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Non-recursive variant of detail::length() for compilers with maximum recursion depth in compile-time constexpr evaluation. #29

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Merged
merged 1 commit into from
Jan 25, 2020
Merged

Non-recursive variant of detail::length() for compilers with maximum recursion depth in compile-time constexpr evaluation. #29

merged 1 commit into from
Jan 25, 2020

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AdmiralCurtiss
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I ran into this in C++14 when creating a string_view with a very long string. For example,

namespace {
    constexpr ::nonstd::string_view tmp = ::nonstd::string_view(
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
    );
}

On MSVC 2019 in C++14 mode this gives:

error C2131: expression did not evaluate to a constant
  note: failure was caused by evaluation exceeding call depth limit of 512 (/constexpr:depth<NUMBER>)

And on g++ 9.2.1 this gives

error: ‘constexpr’ evaluation depth exceeds maximum of 512 (use ‘-fconstexpr-depth=’ to increase the maximum)

I don't think we can fix this for C++11, but when we have C++14 we have better ways to write a constexpr length() of a nullterminated string, so might as well do so to work around this problem.

@AdmiralCurtiss
Non-recursive variant of detail::length() for compilers with maximum …
b42d766 
…recursion depth in compile-time constexpr evaluation.
@martinmoene martinmoene merged commit 5990f16 into martinmoene:master Jan 25, 2020
@AdmiralCurtiss AdmiralCurtiss deleted the max-constexpr-recursion-depth branch January 25, 2020 14:43
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@AdmiralCurtiss @martinmoene