[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](f52c4f1afd/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.

crates/ty_python_semantic/src/types/function.rs

@@ -76,8 +76,9 @@ use crate::types::narrow::ClassInfoConstraintFunction;
 use crate::types::signatures::{CallableSignature, Signature};
 use crate::types::visitor::any_over_type;
 use crate::types::{
-    BoundMethodType, CallableType, DeprecatedInstance, DynamicType, KnownClass, Type, TypeMapping,
-    TypeRelation, TypeTransformer, TypeVarInstance, UnionBuilder, walk_type_mapping,
+    BoundMethodType, CallableType, ClassLiteral, ClassType, DeprecatedInstance, DynamicType,
+    KnownClass, Truthiness, Type, TypeMapping, TypeRelation, TypeTransformer, TypeVarInstance,
+    UnionBuilder, walk_type_mapping,
 };
 use crate::{Db, FxOrderSet, ModuleName, resolve_module};
 
@@ -882,6 +883,96 @@ impl<'db> FunctionType<'db> {
     }
 }
 
+/// Evaluate an `isinstance` call. Return `Truthiness::AlwaysTrue` if we can definitely infer that
+/// this will return `True` at runtime, `Truthiness::AlwaysFalse` if we can definitely infer
+/// that this will return `False` at runtime, or `Truthiness::Ambiguous` if we should infer `bool`
+/// instead.
+fn is_instance_truthiness<'db>(
+    db: &'db dyn Db,
+    ty: Type<'db>,
+    class: ClassLiteral<'db>,
+) -> Truthiness {
+    let is_instance = |ty: &Type<'_>| {
+        if let Type::NominalInstance(instance) = ty {
+            if instance
+                .class
+                .is_subclass_of(db, ClassType::NonGeneric(class))
+            {
+                return true;
+            }
+        }
+        false
+    };
+
+    let always_true_if = |test: bool| {
+        if test {
+            Truthiness::AlwaysTrue
+        } else {
+            Truthiness::Ambiguous
+        }
+    };
+
+    match ty {
+        Type::Union(..) => {
+            // We do not handle unions specifically here, because something like `A | SubclassOfA` would
+            // have been simplified to `A` anyway
+            Truthiness::Ambiguous
+        }
+        Type::Intersection(intersection) => always_true_if(
+            intersection
+                .positive(db)
+                .iter()
+                .any(|element| is_instance_truthiness(db, *element, class).is_always_true()),
+        ),
+
+        Type::NominalInstance(..) => always_true_if(is_instance(&ty)),
+
+        Type::BooleanLiteral(..)
+        | Type::BytesLiteral(..)
+        | Type::IntLiteral(..)
+        | Type::StringLiteral(..)
+        | Type::LiteralString
+        | Type::ModuleLiteral(..)
+        | Type::EnumLiteral(..) => always_true_if(
+            ty.literal_fallback_instance(db)
+                .as_ref()
+                .is_some_and(is_instance),
+        ),
+
+        Type::Tuple(..) => always_true_if(class.is_known(db, KnownClass::Tuple)),
+
+        Type::FunctionLiteral(..) => {
+            always_true_if(is_instance(&KnownClass::FunctionType.to_instance(db)))
+        }
+
+        Type::ClassLiteral(..) => always_true_if(is_instance(&KnownClass::Type.to_instance(db))),
+
+        Type::BoundMethod(..)
+        | Type::MethodWrapper(..)
+        | Type::WrapperDescriptor(..)
+        | Type::DataclassDecorator(..)
+        | Type::DataclassTransformer(..)
+        | Type::GenericAlias(..)
+        | Type::SubclassOf(..)
+        | Type::ProtocolInstance(..)
+        | Type::SpecialForm(..)
+        | Type::KnownInstance(..)
+        | Type::PropertyInstance(..)
+        | Type::AlwaysTruthy
+        | Type::AlwaysFalsy
+        | Type::TypeVar(..)
+        | Type::BoundSuper(..)
+        | Type::TypeIs(..)
+        | Type::Callable(..)
+        | Type::Dynamic(..)
+        | Type::Never => {
+            // We could probably try to infer more precise types in some of these cases, but it's unclear
+            // if it's worth the effort.
+            Truthiness::Ambiguous
+        }
+    }
+}
+
 fn signature_cycle_recover<'db>(
     _db: &'db dyn Db,
     _value: &CallableSignature<'db>,
@@ -1228,21 +1319,26 @@ impl KnownFunction {
             }
 
             KnownFunction::IsInstance | KnownFunction::IsSubclass => {
-                let [_, Some(Type::ClassLiteral(class))] = parameter_types else {
+                let [Some(first_arg), Some(Type::ClassLiteral(class))] = parameter_types else {
                     return;
                 };
-                let Some(protocol_class) = class.into_protocol_class(db) else {
-                    return;
-                };
-                if protocol_class.is_runtime_checkable(db) {
-                    return;
+
+                if let Some(protocol_class) = class.into_protocol_class(db) {
+                    if !protocol_class.is_runtime_checkable(db) {
+                        report_runtime_check_against_non_runtime_checkable_protocol(
+                            context,
+                            call_expression,
+                            protocol_class,
+                            self,
+                        );
+                    }
+                }
+
+                if self == KnownFunction::IsInstance {
+                    overload.set_return_type(
+                        is_instance_truthiness(db, *first_arg, *class).into_type(db),
+                    );
                 }
-                report_runtime_check_against_non_runtime_checkable_protocol(
-                    context,
-                    call_expression,
-                    protocol_class,
-                    self,
-                );
             }
 
             known @ (KnownFunction::DunderImport | KnownFunction::ImportModule) => {