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## Summary
Add more precise type inference for a limited set of `isinstance(…)`
calls, i.e. return `Literal[True]` if we can be sure that this is the
correct result. This improves exhaustiveness checking / reachability
analysis for if-elif-else chains with `isinstance` checks. For example:
```py
def is_number(x: int | str) -> bool: # no "can implicitly return `None` error here anymore
if isinstance(x, int):
return True
elif isinstance(x, str):
return False
# code here is now detected as being unreachable
```
This PR also adds a new test suite for exhaustiveness checking.
## Test Plan
New Markdown tests
### Ecosystem analysis
The removed diagnostics look good. There's [one
case](
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| .. | ||
| annotation.md | ||
| builtins.md | ||
| callable_instance.md | ||
| constructor.md | ||
| dunder.md | ||
| dunder_import.md | ||
| function.md | ||
| getattr_static.md | ||
| invalid_syntax.md | ||
| methods.md | ||
| never.md | ||
| overloads.md | ||
| str_startswith.md | ||
| subclass_of.md | ||
| union.md | ||