Fix round-trip assertion for fractional floating-point formats - #134106
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tannergooding merged 1 commit intoSep 22, 2026
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Copilot review overview
🟢 Approval recommended
The assertion is corrected and comprehensive regression coverage is included.
Review effort: Lite
Findings: None
What changed in this PR
Fixes an incorrect checked/debug round-trip assertion for fractional floating-point formatting without changing release output.
Changes:
- Restricts assertions to significant-digit formats.
- Adds 48 regression cases across four floating-point types.
- Covers
ToString, UTF-16/UTF-8TryFormat, and insufficient buffers.
| File | Description |
|---|---|
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/SingleTests.cs |
Adds float regression cases. |
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Numerics/BFloat16Tests.cs |
Adds BFloat16 regression cases. |
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/HalfTests.cs |
Adds Half regression cases. |
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/DoubleTests.cs |
Adds double regression cases. |
src/libraries/System.Private.CoreLib/src/System/Number.Formatting.cs |
Corrects assertion applicability. |
lewing
approved these changes
Sep 16, 2026
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/ba-g unrelated timeouts |
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This was just an invalid assert, not backporting. |
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Number.FormatFloatchecks that its intermediate decimal representation round-trips when the requested precision reachesMaxRoundTripDigits. That threshold counts significant digits, butF,N,C, andPrequest fractional digits. Leading fractional zeros can consume the requested precision, so correctly rounded output can fail the assertion in debug/checked builds.Restrict the round-trip assertion to significant-digit modes, retaining the check for shortest-round-trip formatting and sufficiently precise significant-digit formatting. Formatting output and release behavior are unchanged.
Add 48 regression cases for
double,float,Half, andBFloat16, covering fractional formats, precision boundaries, negative values, subnormals, and signed zero. Each case checksToStringand UTF-16/UTF-8TryFormat, including insufficient destination space.Validation on Windows x64: reproduced the original assertion with the new tests, rebuilt checked CoreLib, and passed all 6,367 tests in the four affected numeric test classes, including all 48 new cases.
Resolves #134101
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This PR description and changes were drafted with GitHub Copilot.