feat(checker): evaluate constraints statically on literals

midas/checker/builtins.py

@@ -15,7 +15,7 @@ if TYPE_CHECKING:
 
 
 BUILTIN_SUBTYPES: dict[str, set[str]] = {
-    "object": {"float", "list", "dict"},
+    "object": {"float", "list", "dict", "str"},
     "float": {"int"},
     "int": {"bool"},
 }

midas/checker/python.py

@@ -1,11 +1,13 @@
 import ast
 import logging
 from dataclasses import dataclass
-from typing import Optional
+from typing import Any, Optional
 
 import midas.ast.python as p
 from midas.ast.location import Location
+from midas.ast.printer import MidasPrinter
 from midas.checker.environment import Environment
+from midas.checker.evaluator import Evaluator
 from midas.checker.operators import (
     PY_COMPARATOR_METHODS,
     PY_OPERATOR_METHODS,
@@ -18,6 +20,8 @@ from midas.checker.resolver import Resolver
 from midas.checker.types import (
     AliasType,
     AppliedType,
+    BaseType,
+    ConstraintType,
     Function,
     GenericType,
     OverloadedFunction,
@@ -538,8 +542,12 @@ class PythonTyper(
         return UnknownType()
 
     def visit_cast_expr(self, expr: p.CastExpr) -> Type:
-        _ = self.type_of(expr.expr)
-        return self.resolve_type_expr(expr.type)
+        subject_type: Type = self.type_of(expr.expr)
+        target_type: Type = self.resolve_type_expr(expr.type)
+        is_lit, lit_value = self._get_literal(expr.expr)
+        if is_lit:
+            self._evaluate_cast_statically(expr, subject_type, target_type, lit_value)
+        return target_type
 
     def visit_ternary_expr(self, expr: p.TernaryExpr) -> Type:
         test_type: Type = self.type_of(expr.test)
@@ -1108,3 +1116,83 @@ class PythonTyper(
                 return p.BaseType(location=location, base=name, param=None)
             case _:
                 raise NotImplementedError
+
+    def _get_literal(self, expr: p.Expr) -> tuple[bool, Any]:
+        match expr:
+            case p.LiteralExpr(value=value):
+                return True, value
+
+            case p.ListExpr(items=items):
+                values: list[Any] = []
+                for item in items:
+                    is_lit, value = self._get_literal(item)
+                    if not is_lit:
+                        return False, None
+                    values.append(value)
+                return True, values
+
+            case p.DictExpr(keys=keys, values=values):
+                pairs: list[tuple[Any, Any]] = []
+                for key, value in zip(keys, values):
+                    key_val = None
+                    if key is not None:
+                        is_lit, key_val = self._get_literal(key)
+                        if not is_lit:
+                            return False, None
+
+                    is_lit, value_val = self._get_literal(value)
+                    if not is_lit:
+                        return False, None
+
+                    if key is None:
+                        # TODO: check that value is always a dict
+                        assert isinstance(value_val, dict)
+                        pairs.extend(value_val.items())
+                    else:
+                        pairs.append((key_val, value_val))
+                return True, dict(pairs)
+
+            case _:
+                return False, None
+
+    def _evaluate_cast_statically(
+        self, expr: p.CastExpr, subject_type: Type, target_type: Type, lit_value: Any
+    ):
+        match target_type:
+            case AliasType(type=base):
+                self._evaluate_cast_statically(expr, subject_type, base, lit_value)
+
+            case AppliedType(name=name, args=args, body=body):
+                generic: Type = self.types.get_type(name)
+                assert isinstance(generic, GenericType)
+                for param, arg in zip(generic.params, args):
+                    pass
+                self._evaluate_cast_statically(expr, subject_type, body, lit_value)
+
+            case ConstraintType(type=base, constraint=constraint):
+                self._evaluate_cast_statically(expr, subject_type, base, lit_value)
+                evaluator = Evaluator(self.types)
+                evaluator.set_value("_", lit_value)
+                res = evaluator.evaluate(constraint)
+                if not res:
+                    printer = MidasPrinter()
+                    constraint_str: str = printer.print(constraint)
+                    self.reporter.error(
+                        expr.location,
+                        f"Value {lit_value!r} does not fit constraint '{constraint_str}'",
+                    )
+
+            case BaseType():
+                # TODO: do we want to allow cast(float, int)? would require runtime conversion
+                if not self.types.is_subtype(
+                    subject_type, target_type
+                ) or not self.types.is_subtype(target_type, subject_type):
+                    self.reporter.error(
+                        expr.location,
+                        f"Value {lit_value!r} of type {subject_type} cannot be cast as {target_type}",
+                    )
+
+            case _:
+                self.reporter.info(
+                    expr.location, f"Cannot evaluate cast to {target_type} statically"
+                )