C17 7.22.1.3 and 7.29.4.1.1 interpret NAN(n-char-sequence) as a quiet
NaN. With MinGW-w64, std::strtod, strtof and strtold are mingw-w64's
gdtoa functions, which copy the hex digits into the significand
without the quiet bit, and for long double without the x87 integer
bit: strtod("nan(0x123)") returned the signaling NaN
0x7ff0000000000123 and strtold a pseudo-NaN. A payload whose only set
bit is the implicit or x87 integer bit gave infinity:
strtof("nan(0x800000)"), strtod("nan(0x10000000000000)") and
strtold("nan(0x8000000000000000)") returned +inf. glibc returns quiet
NaNs that keep the payload.
C17 7.12.11.2p2 makes nan("n-char-sequence") equivalent to
strtod("NAN(n-char-sequence)", NULL), and nan and nanf already return
the console's fixed quiet NaNs, ignoring the tag. ParseFloat, which
strtod, strtof, strtold, atof and the wide variants share, now returns
the same values, 0x7ff8000000000000 and 0x7fc00000, negated after a
minus sign, on Windows and Linux. strtold returns the x87 default
quiet NaN, since no console nanl was observed.
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