C4702: unreachable code #65

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1. subbota-a commented on Mar 21, 2024

   

   subbota-a

   on Mar 21, 2024

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   Me too
2. szaszm commented on Apr 23, 2024

   

   szaszm

   on Apr 23, 2024

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   How does that work? That line is a comment, there is no code there: https://github.com/martinmoene/expected-lite/blob/v0.6.3/include/nonstd/expected.hpp#L2142
   Is it any different in your build directory?
3. np-c0mp1ete commented on May 22, 2024

   

   np-c0mp1ete

   on May 22, 2024

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   Hello!

   Repro:

   #include "expected.hpp"
   
   int main()
   {
       nonstd::expected<int, int> ei = 0;
       return ei.value();
   }
   

   Compile using cl /O2 /W4 /WX main.cpp. expected-lite version is 0.6.3.

   Output:

   Microsoft (R) C/C++ Optimizing Compiler Version 19.39.33523 for x64
   Copyright (C) Microsoft Corporation.  All rights reserved.
   
   main.cpp
   C:\test\expected.hpp(1803) : error C2220: the following warning is treated as an error
   C:\test\expected.hpp(1803) : warning C4702: unreachable code
   
4. martinmoene commented on May 29, 2024

   

   martinmoene

   on May 29, 2024

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   On Compiler Explorer:

   • MSVC v19.latest: -O2 -W4 -WX -Dnsel_CONFIG_SELECT_EXPECTED=nsel_EXPECTED_NONSTD: C4702: unreachable code.
   • clang 18.1.0: -O2 -Wall -Wunreachable-code-aggressive -Dnsel_CONFIG_SELECT_EXPECTED=nsel_EXPECTED_NONSTD: no warning.

   I'm not sure what to think of MSVC's warning.
5. szaszm commented on May 29, 2024

   

   szaszm

   on May 29, 2024

   Last edited by szaszm

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   Ah, at first I thought line 2142 is with the 0.6.3 release branch, but apparently it's on master, so there is actual code there. So the correct reference is

   • https://github.com/martinmoene/expected-lite/blob/v0.6.3/include/nonstd/expected.hpp#L1803
   • https://github.com/martinmoene/expected-lite/blob/master/include/nonstd/expected.hpp#L2142

   Maybe MSVC doesn't like the value() call next to the comma operator after rethrow, which is actually unreachable. Maybe a random throw expression instead of value() call would silence it, but I can't test this now.

   A few other wrong ideas I had:

   • immediately invoked lambda that calls rethrow, and lies about its return type. That would produce another warning about the function not returning anything, because rethrow is not actually throwing.
   • changing rethrow to lie that its return type is value_type&. That would generate a similar warning inside rethrow, because it doesn't return that type, nor does it throw. assert and RaiseException look like normal function calls to the control flow analyzer.
   • change to an if statement. This wouldn't work with C++11 compilers in constexpr context.

   👍React with 👍1Reacted by Martin Moene
6. martinmoene commented on May 30, 2024

   

   martinmoene

   on May 30, 2024

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   Thanks for your helpful thoughts to advance this issue.

   👍React with 👍1Reacted by Márton Szász
7. martinmoene

   added 2 commits that reference this issue on May 30, 2024

   Fix (locally disable) MSVC warning C4702: unreachable code (#65, thanks

   ...

   981f2ec

   Release 0.7.0

   ...

   643873e
8. martinmoene

   closed this as completedon Jun 2, 2024