Do not crash when folding constants that overflow

mypy/constant_fold.py

@@ -112,6 +112,9 @@ def constant_fold_binary_op(
 
 
 def constant_fold_binary_int_op(op: str, left: int, right: int) -> int | float | None:
+    # Operands are unbounded ints, so some results do not fit into a float (`/`) or
+    # cannot be built at all (`<<` with a huge count). Folding is an optimization:
+    # when it cannot produce a value, return None and let the expression stand.
     if op == "+":
         return left + right
     if op == "-":
@@ -120,7 +123,10 @@ def constant_fold_binary_int_op(op: str, left: int, right: int) -> int | float |
         return left * right
     elif op == "/":
         if right != 0:
-            return left / right
+            try:
+                return left / right
+            except OverflowError:
+                return None
     elif op == "//":
         if right != 0:
             return left // right
@@ -135,7 +141,10 @@ def constant_fold_binary_int_op(op: str, left: int, right: int) -> int | float |
         return left ^ right
     elif op == "<<":
         if right >= 0:
-            return left << right
+            try:
+                return left << right
+            except (OverflowError, ValueError):
+                return None
     elif op == ">>":
         if right >= 0:
             return left >> right
@@ -149,22 +158,27 @@ def constant_fold_binary_int_op(op: str, left: int, right: int) -> int | float |
 
 def constant_fold_binary_float_op(op: str, left: int | float, right: int | float) -> float | None:
     assert not (isinstance(left, int) and isinstance(right, int)), (op, left, right)
-    if op == "+":
-        return left + right
-    elif op == "-":
-        return left - right
-    elif op == "*":
-        return left * right
-    elif op == "/":
-        if right != 0:
-            return left / right
-    elif op == "//":
-        if right != 0:
-            return left // right
-    elif op == "%":
-        if right != 0:
-            return left % right
-    elif op == "**":
+    # An int operand here is unbounded, so converting it to a float can overflow.
+    # `**` already guards against this; the other operations get the same treatment.
+    try:
+        if op == "+":
+            return left + right
+        elif op == "-":
+            return left - right
+        elif op == "*":
+            return left * right
+        elif op == "/":
+            if right != 0:
+                return left / right
+        elif op == "//":
+            if right != 0:
+                return left // right
+        elif op == "%":
+            if right != 0:
+                return left % right
+    except OverflowError:
+        return None
+    if op == "**":
         if (left < 0 and isinstance(right, int)) or left > 0:
             try:
                 ret = left**right

mypy/test/testconstantfold.py

@@ -0,0 +1,28 @@
+"""Tests for constant folding of huge operands."""
+
+from __future__ import annotations
+
+from mypy.constant_fold import constant_fold_binary_float_op, constant_fold_binary_int_op
+from mypy.test.helpers import Suite
+
+BIG = 2**2000
+
+
+class ConstantFoldOverflowSuite(Suite):
+    """Folding is an optimization: when a result cannot be built, it must yield None."""
+
+    def test_int_div_overflow(self) -> None:
+        assert constant_fold_binary_int_op("/", BIG, 3) is None
+
+    def test_int_lshift_huge_count(self) -> None:
+        assert constant_fold_binary_int_op("<<", 1, 2**70) is None
+
+    def test_float_ops_with_huge_int(self) -> None:
+        for op in ("+", "-", "*", "/", "//", "%"):
+            assert constant_fold_binary_float_op(op, BIG, 1.0) is None, op
+
+    def test_small_operands_still_fold(self) -> None:
+        assert constant_fold_binary_int_op("/", 6, 3) == 2.0
+        assert constant_fold_binary_int_op("<<", 1, 4) == 16
+        assert constant_fold_binary_float_op("+", 1, 2.5) == 3.5
+        assert constant_fold_binary_float_op("**", 2.0, 3) == 8.0

mypy/test/testtypes.py

@@ -61,6 +61,19 @@
 import mypy.expandtype  # ruff: isort: skip
 
 
+class LiteralTypeReprSuite(Suite):
+    def setUp(self) -> None:
+        self.fx = TypeFixture()
+
+    def test_value_repr_of_huge_int(self) -> None:
+        # repr() of an int is limited by sys.set_int_max_str_digits(); a literal built
+        # from a folded power can exceed it and used to raise ValueError.
+        huge = LiteralType(2**100000, self.fx.a)
+        rendered = huge.value_repr()
+        assert rendered.startswith("0x")
+        assert int(rendered, 16) == 2**100000
+
+
 class TypesSuite(Suite):
     def setUp(self) -> None:
         self.x = UnboundType("X")  # Helpers

mypy/types.py

@@ -3396,7 +3396,13 @@ def value_repr(self) -> str:
         if isinstance(self.value, SentinelValue):
             return self.value.name
 
-        raw = repr(self.value)
+        try:
+            raw = repr(self.value)
+        except ValueError:
+            # int -> str conversion is limited by sys.set_int_max_str_digits(); a literal
+            # type built from a folded power can exceed it. Fall back to a lossless form.
+            assert isinstance(self.value, int)
+            raw = hex(self.value)
         fallback_name = self.fallback.type.fullname
 
         # If this is backed by an enum,