[all-commits] [llvm/llvm-project] 1c9990: [CUDA][HIP] Fix overriding of constexpr virtual fu...

[Yaxun (Sam) Liu via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 1c999072221371c9008cd0aec80e387777378de0
      https://github.com/llvm/llvm-project/commit/1c999072221371c9008cd0aec80e387777378de0
  Author: Yaxun (Sam) Liu <yaxun.liu at amd.com>
  Date:   2025-01-09 (Thu, 09 Jan 2025)

  Changed paths:
    M clang/docs/ReleaseNotes.rst
    M clang/lib/Sema/SemaOverload.cpp
    A clang/test/SemaCUDA/constexpr-virtual-func.cu

  Log Message:
  -----------
  [CUDA][HIP] Fix overriding of constexpr virtual function (#121986)

In C++20 constexpr virtual function is allowed. In C++17 although
non-pure virtual function is not allowed to be constexpr, pure virtual
function is allowed to be constexpr and is allowed to be overriden by
non-constexpr virtual function in the derived class.

The following code compiles as C++:

```
class A
{
public:
    constexpr virtual int f() = 0;
};

class B : public A
{
public:
    int f() override
    {
        return 42;
    }
};
```

However, it fails to compile as CUDA or HIP code. The reason: A::f() is
implicitly host device function whereas B::f() is a host function. Since
they have different targets, clang does not treat B::f() as an override
of A::f(). Instead, it treats B::f() as a name-hiding non-virtual
function for A::f(), and diagnoses it.

This causes any CUDA/HIP program using C++ standard header file
`<format>` from g++-13 to fail to compile since such usage patten show
up there:

```
/usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/format:3564:34: error: non-virtual member function marked 'override' hides virtual member function
 3564 |       _M_format_arg(size_t __id) override
      |                                  ^
/usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/format:3538:30: note: hidden overloaded virtual function 'std::__format::_Scanner<char>::_M_format_arg' declared here
 3538 |       constexpr virtual void _M_format_arg(size_t __id) = 0;
      |                              ^
```

This is a serious issue and there is no workaround.

This patch allows non-constexpr function to override constexpr virtual
function for CUDA and HIP. This should be OK since non-constexpr
function without explicit host or device attribute can only be called in
host functions.

Fixes: SWDEV-507350



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