Literal types tracking issue #5935

This issue tracks TODOs for literal types.

Core work

• Modify the parser so it can handle literal expressions in types (Add basic end-to-end support for Literal types #5947)
• Make any necessary changes to semantic analysis (see below) (Add basic end-to-end support for Literal types #5947)
• Implement the core type-checking logic (also see below)
• Implement logic for determining when and when not to infer Literal (Make literal exprs have inferred type of 'Literal' based on context #5990)
• Validate that everything works in incremental mode and with the mypy daemon (Add tests for Literal types with incremental and fine-grained mode #6075)
• Add "extra" features (again see below)
• Write docs

Parsing/semantic analysis follow-up tasks

• Distinguish between byte strings, native strings, and unicode in Python 2 and Python 3 (Add support for byte and unicode Literal strings #6087)
• Make sure everything still works with from __future__ import unicode_literals (Add support for byte and unicode Literal strings #6087)
• Add more tests for forward references and related shenanigans
• Add logic to make sure literal expressions are coerced to the correct type based on context (Make literal exprs have inferred type of 'Literal' based on context #5990)

Error messages

• Add better error messages for complex numbers (Improve error messages related to literal types #6149)
• Add better error messages for weird expressions in general (e.g. disallowing Literal[1 + 2]. This will probably involve moving all invalid type checking into the semantic analysis layer? (Improve error messages related to literal types #6149)
• Resolve Regression: error messages for literal expressions in invalid places are worse #5989 (Improve error messages related to literal types #6149)

Implementing and testing type visitors

Implement and write tests that exercise the following visitors. (Introduced by #5934)

• constraints.ConstraintBuilderVisitor (Make literal exprs have inferred type of 'Literal' based on context #5990)
• join.TypeJoinVisitor (Make literal exprs have inferred type of 'Literal' based on context #5990)
• meet.TypeMeetVisitor (Implement meet for literal types; add tests for 'is_same_type' #6043)
• subtypes.ProperSubtypeVisitor (Add basic end-to-end support for Literal types #5947)
• subtypes.SubtypeVisitor (Add basic end-to-end support for Literal types #5947)
• typeanal.TypeAnalyzer (Add basic end-to-end support for Literal types #5947)
• typeanal.TypeAnalyzerPass3 (Add basic end-to-end support for Literal types #5947)
• types.TypeStrVisitor (Add basic end-to-end support for Literal types #5947)
• type_visitor.TypeVisitor (indirectly, by other tests)
• type_visitor.TypeTranslator (indirectly, by other tests)

The following have preliminary implementations, but are missing associated tests:

• erasetype.EraseTypeVisitor (indirectly, by Add tests for literals and generics #6035)
• expandtype.ExpandTypeVisitor (indirectly, by Add tests for literals and generics #6035)
• fixup.TypeFixer (important for Enums)
• sametypes.SameTypeVisitor (Implement meet for literal types; add tests for 'is_same_type' #6043)
• server.astdiff.SnapshotTypeVisitor (Add tests for Literal types with incremental and fine-grained mode #6075)
• server.deps.TypeTriggersVisitor (Add tests for Literal types with incremental and fine-grained mode #6075)

Other checks to add in the type-checking layer

• Make sure we cannot subclass Literals (Prohibit some illegal uses of Literal #6034)
• Blacklist constructs like Type[Literal[4]]
  • Actually, should we? Maybe we should treat Type[Literal[4]] as int instead -- this might make certain TypeVar shenanigans more smooth. Need to discuss.
  • Actually, the above idea seems like a bad idea: if we do x: Literal[3], doing type(x) ought to return a value with type Type[int], not just int.
• Blacklist using Literal[...] in isinstance and issubclass checks (Prohibit some illegal uses of Literal #6034)
• Validate Literal works as expected when used in generics (Add tests for literals and generics #6035)
• Consider having mypy warn if somebody tries doing literally TypeVar('T', bound=Literal[4])? Need to discuss first.

"Extra" features to add

• Intelligent indexing of tuples, NamedTuples, TypedDict, getattr, and setattr (Add intelligent indexing of tuples, NamedTuples, and TypedDict #6124)
  • While I'm at it, design a more intelligent "macro replace" system and try and fix the misc errors we get when using --disallow-any-expr?
• Type narrowing with equality and inequality checks
• Interactions with Final (Add interactions between Literal and Final #6081)
• Add custom type guards (see Ivan's comment below, also type refinement in function call #5206)
• Treat bool as a union of Literal[True] and Literal[False] (see Treat bool as equivalent to Literal[True, False] to allow better type inference #6113)

Things to do before mypy 0.660 is released if literal types aren't going to make it

• Address Regression: error messages for literal expressions in invalid places are worse #5989 -- restore old error messages, and expunge any reference to literal types in any error messages we added. (Remove RawLiteralType synthetic type #6121)

Other things

• Try replacing one of the functions we currently manage using plugins with literal types instead, and see what happens.
  • I tried replacing the "open(...)" plugin and re-ran mypy on one of our smaller internal codebases. Out of approximately 680 calls to open, about 610 of them had 2 or more arguments. We inferred the correct type for about 560 of those. About another 20 of these were passing in string variable arguments as the mode. The remaining 30 require a little more investigation -- some people were passing in **kwargs or **args in as the argument, etc...

Things that have been deferred to after this project is completed

• Adding support for enums in literal types (and while I'm at it, I might as well investigate overhauling enums in general). Related tasks:
  • Treat enum classes as roughly a union of their values
  • Better error messages for bad imports (e.g. enums with a coerced type of 'Any')
• In error messages, we seem to quote types using either single quotes or double quotes. This can sometimes lead to messy-looking error messages (e.g. Revealed type is 'Literal['foo']'). We should standardize on a specific quoting scheme + avoid this weird "nesting" issue.
• Make sure we report an error when somebody tries doing Literal[3]() (and Final[int]() and Protocol[blah]()...)