Add NoReturn overload to builtins.sum

[AlexWaygood]

sum fails if you try to pass an iterable of strings to it:

>>> sum(('a', 'b', 'c'), start='d')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: sum() can't sum strings [use ''.join(seq) instead]

As pointed out by @Akuli, there's no ideal way to describe this in the stubs without python/typing#1043. However, a NoReturn overload is better than nothing.

Helps with #7574

[JelleZijlstra]

Not sure this is an improvement. NoReturn makes mypy silently discard code as unreachable, and pyright doesn't give an error if you use it either.

Consider this code:

from typing import NoReturn

def f() -> NoReturn:
    raise Exception

def capybara() -> None:
    f()
    1 + "1"

Mypy type checks it without errors. Pyright detects the 1 + "1", but says nothing about the f() call.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

sockeye (https://github.com/awslabs/sockeye)
+ sockeye/data_io.py:673: error: Argument "num_sents_per_bucket" to "DataStatistics" has incompatible type "List[NoReturn]"; expected "List[int]"
+ sockeye/data_io.py:673: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
+ sockeye/data_io.py:673: note: Consider using "Sequence" instead, which is covariant

xarray (https://github.com/pydata/xarray)
+ xarray/core/formatting.py:195: error: Value of type "NoReturn" is not indexable  [index]

manticore (https://github.com/trailofbits/manticore)
+ manticore/core/smtlib/solver.py:297: error: Need type annotation for "lparen"
+ manticore/core/smtlib/solver.py:297: error: Need type annotation for "rparen"
+ manticore/core/smtlib/solver.py:298: error: Unsupported left operand type for == ("NoReturn")

[AlexWaygood]

NoReturn makes mypy silently discard code as unreachable

I hoped that the implication for reachability was the advantage of this proposal -- that at least mypy would start raising errors for "unreachable code" if people had the --warn-unreachable setting switched on.

But I had forgotten that mypy simply starts ignoring type errors if it considers the code that contains the type error to be unreachable.

[AlexWaygood]

Too many mypy_primer false positives, and no clear indication that this would meaningfully improve things.

[AlexWaygood]

NoReturn makes mypy silently discard code as unreachable

Maybe we should reconsider NoReturn overloads like this one:

typeshed/stdlib/datetime.pyi

Lines 73 to 81 in 17b2f31

	if sys.version_info >= (3, 8):
	def __add__(self: Self, __other: timedelta) -> Self: ...
	def __radd__(self: Self, __other: timedelta) -> Self: ...
	@overload
	def __sub__(self: Self, __other: timedelta) -> Self: ...
	@overload
	def __sub__(self, __other: datetime) -> NoReturn: ...
	@overload
	def __sub__(self: _D, __other: _D) -> timedelta: ...