anticrisis / vs-temp-ref-method-invoke

About

This repository demonstrates a code generation bug in Visual Studio C++ 2019 when using an x86-Release build and the gsl::at function.

Please see comments in the source file. The bug only occurs with a Release build.

To reproduce

Update

August 18, 2019: The problem no longer occurs in Visual Studio 16.2.2. Thank you for fixing the bug.

Older updates follow

Code generation in Visual Studio Version 16.3.0 Preview 1.0 has changed, and the problem no longer occurs. The problem still does occur in Version 16.2.0.

Set up dependencies

This uses a bootstrap and vcpkg setup that I made. It runs git submodule update and then runs vcpkg's bootstrap script. Then, it installs the dependencies in packages.txt

From Powershell:

PS> .\bootstrap.ps1
...
PS> .\install-deps.ps1

Run the test

1. Open folder in Visual Studio.
2. Configure CMake.
3. Select x86-Release configuration.
4. Run.

The checked in version will either thrown an exception or crash with a memory access violation inside the VectorOwner::Check function.

Possible factors

1. Use of gsl:at on line 69. If this use is replaced with a direct array access, the bug does not occur.

    // this version uses gsl::at, and triggers the later corruption error
    const auto& GetFoo(int i) const noexcept { return gsl::at(foos, i); }

    // use this version to prevent corruption in all cases
    //const auto& GetFoo(int i) const noexcept { return foos[i]; }

2. Accessing a member function via a temporary this pointer. The crash on line 100 does not occur if the result of calling handle.Get() is placed in a local variable before calling Run. A successful case is shown on lines 96-97.

    // this call to VectorOwner::Run succeeds in all cases, because a
    // temporary is not used
    auto& vo = handle.Get();
    vo.Run(f);

    // this next call fails in its call to Check() if GetFoo uses gsl::at
    handle.Get().Run(f);