Typescript: New error: Property 'value' does not exist on type 'never'

[amcasey]

https://github.com/linkedin/css-blocks/blob/6023475e9ab9a01cda6daa71986a9b4932c16ecd/packages/@css-blocks/core/src/BlockTree/Block.ts#L443

packages/@css-blocks/core/src/BlockTree/Block.ts:443:48 - error TS2339: Property 'value' does not exist on type 'never'.

443           let klass = otherBlock.getClass(next.value);
                                                   ~~~~~

packages/@css-blocks/core/src/BlockTree/Block.ts:445:32 - error TS2339: Property 'next' does not exist on type 'never'.

445             let another = next.next();
                                   ~~~~

To repro:
1) yarn
2) tsc -b -f packages

[RyanCavanaugh]

This appears to be a correct error. The relevant code looks like this (highlights for emphasis):

    let next = node.next();
    if (isRootNode(node) && next && isAttributeNode(next) && typeof next.namespace === "string") {
//                                  ^^^^^^^^^^^^^^^^^^^^^
      let otherBlock = this.getReferencedBlock(next.namespace);
      if (otherBlock) {
        if (next && isClassNode(next)) {
//                  ^^^^^^^^^^^^^^^^^
          let klass = otherBlock.getClass(next.value);

isAttributeNode returns arg is Attribute where Attribute is { type: "Attribute" }. isClassNode returns arg is ClassName where ClassName is { type: "Class" }. I don't see any other out-of-band assignments to next; I think this code actually does have a bug in it either from a runtime perspective or from an incorrect typings perspective - no value could satisfy both of those conditions.

[RyanCavanaugh]

cc file authors @chriseppstein @ramitha @amiller-gh for commentary

[amcasey]

@RyanCavanaugh Is there some way we could make it easier to work backwards from the error to the problem? Maybe the if block could report that its condition is unsatisfiable?

[chriseppstein]

I'll look into this right now.

[amiller-gh]

Looks like the error was introduced in https://github.com/linkedin/css-blocks/commit/b9c493866f0cf6916e0ce03a3c9988830e3b2c44

This is what the isAttributeNode() type check was previously: https://github.com/linkedin/css-blocks/blame/3dcd62065bc2a56ce2367f6a813379d06f16cbc2/packages/@css-blocks/core/src/BlockTree/Block.ts#L397

[chriseppstein]

I agree with @RyanCavanaugh that this code block isn't reachable and I think the error is very helpful... I wish I would have had it earlier! I agree with @amcasey that an error telling me that one of the branches of the if statement isn't reachable would be a slightly nicer error, but I think I would have been able to sort out my mistake based on the error as-is.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.