Support lambda types in union types

[liufengyun]

Support lambda types in union types

Summary

Now a union type like Int | (Int => Int) is valid. Only one lambda type may appear in a union type, similar to the restrict that only one branch may be numeric.

Checklist

• Added / updated tests under tests/pos/ or tests/warn/
• Docs updated if the change affects user-visible behavior
• All commits are signed off (why?)

How to sign off commits

Use git commit -s to add the Signed-off-by line automatically:

To add a sign-off to the last commit retroactively:

git commit --amend -s --no-edit

To add sign-off to the last 3 commits:

git rebase --signoff HEAD~3

Security impact

No

Compatibility impact

• Source compatibility: existing code continue to compile
• SAST compatibility
  • forward compatibility: new libraries can be used by old compiler
  • backward comopatibility: old libraries can be used by new compiler
• Standard Library compatibility:
  • forward compatibility: new code can work with old stdlib
  • backward compatibility: old code work with the new stdlib
• Runtime Library compatibility:
  • forward compatibility: new code can work with old runtime
  • backward compatibility: old code work with the new runtime
• Build tool
  • Build spec compatibility: old projects continue to build
  • Joy package compatibility: old .joy can be consumed by new build tool

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Implementation checklist (for new language features)

Expand

• specify syntax (syntax-summary.md)
• token & scanner
• ast & parsing syntax
• sast
  • sast & sast operations
  • raw printer and pickling test
  • flags & symbols & name table
  • types & type operations
  • subtyping
  • encoding & decoding
• type checking
  • namer
  • expression typer
  • flow typer
  • view checker
  • adaptation
  • auto resolution
• context params
  • analysis and check
  • semantic transform
• pattern match
  • pattern typer
  • flow typing
  • determinism check
  • exhaustivity check
  • semantic transform
• phases
  • closure conversion
  • tail-rec optimization
  • erasure
• platforms
  • interpreter
  • js backend
  • ruby backend
  • python backend
  • stack machine
  • register machine
• tools
  • emacs
  • vscode
  • vim
  • js code highlight
• documentation
  • design doc
• tests
  • positive tests
  • negative tests