open-source-parsers / jsoncpp / 27651508513

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fix: accept subnormal and underflowing doubles when parsing (#1427)

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

Treat errno == ERANGE in the failure path as success and keep the value
operator>> produced (the correctly-rounded subnormal, zero on underflow,
or infinity on overflow). Genuine junk leaves errno unset and is still
rejected. Applied to both Reader and OurReader, which share this logic.

Adds CharReaderTest/parseSubnormalAndUnderflow covering subnormals, a
writer round-trip, underflow-to-zero, and continued rejection of junk.

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