Add test cases for pow that are meant to fail a type check (#7760)

test_cases/stdlib/builtins/test_pow.py

@@ -1,3 +1,5 @@
+# pyright: reportUnnecessaryTypeIgnoreComment=true
+
 from decimal import Decimal
 from fractions import Fraction
 from typing import Any, NoReturn
@@ -54,7 +56,8 @@
 assert_type(Decimal("4.6") ** 7, Decimal)
 
 # These would ideally be more precise, but `Any` is acceptable
-# They have to be `Any` due to the fact that type-checkers can't distinguish between positive and negative numbers for the second argument to `pow()`
+# They have to be `Any` due to the fact that type-checkers can't distinguish
+# between positive and negative numbers for the second argument to `pow()`
 #
 # int for positive 2nd-arg, float otherwise
 assert_type(pow(4, 65), Any)
@@ -71,3 +74,18 @@
 assert_type(pow(4.7, 9.2, None), Any)
 # See #7046 -- float for a positive 1st arg, complex otherwise
 assert_type((-95) ** 8.42, Any)
+
+# All of the following cases should fail a type-checker.
+#
+# mypy/pyright will emit errors if any of them do not fail:
+# - We use --warn-unused-ignores for mypy when checking this subdirectory;
+# - For pyright, we have reportUnnecessaryTypeIgnoreComment=true at the top of this file
+pow(1.9, 4, 6)  # type: ignore[misc]
+pow(4, 7, 4.32)  # type: ignore[misc]
+pow(6.2, 5.9, 73)  # type: ignore[misc]
+pow(complex(6), 6.2, 7)  # type: ignore[misc]
+pow(Fraction(), 5, 8)  # type: ignore[call-overload]
+Decimal("8.7") ** 3.14  # type: ignore[operator]
+
+# TODO: This fails at runtime, but currently passes mypy and pyright:
+pow(Decimal("8.5"), 3.21)

tests/mypy_test.py

@@ -14,6 +14,7 @@
 import argparse
 import os
 import re
+import shutil
 import subprocess
 import sys
 import tempfile
@@ -375,7 +376,11 @@ def test_the_test_cases(code: int, major: int, minor: int, args: argparse.Namesp
     if args.dry_run:
         this_code = 0
     else:
-        this_code = subprocess.run([sys.executable, "-m", "mypy", "test_cases", *flags]).returncode
+        # --warn-unused-ignores doesn't work for files inside typeshed.
+        # SO, to work around this, we copy the test_cases directory into a TemporaryDirectory.
+        with tempfile.TemporaryDirectory() as td:
+            shutil.copytree(Path("test_cases"), Path(td) / "test_cases")
+            this_code = subprocess.run([sys.executable, "-m", "mypy", td, *flags]).returncode
     code = max(code, this_code)
     return TestResults(code, num_test_case_files)