

All errors above are reported in JS files as they result in parse errors when executed.

Actual behavior:

The above file has no errors. They are not reported as syntax errors because TypeScript's parser is more permissive than the spec in many cases. Since grammar errors are only reported during type-checking they are ignored in JS files without // @ts-check.

Other JS parsers, e.g. javascript VMs, emit a parse error for the code above.

Related Issues:
#6107 suggests moving grammar errors to the binding phase
#6802

Reproduce the examples in the TypeScript parser and compare JavaScript diagnostics with and without // @ts-check. Start by tracing grammar errors through parsing, type-checking, and the related binding phase discussed in issues #6107 and #6802. Done means the listed invalid constructs produce syntax diagnostics in JavaScript files without requiring type-checking.