wait_chain: reject empty strategy list with ValueError instead of IndexError

[HrachShah]

Summary

wait_chain() accepts a variadic list of strategies, but did not validate that the list is non-empty. The bad configuration only surfaced at the first call site of the resulting wait function, where self.strategies[-1] raised IndexError: tuple index out of range. This is confusing because the programmer error is at construction time, not at the moment the wait function is invoked inside a running retry.

This change makes wait_chain() raise a clear ValueError("wait_chain() requires at least one strategy") at construction time.

Repro of the pre-fix behaviour

>>> import tenacity
>>> fn = tenacity.wait_chain()
>>> from tenacity import RetryCallState
>>> s = RetryCallState(None, None, (), {})
>>> s.attempt_number = 1
>>> s.set_result(None)
>>> fn(s)
Traceback (most recent call last):
  ...
IndexError: tuple index out of range

With the patch, the same sequence raises ValueError on the wait_chain() call itself.

Changes

• tenacity/wait.py: wait_chain.__init__ now raises ValueError when *strategies is empty.
• tests/test_tenacity.py: added test_wait_chain_requires_at_least_one_strategy, which covers wait_chain() directly, Retrying(... wait=wait_chain()) at construction, and confirms a non-empty chain still works.
• releasenotes/notes/fix-wait-chain-empty-strategies-...yaml: short user-facing note.

Tests

• New test fails on the pre-fix code (AssertionError: ValueError not raised) and passes on the patched code.
• Full tests/test_tenacity.py run: 128 passed.
• ruff check: clean.
• ruff format --check: clean.

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-07-01 08:52 UTC · Rule: default · triggered by rule autoqueue
• ✅ Checks skipped · PR is already up-to-date
• ✅ Merged — 2026-07-01 08:52 UTC · at ce53bc85245373a1b8b605ae78f5f2bc2de46fe2 · squash

This pull request spent 10 seconds in the queue, including 2 seconds running CI.

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