[ty] no more diverging query cycles in type expressions (#20359)

## Summary

Use `Type::Divergent` to short-circuit diverging types in type
expressions. This avoids panicking in a wide variety of cases of
recursive type expressions.

Avoids many panics (but not yet all -- I'll be tracking down the rest)
from https://github.com/astral-sh/ty/issues/256 by falling back to
Divergent. For many of these recursive type aliases, we'd like to
support them properly (i.e. really understand the recursive nature of
the type, not just fall back to Divergent) but that will be future work.

This switches `Type::has_divergent_type` from using `any_over_type` to a
custom set of visit methods, because `any_over_type` visits more than we
need to visit, and exercises some lazy attributes of type, causing
significantly more work. This change means this diff doesn't regress
perf; it even reclaims some of the perf regression from
https://github.com/astral-sh/ruff/pull/20333.

## Test Plan

Added mdtest for recursive type alias that panics on main.

Verified that we can now type-check `packaging` (and projects depending
on it) without panic; this will allow moving a number of mypy-primer
projects from `bad.txt` to `good.txt` in a subsequent PR.

crates/ty_python_semantic/resources/mdtest/implicit_type_aliases.md

@@ -0,0 +1,45 @@
+# Implicit type aliases
+
+Implicit type aliases are the earliest form of type alias, introduced in PEP 484. They have no
+special marker, just an ordinary assignment statement.
+
+## Basic
+
+We support simple type aliases with no extra effort, when the "value type" of the RHS is still a
+valid type for use in a type expression:
+
+```py
+MyInt = int
+
+def f(x: MyInt):
+    reveal_type(x)  # revealed: int
+
+f(1)
+```
+
+## Recursive
+
+### Old union syntax
+
+```py
+from typing import Union
+
+Recursive = list[Union["Recursive", None]]
+
+def _(r: Recursive):
+    reveal_type(r)  # revealed: list[Divergent]
+```
+
+### New union syntax
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+```py
+Recursive = list["Recursive" | None]
+
+def _(r: Recursive):
+    reveal_type(r)  # revealed: list[Divergent]
+```

crates/ty_python_semantic/src/types.rs

@@ -6496,10 +6496,7 @@ impl<'db> Type<'db> {
     }
 
     pub(super) fn has_divergent_type(self, db: &'db dyn Db, div: Type<'db>) -> bool {
-        any_over_type(db, self, &|ty| match ty {
+        any_over_type(db, self, &|ty| ty == div, false)
-            Type::Dynamic(DynamicType::Divergent(_)) => ty == div,
-            _ => false,
-        })
     }
 }
 
@@ -7445,10 +7442,27 @@ fn walk_type_var_type<'db, V: visitor::TypeVisitor<'db> + ?Sized>(
     typevar: TypeVarInstance<'db>,
     visitor: &V,
 ) {
-    if let Some(bounds) = typevar.bound_or_constraints(db) {
+    if let Some(bound_or_constraints) = if visitor.should_visit_lazy_type_attributes() {
-        walk_type_var_bounds(db, bounds, visitor);
+        typevar.bound_or_constraints(db)
+    } else {
+        match typevar._bound_or_constraints(db) {
+            _ if visitor.should_visit_lazy_type_attributes() => typevar.bound_or_constraints(db),
+            Some(TypeVarBoundOrConstraintsEvaluation::Eager(bound_or_constraints)) => {
+                Some(bound_or_constraints)
             }
-    if let Some(default_type) = typevar.default_type(db) {
+            _ => None,
+        }
+    } {
+        walk_type_var_bounds(db, bound_or_constraints, visitor);
+    }
+    if let Some(default_type) = if visitor.should_visit_lazy_type_attributes() {
+        typevar.default_type(db)
+    } else {
+        match typevar._default(db) {
+            Some(TypeVarDefaultEvaluation::Eager(default_type)) => Some(default_type),
+            _ => None,
+        }
+    } {
         visitor.visit_type(db, default_type);
     }
 }
@@ -9877,6 +9891,9 @@ fn walk_type_alias_type<'db, V: visitor::TypeVisitor<'db> + ?Sized>(
     type_alias: TypeAliasType<'db>,
     visitor: &V,
 ) {
+    if !visitor.should_visit_lazy_type_attributes() {
+        return;
+    }
     match type_alias {
         TypeAliasType::PEP695(type_alias) => {
             walk_pep_695_type_alias(db, type_alias, visitor);

