Fix C26451 warnnings in to_chars.hpp

[dota17]

Refer to #1911

Warning C26451 Arithmetic overflow: Using operator '-' on a 4 byte value and then casting the result to a 8 byte value. Cast the value to the wider type before calling operator '-' to avoid overflow (io.2).
.\nlohmann\detail\conversions\to_chars.hpp 993
(also line 997, 1007, 1009, 1017, 1021, 1036 and 1038

In to_chars.hpp, there were asserts before operator -.
This PR will disable warning C26451.

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[coveralls]

Coverage remained the same at 100.0% when pulling 096714f on dota17:dota17-disable into 973c52d on nlohmann:develop.

[nlohmann]

Is there no way to fix the warning?

[dota17]

Honestly: Can't we just use the original code and suppress the warning from the msvc analyzer? Those casts add absolutely nothing in terms of correctness, readability or safety to the code.

What MSVC complains about is that computation is done in 32 bits first and then the result is widened to 64bits (a.k.a. std::size_t). Such a warning makes sense in circumstances where people expect the result of a computation might need more than 32 bits and thus assign the result to a 64 bit variable, but forget that the computation itself (and resulting overflow) still happens in 32 bits.

From #1969 (comment), i agree with @MBalszun .
There were asserts before operator - so that we can makesure that the computation itself (and resulting overflow) still happens in 32 bits.

But if you dont want this, i will take a new version out.

[MBalszun]

Assuming this warning needs to be tackled at all and not suppressed, wouldn't it be easiest to just turn k and n into variables of std::size_t or std::ptrdiff_t if they need to be signed instead of casting them to those types all over the code?

[dota17]

Assuming this warning needs to be tackled at all and not suppressed, wouldn't it be easiest to just turn k and n into variables of std::size_t or std::ptrdiff_t if they need to be signed instead of casting them to those types all over the code?

Good idea.
I will try and test.

No, it is not ok for

json/include/nlohmann/detail/conversions/to_chars.hpp

Line 1042 in 19843b0

return append_exponent(buf, n - 1);

json/include/nlohmann/detail/conversions/to_chars.hpp

Line 923 in 19843b0

inline char* append_exponent(char* buf, int e)

which raises ...possible loss of data.

It is wried if we change append_exponent(buf, n - 1) to append_exponent(buf, len + decimal_exponent - 1). But if nobody against this, it will be ok.

[dota17]

There are some errors in CI. And i tried to fix it.
However, the errors in #2004, #2002 are same as this issue, especially #2004 just add a yml file and don't change any code.

[dota17]

Ready to go.

[nlohmann]

Looks good to me.

[nlohmann]

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🔖 Release item

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Type

• ✨ New Feature
• 🐛 Bug Fix
• ⚡️ Improvement
• 🔨 Further Change
• 🔥 Deprecated function

Description

• Fix a compiler warning in MSVC. Fix C26451 warnnings in to_chars.hpp #1967 Compile warnings on MSVC 14.2 #1911

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[nlohmann]

Thanks!