Report use before definitely defined errors

[falsandtru]

This compiled code will throw a ReferenceError exception by native javascript engine if Class syntax is implemented. It means that TS compiler generate the invalid code. Should be disallow the use of a class before definition like a Let and Const Declaration because Class Declaration does not specified as hoisting target by es6 spec.

a; // ts: pass.
   // js: pass.
var a;
b; // ts: compile error
   // js: runtime error(ReferenceError).
let b;
c; // ts: compile error
   // js: runtime error(ReferenceError).
const c = void 0;
f(); // ts: pass.
     // js: pass.
function f(){}

new Foo(); // ts: pass.
           // js: runtime error(ReferenceError)!
           // Class Declaration is not hoistable.
           // should be a compile error before throw a runtime error.
class Foo {}

http://www.ecma-international.org/ecma-262/6.0/#sec-ecmascript-language-statements-and-declarations
http://www.ecma-international.org/ecma-262/6.0/#sec-variable-statement
http://www.2ality.com/2015/02/es6-classes-final.html

I think, this issue is different from #2854 because this issue is based on spec.

[DanielRosenwasser]

I think @vladima's CFA work might catch this.

[falsandtru]

What is CFA? Latest commit does not detect this error.

[basarat]

What is CFA

Code Flow Analysis. Fancy term for code reach-ability checking.

Latest commit does not detect this error.

The work is not on master. Here is one PR : #4788 ... there might be others 🌹

[falsandtru]

@basarat thanks! I tested on #4788 but could not detect for now. I think reach-ability check and declaration(and evaluation order) check are different.

[mhegazy]

this is not CFA; CFA is detecting unreachable code, e.g. a statement after a return. what this issue is about is Definite Assignment Analysis. you want to know that a variable is definitely assigned to before the first use site.

I think, this issue is different from #2854 because this issue is based on spec.

the JS spec talks about runtime errors. these can not all be detected at design time easily. issue #2854 is not the right issue. i will update this issue title to reflect the request.

[RyanCavanaugh]

@mhegazy will refresh his PR and implement this for extends clauses and import declarations