open-source-parsers / jsoncpp / 27653893209

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fix: accept subnormal doubles when parsing (#1427) (#1695)

decodeDouble parsed numbers via `istringstream >> double`. For a
subnormal value such as `3.2114e-312`, operator>> sets failbit (the
result underflowed) even though it produced the correctly-rounded value.
The failure path only special-cased overflow, so subnormals were
rejected as "not a number" -- meaning a value jsoncpp had just serialized
could fail to parse back.

In the failure path, accept the value when it is a subnormal
(std::fpclassify(value) == FP_SUBNORMAL). This keys off the value
operator>> produces, which is the correctly-rounded subnormal on
libstdc++, libc++, and MSVC, so it needs no errno/eof heuristics. It
deliberately does not accept results that round to zero, so malformed
numbers like "0e" / "0e+" (jsonchecker fail29/fail30) and other junk are
still rejected. Applied to both Reader and OurReader.

Adds CharReaderTest/parseSubnormal covering subnormals, a writer
round-trip, and continued rejection of malformed numbers.

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