Erroring on Types and Namespaces Named undefined

#57573
#57575

• You can currently create variables, types, and namespaces named undefined.
• Technically this is implemented to-spec - you can rename a variable undefined.
• We could be more extreme about this.
• We're okay with forbidding in type names (e.g. type parameters, type aliases, interfaces...), but not in namespaces and variables.

Understanding no-default-lib

#57524

• What does no-default-lib mean?
• Originally, we wanted people to be able to specify an alternative lib.
  • We created no-default-lib to say "I have my own lib, don't provide one."
  • This precedes noLib and the lib array.
• Who uses it?
  • Deno uses it - kinda makes sense?
    • Let's ask Deno folks about why they need it.
  • Some people on GitHub are using it in questionable ways.
• What is it supposed to do when someone has a lib array set?
• This is spooky action at a distance. The fact that this can appear in a .d.ts file and override your compiler settings is extremely sketchy.
• We want to deprecate this.
• Wait we remember now - skipDefaultLibCheck also uses these directives to skip full type-checking of these files.
  • But we also have skipLibCheck
    • But you don't always want that because hand-authored .d.ts files should be checked.
• We would like to come up with an alternative, we want to deprecate this (6.0 Deprecation List #54500)

Type Predicate Function Inference

#57465

• We like the PR, we just want to hunt down that 1% performance regression.