crates/ty_python_semantic/src/types/function.rs

@@ -1499,8 +1499,8 @@ impl KnownFunction {
                 let contains_unknown_or_todo =
                     |ty| matches!(ty, Type::Dynamic(dynamic) if dynamic != DynamicType::Any);
                 if source_type.is_equivalent_to(db, *casted_type)
-                    && !any_over_type(db, *source_type, &contains_unknown_or_todo)
+                    && !any_over_type(db, *source_type, &contains_unknown_or_todo, true)
-                    && !any_over_type(db, *casted_type, &contains_unknown_or_todo)
+                    && !any_over_type(db, *casted_type, &contains_unknown_or_todo, true)
                 {
                     if let Some(builder) = context.report_lint(&REDUNDANT_CAST, call_expression) {
                         builder.into_diagnostic(format_args!(

crates/ty_python_semantic/src/types/infer.rs

@@ -58,6 +58,9 @@ mod builder;
 #[cfg(test)]
 mod tests;
 
+/// How many fixpoint iterations to allow before falling back to Divergent type.
+const ITERATIONS_BEFORE_FALLBACK: u32 = 10;
+
 /// Infer all types for a [`ScopeId`], including all definitions and expressions in that scope.
 /// Use when checking a scope, or needing to provide a type for an arbitrary expression in the
 /// scope.
@@ -111,12 +114,18 @@ pub(crate) fn infer_definition_types<'db>(
 }
 
 fn definition_cycle_recover<'db>(
-    _db: &'db dyn Db,
+    db: &'db dyn Db,
     _value: &DefinitionInference<'db>,
-    _count: u32,
+    count: u32,
-    _definition: Definition<'db>,
+    definition: Definition<'db>,
 ) -> salsa::CycleRecoveryAction<DefinitionInference<'db>> {
+    if count == ITERATIONS_BEFORE_FALLBACK {
+        salsa::CycleRecoveryAction::Fallback(DefinitionInference::cycle_fallback(
+            definition.scope(db),
+        ))
+    } else {
         salsa::CycleRecoveryAction::Iterate
+    }
 }
 
 fn definition_cycle_initial<'db>(
@@ -207,9 +216,6 @@ fn infer_expression_types_impl<'db>(
     .finish_expression()
 }
 
-/// How many fixpoint iterations to allow before falling back to Divergent type.
-const ITERATIONS_BEFORE_FALLBACK: u32 = 10;
-
 fn expression_cycle_recover<'db>(
     db: &'db dyn Db,
     _value: &ExpressionInference<'db>,
@@ -623,6 +629,22 @@ impl<'db> DefinitionInference<'db> {
         }
     }
 
+    fn cycle_fallback(scope: ScopeId<'db>) -> Self {
+        let _ = scope;
+
+        Self {
+            expressions: FxHashMap::default(),
+            bindings: Box::default(),
+            declarations: Box::default(),
+            #[cfg(debug_assertions)]
+            scope,
+            extra: Some(Box::new(DefinitionInferenceExtra {
+                cycle_recovery: Some(CycleRecovery::Divergent(scope)),
+                ..DefinitionInferenceExtra::default()
+            })),
+        }
+    }
+
     pub(crate) fn expression_type(&self, expression: impl Into<ExpressionNodeKey>) -> Type<'db> {
         self.try_expression_type(expression)
             .unwrap_or_else(Type::unknown)

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -8785,14 +8785,19 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                         *typevar,
                     )
                     .ok_or(GenericContextError::InvalidArgument)
-                } else if any_over_type(self.db(), *typevar, &|ty| match ty {
+                } else if any_over_type(
+                    self.db(),
+                    *typevar,
+                    &|ty| match ty {
                         Type::Dynamic(DynamicType::TodoUnpack) => true,
                         Type::NominalInstance(nominal) => matches!(
                             nominal.known_class(self.db()),
                             Some(KnownClass::TypeVarTuple | KnownClass::ParamSpec)
                         ),
                         _ => false,
-                }) {
+                    },
+                    true,
+                ) {
                     Err(GenericContextError::NotYetSupported)
                 } else {
                     if let Some(builder) =

