[matt-edmondson]

Fixes #177

What was wrong

Parse(ReadOnlySpan<char>, NumberStyles, IFormatProvider?) never read AllowHexSpecifier or AllowBinarySpecifier. Input made only of decimal digits was read as decimal, so T.Parse("10", NumberStyles.HexNumber, …) returned 10 for PreciseNumber with no error. BigInteger returns 16 for the same call, and decimal and double throw.

The fix

The issue offered two options. This PR takes the one the triage recommended: refuse both styles with an ArgumentException, as decimal and double do. The check runs at the top of Parse. TryParse catches only FormatException and OverflowException, so it throws the same exception rather than returning false, which also matches decimal.TryParse.

NumberStyles.AllowBinarySpecifier only exists from .NET 8 onward. On the net7.0 target the mask uses its raw value (0x400), so a caller there who passes it is still refused.

Tests

ParseAndTryParse_RejectHexAndBinaryStylesAsDecimalDoes covers HexNumber, AllowHexSpecifier, BinaryNumber and AllowBinarySpecifier. For each one it checks that decimal.Parse throws, then asserts that PreciseNumber's string and span Parse, its string and span TryParse, and a generic INumberBase<T>.Parse all throw ArgumentException.

• Without the fix, all 5 cases fail: four parse silently and "A" throws FormatException. With the fix they pass.
• The full suite passes: 521/521 on net10.0. The solution builds for every target framework with no warnings.

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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