Rework ExceptionInfo to not require manual __init__ call

Mypy doesn't like calling __init__() in this way.

src/_pytest/_code/code.py

@@ -427,11 +427,16 @@ def from_current(
         return cls(tup, _striptext)
 
     @classmethod
-    def for_later(cls) -> "ExceptionInfo[_E]":
+    def unfilled(cls) -> "ExceptionInfo[_E]":
         """return an unfilled ExceptionInfo
         """
         return cls(None)
 
+    def fill_unfilled(self, exc_info: Tuple["Type[_E]", _E, TracebackType]) -> None:
+        """fill an unfilled ExceptionInfo"""
+        assert self._excinfo is None, "ExceptionInfo was already filled"
+        self._excinfo = exc_info
+
     @property
     def type(self) -> "Type[_E]":
         """the exception class"""

src/_pytest/python_api.py

@@ -731,7 +731,7 @@ def __init__(
         self.excinfo = None  # type: Optional[_pytest._code.ExceptionInfo[_E]]
 
     def __enter__(self) -> _pytest._code.ExceptionInfo[_E]:
-        self.excinfo = _pytest._code.ExceptionInfo.for_later()
+        self.excinfo = _pytest._code.ExceptionInfo.unfilled()
         return self.excinfo
 
     def __exit__(
@@ -744,9 +744,13 @@ def __exit__(
         if exc_type is None:
             fail(self.message)
         assert self.excinfo is not None
-        # Type ignored because mypy doesn't like calling __init__ directly like this.
-        self.excinfo.__init__((exc_type, exc_val, exc_tb))  # type: ignore
-        suppress_exception = issubclass(self.excinfo.type, self.expected_exception)
-        if self.match_expr is not None and suppress_exception:
+        if not issubclass(exc_type, self.expected_exception):
+            return False
+        # Cast to narrow the exception type now that it's verified.
+        exc_info = cast(
+            Tuple["Type[_E]", _E, TracebackType], (exc_type, exc_val, exc_tb)
+        )
+        self.excinfo.fill_unfilled(exc_info)
+        if self.match_expr is not None:
             self.excinfo.match(self.match_expr)
-        return suppress_exception
+        return True

testing/code/test_excinfo.py

@@ -339,8 +339,8 @@ def test_excinfo_repr_str():
     assert str(excinfo) == "<ExceptionInfo ValueError tblen=4>"
 
 
-def test_excinfo_for_later():
-    e = ExceptionInfo.for_later()
+def test_excinfo_unfilled():
+    e = ExceptionInfo.unfilled()
     assert "for raises" in repr(e)
     assert "for raises" in str(e)