Add use-before-def error for uninitialized property

[rbuckton]

Reports an error when referencing an uninitialized instance property from the initializer of another instance property when in --strictNullChecks and --strictPropertyInitialization.

Fixes #31223

[weswigham]

Should we tack on another related span to this message that says where num is initialized, since that's what the error is about?

[weswigham]

Seems good - I have a comment on the related span we're reporting (it seems to point at a not-so-useful location), but that's probably an existing issue (should we open a bug for it?).

[sandersn]

@rbuckton is this ready to merge after it's brought up to date with master?

[rbuckton]

The failing test has an interesting case:

    class A {
        a: number;
        b = this.a;  // Error
                 ~
!!! error TS2729: Property 'a' is used before its initialization.
!!! related TS2728 tests/cases/compiler/initializerWithThisPropertyAccess.ts:2:5: 'a' is declared here.
        c = () => this.a;
        d = (new A()).a;
                      ~
!!! error TS2729: Property 'a' is used before its initialization.
!!! related TS2728 tests/cases/compiler/initializerWithThisPropertyAccess.ts:2:5: 'a' is declared here.
        constructor() {
            this.a = 1;
        }
    }

This first case of b = this.a is correctly an error. What's not clear is whether the second case of d = (new A()).a should be an error. Perhaps I need to restrict the error to this references (or references whose type is this)? Or should it stand?

[rbuckton]

I'm accepting the error as is for now since its still essentially true (its initialization is circular, so no 'a' will ever be initialized when trying to initialize d...).

[rbuckton]

This was superceded by #38030