[ty] Reachability analysis for isinstance(…) branches (#19503)

## 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](https://github.com/pytorch/vision/blob/f52c4f1afd7dec25cbe7b98bcf1cbc840298e8da/torchvision/io/video_reader.py#L125-L143)
where a "true positive" is removed in unreachable code. `src` is
annotated as being of type `str`, but there is an `elif isinstance(src,
bytes)` branch, which we now detect as unreachable. And so the
diagnostic inside that branch is silenced. I don't think this is a
problem, especially once we have a "graying out" feature, or a lint that
warns about unreachable code.

Author: sharkdp

crates/ty_python_semantic/resources/mdtest/call/builtins.md

@@ -105,3 +105,59 @@ str("Müsli", "utf-8")
 # error: [invalid-argument-type] "Argument to class `str` is incorrect: Expected `str`, found `Literal[b"utf-8"]`"
 str(b"M\xc3\xbcsli", b"utf-8")
 ```
+
+## Calls to `isinstance`
+
+We infer `Literal[True]` for a limited set of cases where we can be sure that the answer is correct,
+but fall back to `bool` otherwise.
+
+```py
+from enum import Enum
+from types import FunctionType
+
+class Answer(Enum):
+    NO = 0
+    YES = 1
+
+reveal_type(isinstance(True, bool))  # revealed: Literal[True]
+reveal_type(isinstance(True, int))  # revealed: Literal[True]
+reveal_type(isinstance(True, object))  # revealed: Literal[True]
+reveal_type(isinstance("", str))  # revealed: Literal[True]
+reveal_type(isinstance(1, int))  # revealed: Literal[True]
+reveal_type(isinstance(b"", bytes))  # revealed: Literal[True]
+reveal_type(isinstance(Answer.NO, Answer))  # revealed: Literal[True]
+
+reveal_type(isinstance((1, 2), tuple))  # revealed: Literal[True]
+
+def f(): ...
+
+reveal_type(isinstance(f, FunctionType))  # revealed: Literal[True]
+
+reveal_type(isinstance("", int))  # revealed: bool
+
+class A: ...
+class SubclassOfA(A): ...
+class B: ...
+
+reveal_type(isinstance(A, type))  # revealed: Literal[True]
+
+a = A()
+
+reveal_type(isinstance(a, A))  # revealed: Literal[True]
+reveal_type(isinstance(a, object))  # revealed: Literal[True]
+reveal_type(isinstance(a, SubclassOfA))  # revealed: bool
+reveal_type(isinstance(a, B))  # revealed: bool
+
+s = SubclassOfA()
+reveal_type(isinstance(s, SubclassOfA))  # revealed: Literal[True]
+reveal_type(isinstance(s, A))  # revealed: Literal[True]
+
+def _(x: A | B):
+    reveal_type(isinstance(x, A))  # revealed: bool
+
+    if isinstance(x, A):
+        pass
+    else:
+        reveal_type(x)  # revealed: B & ~A
+        reveal_type(isinstance(x, B))  # revealed: Literal[True]
+```

crates/ty_python_semantic/resources/mdtest/exhaustiveness_checking.md

@@ -0,0 +1,238 @@
+# Exhaustiveness checking
+
+```toml
+[environment]
+python-version = "3.11"
+```
+
+## Checks on literals
+
+```py
+from typing import Literal, assert_never
+
+def if_else_exhaustive(x: Literal[0, 1, "a"]):
+    if x == 0:
+        pass
+    elif x == 1:
+        pass
+    elif x == "a":
+        pass
+    else:
+        no_diagnostic_here
+
+        assert_never(x)
+
+def if_else_exhaustive_no_assertion(x: Literal[0, 1, "a"]) -> int:
+    if x == 0:
+        return 0
+    elif x == 1:
+        return 1
+    elif x == "a":
+        return 2
+
+def if_else_non_exhaustive(x: Literal[0, 1, "a"]):
+    if x == 0:
+        pass
+    elif x == "a":
+        pass
+    else:
+        this_should_be_an_error  # error: [unresolved-reference]
+
+        # this diagnostic is correct: the inferred type of `x` is `Literal[1]`
+        assert_never(x)  # error: [type-assertion-failure]
+
+def match_exhaustive(x: Literal[0, 1, "a"]):
+    match x:
+        case 0:
+            pass
+        case 1:
+            pass
+        case "a":
+            pass
+        case _:
+            # TODO: this should not be an error
+            no_diagnostic_here  # error: [unresolved-reference]
+
+            assert_never(x)
+
+# TODO: there should be no error here
+# error: [invalid-return-type] "Function can implicitly return `None`, which is not assignable to return type `int`"
+def match_exhaustive_no_assertion(x: Literal[0, 1, "a"]) -> int:
+    match x:
+        case 0:
+            return 0
+        case 1:
+            return 1
+        case "a":
+            return 2
+
+def match_non_exhaustive(x: Literal[0, 1, "a"]):
