Detect usage of block-scoped variables from earlier case blocks

[RyanCavanaugh]

Fixes #19503

Note that this has the possibility to incur a false positive:

let sHasBeenSet = false;
switch (n) {
    case 0:
        let s = "ok";
        sHasBeenSet = true;
    case 1:
        if (sHasBeenSet) {
            s; // ok by construction
        }
        break;
}

[DanielRosenwasser]

Looks good, just thinking about ways we can give some more context to the error when possible.

[DanielRosenwasser]

Add a "'0' is declared here" related span to the valueDeclaration.

[DanielRosenwasser]

case needs to be quoted

Suggested change

	"Variable '{0}' is declared in a prior case block.": {
	"Variable '{0}' is declared in a prior 'case' block.": {

It's tough to give some more context, but I think it could be helpful. Maybe something like

Suggested change

	"Variable '{0}' is declared in a prior case block.": {
	"Variable '{0}' is declared in a prior 'case' block and cannot be referenced if that declaration does not execute.": {

[RyanCavanaugh]

@typescript-bot user test this

[typescript-bot]

Heya @RyanCavanaugh, I've started to run the parallelized community code test suite on this PR at e223f1b. You can monitor the build here.

[typescript-bot]

The user suite test run you requested has finished and failed. I've opened a PR with the baseline diff from master.

[RyanCavanaugh]

@typescript-bot test this

[typescript-bot]

Heya @RyanCavanaugh, I've started to run the extended test suite on this PR at e223f1b. You can monitor the build here.

[fatcerberus]

FWIW, despite technically being a false positive, I find the sample code in the OP to be... somewhat suspicious, to put it lightly. Fallthrough is a thing, but cases still look like blocks regardless. If I found that in a code review, I'd immediately suggest moving the declaration of s outside of the switch.

[sandersn]

@RyanCavanaugh do you want to come back to this? It's been sitting for a long time.

[RyanCavanaugh]

I'm not super keen on this one to be honest.