Make ExceptionInfo generic in the exception type

This way, in

    with pytest.raises(ValueError) as cm:
        ...

cm.value is a ValueError and not a BaseException.

src/_pytest/python_api.py

@@ -10,10 +10,13 @@ from numbers import Number
 from types import TracebackType
 from typing import Any
 from typing import Callable
+from typing import cast
+from typing import Generic
 from typing import Optional
 from typing import overload
 from typing import Pattern
 from typing import Tuple
+from typing import TypeVar
 from typing import Union
 
 from more_itertools.more import always_iterable
@@ -537,33 +540,35 @@ def _is_numpy_array(obj):
 
 # builtin pytest.raises helper
 
+_E = TypeVar("_E", bound=BaseException)
+
 
 @overload
 def raises(
-    expected_exception: Union["Type[BaseException]", Tuple["Type[BaseException]", ...]],
+    expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
     *,
     match: Optional[Union[str, Pattern]] = ...
-) -> "RaisesContext":
+) -> "RaisesContext[_E]":
     ...  # pragma: no cover
 
 
 @overload
 def raises(
-    expected_exception: Union["Type[BaseException]", Tuple["Type[BaseException]", ...]],
+    expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
     func: Callable,
     *args: Any,
     match: Optional[str] = ...,
     **kwargs: Any
-) -> Optional[_pytest._code.ExceptionInfo]:
+) -> Optional[_pytest._code.ExceptionInfo[_E]]:
     ...  # pragma: no cover
 
 
 def raises(
-    expected_exception: Union["Type[BaseException]", Tuple["Type[BaseException]", ...]],
+    expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
     *args: Any,
     match: Optional[Union[str, Pattern]] = None,
     **kwargs: Any
-) -> Union["RaisesContext", Optional[_pytest._code.ExceptionInfo]]:
+) -> Union["RaisesContext[_E]", Optional[_pytest._code.ExceptionInfo[_E]]]:
     r"""
     Assert that a code block/function call raises ``expected_exception``
     or raise a failure exception otherwise.
@@ -703,28 +708,30 @@ def raises(
         try:
             func(*args[1:], **kwargs)
         except expected_exception:
-            return _pytest._code.ExceptionInfo.from_current()
+            # Cast to narrow the type to expected_exception (_E).
+            return cast(
+                _pytest._code.ExceptionInfo[_E],
+                _pytest._code.ExceptionInfo.from_current(),
+            )
     fail(message)
 
 
 raises.Exception = fail.Exception  # type: ignore
 
 
-class RaisesContext:
+class RaisesContext(Generic[_E]):
     def __init__(
         self,
-        expected_exception: Union[
-            "Type[BaseException]", Tuple["Type[BaseException]", ...]
-        ],
+        expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
         message: str,
         match_expr: Optional[Union[str, Pattern]] = None,
     ) -> None:
         self.expected_exception = expected_exception
         self.message = message
         self.match_expr = match_expr
-        self.excinfo = None  # type: Optional[_pytest._code.ExceptionInfo]
+        self.excinfo = None  # type: Optional[_pytest._code.ExceptionInfo[_E]]
 
-    def __enter__(self) -> _pytest._code.ExceptionInfo:
+    def __enter__(self) -> _pytest._code.ExceptionInfo[_E]:
         self.excinfo = _pytest._code.ExceptionInfo.for_later()
         return self.excinfo