Unexpected undefined signature declaration in validation of function assignments. Requires some more investigation why signature is undefined, even though the type checker can infer the signature alright.

Playground reproduction (check dev console): https://typescripttolua.github.io/play/#code/CYUwxgNghgTiAEBLAdgFxDAZlMCCiAbiGgM7wDeAUPPFABQCUAXPAQPaLADc18A2gGsQATxYlUMFAHMAuiwBiAV2RhUiNsh4BfSpVCRYCTMtXrk8TGzYAeACrwQAD3TJgZAORR38AD7x3AEbuAHx0YFAQEAE4AiyExKgkfLYyzKwc3JSWNp4hdIzwALzBFFoMPEA

Further discussion in discord: https://discord.com/channels/515854149821267971/515854150257344524/720265042200756226