crates/ty_python_semantic/src/types/infer/builder/type_expression.rs

@@ -21,7 +21,11 @@ use crate::types::{
 impl<'db> TypeInferenceBuilder<'db, '_> {
     /// Infer the type of a type expression.
     pub(super) fn infer_type_expression(&mut self, expression: &ast::Expr) -> Type<'db> {
-        let ty = self.infer_type_expression_no_store(expression);
+        let mut ty = self.infer_type_expression_no_store(expression);
+        let divergent = Type::divergent(self.scope());
+        if ty.has_divergent_type(self.db(), divergent) {
+            ty = divergent;
+        }
         self.store_expression_type(expression, ty);
         ty
     }
@@ -713,7 +717,7 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                 // `infer_expression` (instead of `infer_type_expression`) here to avoid
                 // false-positive `invalid-type-form` diagnostics (`1` is not a valid type
                 // expression).
-                self.infer_expression(&subscript.slice, TypeContext::default());
+                self.infer_expression(slice, TypeContext::default());
                 Type::unknown()
             }
             Type::SpecialForm(special_form) => {
@@ -721,7 +725,7 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
             }
             Type::KnownInstance(known_instance) => match known_instance {
                 KnownInstanceType::SubscriptedProtocol(_) => {
-                    self.infer_type_expression(&subscript.slice);
+                    self.infer_type_expression(slice);
                     if let Some(builder) = self.context.report_lint(&INVALID_TYPE_FORM, subscript) {
                         builder.into_diagnostic(format_args!(
                             "`typing.Protocol` is not allowed in type expressions",
@@ -730,7 +734,7 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     Type::unknown()
                 }
                 KnownInstanceType::SubscriptedGeneric(_) => {
-                    self.infer_type_expression(&subscript.slice);
+                    self.infer_type_expression(slice);
                     if let Some(builder) = self.context.report_lint(&INVALID_TYPE_FORM, subscript) {
                         builder.into_diagnostic(format_args!(
                             "`typing.Generic` is not allowed in type expressions",
@@ -739,7 +743,7 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     Type::unknown()
                 }
                 KnownInstanceType::Deprecated(_) => {
-                    self.infer_type_expression(&subscript.slice);
+                    self.infer_type_expression(slice);
                     if let Some(builder) = self.context.report_lint(&INVALID_TYPE_FORM, subscript) {
                         builder.into_diagnostic(format_args!(
                             "`warnings.deprecated` is not allowed in type expressions",
@@ -748,7 +752,7 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     Type::unknown()
                 }
                 KnownInstanceType::Field(_) => {
-                    self.infer_type_expression(&subscript.slice);
+                    self.infer_type_expression(slice);
                     if let Some(builder) = self.context.report_lint(&INVALID_TYPE_FORM, subscript) {
                         builder.into_diagnostic(format_args!(
                             "`dataclasses.Field` is not allowed in type expressions",
@@ -757,7 +761,7 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     Type::unknown()
                 }
                 KnownInstanceType::ConstraintSet(_) => {
-                    self.infer_type_expression(&subscript.slice);
+                    self.infer_type_expression(slice);
                     if let Some(builder) = self.context.report_lint(&INVALID_TYPE_FORM, subscript) {
                         builder.into_diagnostic(format_args!(
                             "`ty_extensions.ConstraintSet` is not allowed in type expressions",
@@ -766,7 +770,7 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     Type::unknown()
                 }
                 KnownInstanceType::TypeVar(_) => {
-                    self.infer_type_expression(&subscript.slice);
+                    self.infer_type_expression(slice);
                     todo_type!("TypeVar annotations")
                 }
                 KnownInstanceType::TypeAliasType(type_alias @ TypeAliasType::PEP695(_)) => {
@@ -831,8 +835,7 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                             .unwrap_or(Type::unknown())
                     }
                     None => {
-                        // TODO: Once we know that e.g. `list` is generic, emit a diagnostic if you try to
+                        // TODO: emit a diagnostic if you try to specialize a non-generic class.
-                        // specialize a non-generic class.
                         self.infer_type_expression(slice);
                         todo_type!("specialized non-generic class")
                     }
@@ -1519,13 +1522,18 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     // `Callable[]`.
                     return None;
                 }
-                if any_over_type(self.db(), self.infer_name_load(name), &|ty| match ty {
+                if any_over_type(
+                    self.db(),
+                    self.infer_name_load(name),
+                    &|ty| match ty {
                         Type::Dynamic(DynamicType::TodoPEP695ParamSpec) => true,
                         Type::NominalInstance(nominal) => {
                             nominal.has_known_class(self.db(), KnownClass::ParamSpec)
                         }
                         _ => false,
-                }) {
+                    },
+                    true,
+                ) {
                     return Some(Parameters::todo());
                 }
             }

