Allow creating ExceptionInfo from existing exc_info for better typing

This way the ExceptionInfo generic parameter can be inferred from the
passed-in exc_info. See for example the replaced cast().

src/_pytest/_code/code.py

@@ -396,6 +396,33 @@ class ExceptionInfo(Generic[_E]):
     _striptext = attr.ib(type=str, default="")
     _traceback = attr.ib(type=Optional[Traceback], default=None)
 
+    @classmethod
+    def from_exc_info(
+        cls,
+        exc_info: Tuple["Type[_E]", "_E", TracebackType],
+        exprinfo: Optional[str] = None,
+    ) -> "ExceptionInfo[_E]":
+        """returns an ExceptionInfo for an existing exc_info tuple.
+
+        .. warning::
+
+            Experimental API
+
+
+        :param exprinfo: a text string helping to determine if we should
+                         strip ``AssertionError`` from the output, defaults
+                         to the exception message/``__str__()``
+        """
+        _striptext = ""
+        if exprinfo is None and isinstance(exc_info[1], AssertionError):
+            exprinfo = getattr(exc_info[1], "msg", None)
+            if exprinfo is None:
+                exprinfo = saferepr(exc_info[1])
+            if exprinfo and exprinfo.startswith(cls._assert_start_repr):
+                _striptext = "AssertionError: "
+
+        return cls(exc_info, _striptext)
+
     @classmethod
     def from_current(
         cls, exprinfo: Optional[str] = None
@@ -411,20 +438,12 @@ class ExceptionInfo(Generic[_E]):
                          strip ``AssertionError`` from the output, defaults
                          to the exception message/``__str__()``
         """
-        tup_ = sys.exc_info()
-        assert tup_[0] is not None, "no current exception"
-        assert tup_[1] is not None, "no current exception"
-        assert tup_[2] is not None, "no current exception"
-        tup = (tup_[0], tup_[1], tup_[2])
-        _striptext = ""
-        if exprinfo is None and isinstance(tup[1], AssertionError):
-            exprinfo = getattr(tup[1], "msg", None)
-            if exprinfo is None:
-                exprinfo = saferepr(tup[1])
-            if exprinfo and exprinfo.startswith(cls._assert_start_repr):
-                _striptext = "AssertionError: "
-
-        return cls(tup, _striptext)
+        tup = sys.exc_info()
+        assert tup[0] is not None, "no current exception"
+        assert tup[1] is not None, "no current exception"
+        assert tup[2] is not None, "no current exception"
+        exc_info = (tup[0], tup[1], tup[2])
+        return cls.from_exc_info(exc_info)
 
     @classmethod
     def for_later(cls) -> "ExceptionInfo[_E]":

src/_pytest/python_api.py

@@ -707,11 +707,11 @@ def raises(
             )
         try:
             func(*args[1:], **kwargs)
-        except expected_exception:
-            # Cast to narrow the type to expected_exception (_E).
-            return cast(
-                _pytest._code.ExceptionInfo[_E],
-                _pytest._code.ExceptionInfo.from_current(),
+        except expected_exception as e:
+            # We just caught the exception - there is a traceback.
+            assert e.__traceback__ is not None
+            return _pytest._code.ExceptionInfo.from_exc_info(
+                (type(e), e, e.__traceback__)
             )
     fail(message)
 

testing/code/test_excinfo.py

@@ -58,7 +58,7 @@ class TWMock:
     fullwidth = 80
 
 
-def test_excinfo_simple():
+def test_excinfo_simple() -> None:
     try:
         raise ValueError
     except ValueError:
@@ -66,6 +66,14 @@ def test_excinfo_simple():
     assert info.type == ValueError
 
 
+def test_excinfo_from_exc_info_simple():
+    try:
+        raise ValueError
+    except ValueError as e:
+        info = _pytest._code.ExceptionInfo.from_exc_info((type(e), e, e.__traceback__))
+    assert info.type == ValueError
+
+
 def test_excinfo_getstatement():
     def g():
         raise ValueError