Parse("1 234,5", fr-FR) throws where decimal returns 1234.5: a typed space doesn't match a no-break-space group separator #174

What's wrong

Many cultures use a no-break space as the group separator:

• U+202F narrow no-break space: fr-FR
• U+00A0 no-break space: ru-RU, pl-PL, sv-SE, cs-CZ, nb-NO, fr-CA, and others

Parse matches the group separator by ordinal comparison only:

if (allowThousands && !hasDecimal && digitCount > 0 && s[i..].StartsWith(groupSeparator, StringComparison.Ordinal))

That means a number typed with an ordinary space (U+0020), which is what a keyboard produces, is rejected.

Every BCL numeric type accepts it. The runtime's number parser deliberately treats U+0020 as matching a U+00A0 or U+202F separator (IsSpaceReplacingChar / MatchChars in Number.Parsing.cs), so decimal, double, and int all parse these strings. Generic code calling INumberBase<T>.Parse(s, provider) therefore works for every built-in type and fails only for PreciseNumber. That includes the provider-only overloads, which use NumberStyles.Any.

PreciseNumberParseCultureTests covers a U+202F separator (NarrowSpaceGroups), but only to round-trip the library's own output. It has no case for typed input.

Reproduction (net10.0, NumberStyles.Number; Any and AllowThousands behave the same)

All inputs below use a plain U+0020 space between digit groups.

var fr = new CultureInfo("fr-FR");
decimal.Parse("1 234,5", fr);          // 1234.5
PreciseNumber.Parse("1 234,5", fr);    // FormatException

Suggested fix / acceptance criteria

• When matching the group separator, and the decimal separator for parity with the BCL, let a U+0020 in the input match a U+00A0 or U+202F in the culture's separator, character by character, the way the runtime's MatchChars does.
• Everything else stays ordinal. For example, de-CH's U+2019 still requires U+2019, and a culture whose separator is not a no-break space does not start accepting spaces.
• If Parse("(1.5)"), Parse("1.5-") and Parse("$1,234.50", NumberStyles.Currency, en-US) throw FormatException: AllowParentheses, AllowTrailingSign and AllowCurrencySymbol are never honoured #152 (currency separators) lands first, apply the same rule to CurrencyGroupSeparator and CurrencyDecimalSeparator.
• Tests:
  • Every failing row above parses to the decimal result.
  • Existing round-trips such as Parse(x.ToString("N2", fr), fr) are unchanged.
  • "1 234" still fails under NumberStyles.Float (no AllowThousands).