Instantiate this for contextually typed type parameters

[sandersn]

Fixes #9673

Previously this parameters were not instantiated when they came from a contextual signature. To fix this, I instantiate the signature in getThisTypeOfSignature. However, the mapper for the instantiation is only available much earlier, when checking the containing function. So I save the mapper on the containing function's signature — that signature is the one that will end up contextually typing this later.

This doesn't look right, though. The instantiation code is signature = instantiateSignature(signature, signature.mapper); and based on other instantiations in the code I don't think signatures with no target are even supposed to be instantiated, and not with their own mapper.

@ahejlsberg or @mhegazy any ideas on how to do this right? I thought about creating a new signature with the correct target/mapper setup, but I'm not sure how to make sure it gets used in place of the original one.

[sandersn]

@ahejlsberg can you look at this? I fixed this right before you got back from vacation, then went on vacation myself and forgot about it.

[sandersn]

The problem may actually be that thisParameter is instantiated too early compared to other parameters on the signature.

[sandersn]

I had to save this block for contextually-typed object literals like this:

interface CI {
  m: number
  method(this: this, n: number);
}
let o: CI = {
  m: 12,
  method(n) {
    return this.m + n;
  }
}

[sandersn]

Based on our in-person discussion, I checked on contextual typing for parameters. It's 1 of 9 cases in getTypeForVariableLikeDeclaration:

                const type = declaration.symbol.name === "this"
                    ? getContextuallyTypedThisType(func)
                    : getContextuallyTypedParameterType(<ParameterDeclaration>declaration);

The contextual typing for an undeclared this is basically the same thing here in checkThisExpression since there is no parameter to get the type of.

[sandersn]

I tried moving the contextual typing out of checkThisExpression to getSignatureFromDeclaration and it passes our tests. @ahejlsberg how does it look to you?

[ahejlsberg]

I think a better name would be getContextualThisParameter.

[sandersn]

done

[ahejlsberg]

👍 Once you make the suggested name change.