[ty]eliminate definitely-impossible types from union in equality narrowing (#20164)

solves https://github.com/astral-sh/ty/issues/939

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/ty_python_semantic/resources/mdtest/narrow/conditionals/in.md

@@ -92,3 +92,106 @@ if (x := f()) in (1,):
 else:
     reveal_type(x)  # revealed: Literal[2, 3]
 ```
+
+## Union with `Literal`, `None` and `int`
+
+```py
+from typing import Literal
+
+def test(x: Literal["a", "b", "c"] | None | int = None):
+    if x in ("a", "b"):
+        # int is included because custom __eq__ methods could make
+        # an int equal to "a" or "b", so we can't eliminate it
+        reveal_type(x)  # revealed: Literal["a", "b"] | int
+    else:
+        reveal_type(x)  # revealed: Literal["c"] | None | int
+```
+
+## Direct `not in` conditional
+
+```py
+from typing import Literal
+
+def test(x: Literal["a", "b", "c"] | None | int = None):
+    if x not in ("a", "c"):
+        # int is included because custom __eq__ methods could make
+        # an int equal to "a" or "b", so we can't eliminate it
+        reveal_type(x)  # revealed: Literal["b"] | None | int
+    else:
+        reveal_type(x)  # revealed: Literal["a", "c"] | int
+```
+
+## bool
+
+```py
+def _(x: bool):
+    if x in (True,):
+        reveal_type(x)  # revealed: Literal[True]
+    else:
+        reveal_type(x)  # revealed: Literal[False]
+
+def _(x: bool | str):
+    if x in (False,):
+        # `str` remains due to possible custom __eq__ methods on a subclass
+        reveal_type(x)  # revealed: Literal[False] | str
+    else:
+        reveal_type(x)  # revealed: Literal[True] | str
+```
+
+## LiteralString
+
+```py
+from typing_extensions import LiteralString
+
+def _(x: LiteralString):
+    if x in ("a", "b", "c"):
+        reveal_type(x)  # revealed: Literal["a", "b", "c"]
+    else:
+        reveal_type(x)  # revealed: LiteralString & ~Literal["a"] & ~Literal["b"] & ~Literal["c"]
+
+def _(x: LiteralString | int):
+    if x in ("a", "b", "c"):
+        reveal_type(x)  # revealed: Literal["a", "b", "c"] | int
+    else:
+        reveal_type(x)  # revealed: (LiteralString & ~Literal["a"] & ~Literal["b"] & ~Literal["c"]) | int
+```
+
+## enums
+
+```py
+from enum import Enum
+
+class Color(Enum):
+    RED = "red"
+    GREEN = "green"
+    BLUE = "blue"
+
+def _(x: Color):
+    if x in (Color.RED, Color.GREEN):
+        # TODO should be `Literal[Color.RED, Color.GREEN]`
+        reveal_type(x)  # revealed: Color
+    else:
+        # TODO should be `Literal[Color.BLUE]`
+        reveal_type(x)  # revealed: Color
+```
+
+## Union with enum and `int`
+
+```py
+from enum import Enum
+
+class Status(Enum):
+    PENDING = 1
+    APPROVED = 2
+    REJECTED = 3
+
+def test(x: Status | int):
+    if x in (Status.PENDING, Status.APPROVED):
+        # TODO should be `Literal[Status.PENDING, Status.APPROVED] | int`
+        # int is included because custom __eq__ methods could make
+        # an int equal to Status.PENDING or Status.APPROVED, so we can't eliminate it
+        reveal_type(x)  # revealed: Status | int
+    else:
+        # TODO should be `Literal[Status.REJECTED] | int`
+        reveal_type(x)  # revealed: Status | int
+```

crates/ty_python_semantic/src/types.rs

@@ -1054,6 +1054,16 @@ impl<'db> Type<'db> {
             || self.is_literal_string()
     }
 
+    pub(crate) fn is_union_with_single_valued(&self, db: &'db dyn Db) -> bool {
+        self.into_union().is_some_and(|union| {
+            union
+                .elements(db)
+                .iter()
+                .any(|ty| ty.is_single_valued(db) || ty.is_bool(db) || ty.is_literal_string())
+        }) || self.is_bool(db)
+            || self.is_literal_string()
+    }
+
     pub(crate) fn into_string_literal(self) -> Option<StringLiteralType<'db>> {
         match self {
             Type::StringLiteral(string_literal) => Some(string_literal),
@@ -9953,14 +9963,6 @@ impl<'db> StringLiteralType<'db> {
     pub(crate) fn python_len(self, db: &'db dyn Db) -> usize {
         self.value(db).chars().count()
     }
-
-    /// Return an iterator over each character in the string literal.
-    /// as would be returned by Python's `iter()`.
-    pub(crate) fn iter_each_char(self, db: &'db dyn Db) -> impl Iterator<Item = Self> {
-        self.value(db)
-            .chars()
-            .map(|c| StringLiteralType::new(db, c.to_string().into_boxed_str()))
-    }
 }
 
 /// # Ordering

crates/ty_python_semantic/src/types/narrow.rs

@@ -615,24 +615,88 @@ impl<'db, 'ast> NarrowingConstraintsBuilder<'db, 'ast> {
         }
     }
 
+    // TODO `expr_in` and `expr_not_in` should perhaps be unified with `expr_eq` and `expr_ne`,
+    // since `eq` and `ne` are equivalent to `in` and `not in` with only one element in the RHS.
     fn evaluate_expr_in(&mut self, lhs_ty: Type<'db>, rhs_ty: Type<'db>) -> Option<Type<'db>> {
