Fix: undefined buffer_snapshot move assignment

[GagaLP]

This fixes a problem I came across while porting Celerity to Windows. The move assignment operator never returned *this, which resulted in really weird bugs on Windows.

[github-actions]

Check-perf-impact results: (ae6918621b46271c2f10d6eb978fe95d)

❓ No new benchmark data submitted. ❓
Please re-run the microbenchmarks and include the results if your commit could potentially affect performance.

[PeterTh]

Surprising that this went unnoticed so long. Can we get a regression test for it? (Ideally one that actually fails on Linux before the fix)

[coveralls]

Coverage Report for CI Build 29335953907

Coverage increased (+0.009%) to 95.076%

Details

• Coverage increased (+0.009%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	7261
Covered Lines:	7156
Line Coverage:	98.55%
Relevant Branches:	3686
Covered Branches:	3252
Branch Coverage:	88.23%
Branches in Coverage %:	Yes
Coverage Strength:	1877963.24 hits per line

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💛 - Coveralls

[GagaLP]

Surprising that this went unnoticed so long. Can we get a regression test for it? (Ideally one that actually fails on Linux before the fix)

This went unnoticed for so long because the move assignment operator was never actually used in our code. Interestingly, we do use the flag -Werror=return-type, which should normally catch this, but since buffer_snapshot is a class template, the compiler only checks a member function's body (including missing-return checks) when it's actually instantiated. Since nothing in our codebase move-assigns a buffer_snapshot, that function was never instantiated, so the compiler never analyzed it and the warning never had a chance to fire.