+    match x:
+        case 0:
+            pass
+        case "a":
+            pass
+        case _:
+            this_should_be_an_error  # error: [unresolved-reference]
+
+            # this diagnostic is correct: the inferred type of `x` is `Literal[1]`
+            assert_never(x)  # error: [type-assertion-failure]
+```
+
+## Checks on enum literals
+
+```py
+from enum import Enum
+from typing import assert_never
+
+class Color(Enum):
+    RED = 1
+    GREEN = 2
+    BLUE = 3
+
+def if_else_exhaustive(x: Color):
+    if x == Color.RED:
+        pass
+    elif x == Color.GREEN:
+        pass
+    elif x == Color.BLUE:
+        pass
+    else:
+        no_diagnostic_here
+
+        assert_never(x)
+
+def if_else_exhaustive_no_assertion(x: Color) -> int:
+    if x == Color.RED:
+        return 1
+    elif x == Color.GREEN:
+        return 2
+    elif x == Color.BLUE:
+        return 3
+
+def if_else_non_exhaustive(x: Color):
+    if x == Color.RED:
+        pass
+    elif x == Color.BLUE:
+        pass
+    else:
+        this_should_be_an_error  # error: [unresolved-reference]
+
+        # this diagnostic is correct: inferred type of `x` is `Literal[Color.GREEN]`
+        assert_never(x)  # error: [type-assertion-failure]
+
+def match_exhaustive(x: Color):
+    match x:
+        case Color.RED:
+            pass
+        case Color.GREEN:
+            pass
+        case Color.BLUE:
+            pass
+        case _:
+            # TODO: this should not be an error
+            no_diagnostic_here  # error: [unresolved-reference]
+
+            assert_never(x)
+
+# TODO: there should be no error here
+# error: [invalid-return-type] "Function can implicitly return `None`, which is not assignable to return type `int`"
+def match_exhaustive_no_assertion(x: Color) -> int:
+    match x:
+        case Color.RED:
+            return 1
+        case Color.GREEN:
+            return 2
+        case Color.BLUE:
+            return 3
+
+def match_non_exhaustive(x: Color):
+    match x:
+        case Color.RED:
+            pass
+        case Color.BLUE:
+            pass
+        case _:
+            this_should_be_an_error  # error: [unresolved-reference]
+
+            # this diagnostic is correct: inferred type of `x` is `Literal[Color.GREEN]`
+            assert_never(x)  # error: [type-assertion-failure]
+```
+
+## `isinstance` checks
+
+```py
+from typing import assert_never
+
+class A: ...
+class B: ...
+class C: ...
+
+def if_else_exhaustive(x: A | B | C):
+    if isinstance(x, A):
+        pass
+    elif isinstance(x, B):
+        pass
+    elif isinstance(x, C):
+        pass
+    else:
+        no_diagnostic_here
+
+        assert_never(x)
+
+def if_else_exhaustive_no_assertion(x: A | B | C) -> int:
+    if isinstance(x, A):
+        return 0
+    elif isinstance(x, B):
+        return 1
+    elif isinstance(x, C):
+        return 2
+
+def if_else_non_exhaustive(x: A | B | C):
+    if isinstance(x, A):
+        pass
+    elif isinstance(x, C):
+        pass
+    else:
+        this_should_be_an_error  # error: [unresolved-reference]
+
+        # this diagnostic is correct: the inferred type of `x` is `B & ~A & ~C`
+        assert_never(x)  # error: [type-assertion-failure]
+
+def match_exhaustive(x: A | B | C):
+    match x:
+        case A():
+            pass
+        case B():
+            pass
+        case C():
+            pass
+        case _:
+            # TODO: this should not be an error
+            no_diagnostic_here  # error: [unresolved-reference]
+
+            assert_never(x)
+
+# TODO: there should be no error here
+# error: [invalid-return-type] "Function can implicitly return `None`, which is not assignable to return type `int`"
+def match_exhaustive_no_assertion(x: A | B | C) -> int:
+    match x:
+        case A():
+            return 0
+        case B():
+            return 1
+        case C():
+            return 2
+
+def match_non_exhaustive(x: A | B | C):
+    match x:
+        case A():
+            pass
+        case C():
+            pass
+        case _:
+            this_should_be_an_error  # error: [unresolved-reference]
+
+            # this diagnostic is correct: the inferred type of `x` is `B & ~A & ~C`
+            assert_never(x)  # error: [type-assertion-failure]
+```