         if lhs_ty.is_single_valued(self.db) || lhs_ty.is_union_of_single_valued(self.db) {
-            if let Type::StringLiteral(string_literal) = rhs_ty {
-                Some(UnionType::from_elements(
-                    self.db,
-                    string_literal
-                        .iter_each_char(self.db)
-                        .map(Type::StringLiteral),
-                ))
-            } else if let Some(tuple_spec) = rhs_ty.tuple_instance_spec(self.db) {
-                // N.B. Strictly speaking this is unsound, since a tuple subclass might override `__contains__`
-                // but we'd still apply the narrowing here. This seems unlikely, however, and narrowing is
-                // generally unsound in numerous ways anyway (attribute narrowing, subscript, narrowing,
-                // narrowing of globals, etc.). So this doesn't seem worth worrying about too much.
-                Some(UnionType::from_elements(self.db, tuple_spec.all_elements()))
+            rhs_ty
+                .try_iterate(self.db)
+                .ok()
+                .map(|iterable| iterable.homogeneous_element_type(self.db))
+        } else if lhs_ty.is_union_with_single_valued(self.db) {
+            let rhs_values = rhs_ty
+                .try_iterate(self.db)
+                .ok()?
+                .homogeneous_element_type(self.db);
+
+            let mut builder = UnionBuilder::new(self.db);
+
+            // Add the narrowed values from the RHS first, to keep literals before broader types.
+            builder = builder.add(rhs_values);
+
+            if let Some(lhs_union) = lhs_ty.into_union() {
+                for element in lhs_union.elements(self.db) {
+                    // Keep only the non-single-valued portion of the original type.
+                    if !element.is_single_valued(self.db)
+                        && !element.is_literal_string()
+                        && !element.is_bool(self.db)
+                    {
+                        builder = builder.add(*element);
+                    }
+                }
+            }
+            Some(builder.build())
         } else {
             None
         }
+    }
+
+    fn evaluate_expr_not_in(&mut self, lhs_ty: Type<'db>, rhs_ty: Type<'db>) -> Option<Type<'db>> {
+        let rhs_values = rhs_ty
+            .try_iterate(self.db)
+            .ok()?
+            .homogeneous_element_type(self.db);
+
+        if lhs_ty.is_single_valued(self.db) || lhs_ty.is_union_of_single_valued(self.db) {
+            // Exclude the RHS values from the entire (single-valued) LHS domain.
+            let complement = IntersectionBuilder::new(self.db)
+                .add_positive(lhs_ty)
+                .add_negative(rhs_values)
+                .build();
+            Some(complement)
+        } else if lhs_ty.is_union_with_single_valued(self.db) {
+            // Split LHS into single-valued portion and the rest. Exclude RHS values from the
+            // single-valued portion, keep the rest intact.
+            let mut single_builder = UnionBuilder::new(self.db);
+            let mut rest_builder = UnionBuilder::new(self.db);
+
+            if let Some(lhs_union) = lhs_ty.into_union() {
+                for element in lhs_union.elements(self.db) {
+                    if element.is_single_valued(self.db)
+                        || element.is_literal_string()
+                        || element.is_bool(self.db)
+                    {
+                        single_builder = single_builder.add(*element);
+                    } else {
+                        rest_builder = rest_builder.add(*element);
+                    }
+                }
+            }
+
+            let single_union = single_builder.build();
+            let rest_union = rest_builder.build();
+
+            let narrowed_single = IntersectionBuilder::new(self.db)
+                .add_positive(single_union)
+                .add_negative(rhs_values)
+                .build();
+
+            // Keep order: first literal complement, then broader arms.
+            let result = UnionBuilder::new(self.db)
+                .add(narrowed_single)
+                .add(rest_union)
+                .build();
+            Some(result)
         } else {
             None
         }
@@ -660,9 +724,7 @@ impl<'db, 'ast> NarrowingConstraintsBuilder<'db, 'ast> {
             ast::CmpOp::Eq => self.evaluate_expr_eq(lhs_ty, rhs_ty),
             ast::CmpOp::NotEq => self.evaluate_expr_ne(lhs_ty, rhs_ty),
             ast::CmpOp::In => self.evaluate_expr_in(lhs_ty, rhs_ty),
-            ast::CmpOp::NotIn => self
-                .evaluate_expr_in(lhs_ty, rhs_ty)
-                .map(|ty| ty.negate(self.db)),
+            ast::CmpOp::NotIn => self.evaluate_expr_not_in(lhs_ty, rhs_ty),
             _ => None,
         }
     }