crates/ty_python_semantic/src/types/instance.rs

@@ -9,6 +9,7 @@ use crate::place::PlaceAndQualifiers;
 use crate::semantic_index::definition::Definition;
 use crate::types::constraints::{ConstraintSet, IteratorConstraintsExtension};
 use crate::types::enums::is_single_member_enum;
+use crate::types::generics::walk_specialization;
 use crate::types::protocol_class::walk_protocol_interface;
 use crate::types::tuple::{TupleSpec, TupleType};
 use crate::types::{
@@ -528,7 +529,20 @@ pub(super) fn walk_protocol_instance_type<'db, V: super::visitor::TypeVisitor<'d
     protocol: ProtocolInstanceType<'db>,
     visitor: &V,
 ) {
+    if visitor.should_visit_lazy_type_attributes() {
         walk_protocol_interface(db, protocol.inner.interface(db), visitor);
+    } else {
+        match protocol.inner {
+            Protocol::FromClass(class) => {
+                if let Some(specialization) = class.class_literal(db).1 {
+                    walk_specialization(db, specialization, visitor);
+                }
+            }
+            Protocol::Synthesized(synthesized) => {
+                walk_protocol_interface(db, synthesized.interface(), visitor);
+            }
+        }
+    }
 }
 
 impl<'db> ProtocolInstanceType<'db> {

crates/ty_python_semantic/src/types/protocol_class.rs

@@ -573,9 +573,12 @@ impl<'a, 'db> ProtocolMember<'a, 'db> {
 
                 let proto_member_as_bound_method = method.bind_self(db);
 
-                if any_over_type(db, proto_member_as_bound_method, &|t| {
+                if any_over_type(
-                    matches!(t, Type::TypeVar(_))
+                    db,
-                }) {
+                    proto_member_as_bound_method,
+                    &|t| matches!(t, Type::TypeVar(_)),
+                    true,
+                ) {
                     // TODO: proper validation for generic methods on protocols
                     return ConstraintSet::from(true);
                 }

crates/ty_python_semantic/src/types/visitor.rs

@@ -23,6 +23,9 @@ use std::cell::{Cell, RefCell};
 /// but it makes it easy for implementors of the trait to do so.
 /// See [`any_over_type`] for an example of how to do this.
 pub(crate) trait TypeVisitor<'db> {
+    /// Should the visitor trigger inference of and visit lazily-inferred type attributes?
+    fn should_visit_lazy_type_attributes(&self) -> bool;
+
     fn visit_type(&self, db: &'db dyn Db, ty: Type<'db>);
 
     fn visit_union_type(&self, db: &'db dyn Db, union: UnionType<'db>) {
@@ -244,18 +247,28 @@ fn walk_non_atomic_type<'db, V: TypeVisitor<'db> + ?Sized>(
 ///
 /// The function guards against infinite recursion
 /// by keeping track of the non-atomic types it has already seen.
+///
+/// The `should_visit_lazy_type_attributes` parameter controls whether deferred type attributes
+/// (value of a type alias, attributes of a class-based protocol, bounds/constraints of a typevar)
+/// are visited or not.
 pub(super) fn any_over_type<'db>(
     db: &'db dyn Db,
     ty: Type<'db>,
     query: &dyn Fn(Type<'db>) -> bool,
+    should_visit_lazy_type_attributes: bool,
 ) -> bool {
     struct AnyOverTypeVisitor<'db, 'a> {
         query: &'a dyn Fn(Type<'db>) -> bool,
         seen_types: RefCell<FxIndexSet<NonAtomicType<'db>>>,
         found_matching_type: Cell<bool>,
+        should_visit_lazy_type_attributes: bool,
     }
 
     impl<'db> TypeVisitor<'db> for AnyOverTypeVisitor<'db, '_> {
+        fn should_visit_lazy_type_attributes(&self) -> bool {
+            self.should_visit_lazy_type_attributes
+        }
+
         fn visit_type(&self, db: &'db dyn Db, ty: Type<'db>) {
             let already_found = self.found_matching_type.get();
             if already_found {
@@ -283,6 +296,7 @@ pub(super) fn any_over_type<'db>(
         query,
         seen_types: RefCell::new(FxIndexSet::default()),
         found_matching_type: Cell::new(false),
+        should_visit_lazy_type_attributes,
     };
     visitor.visit_type(db, ty);
     visitor.found_matching_type.get()