[llvm] [llvm] Initialize function_ref callable state (PR #220561)

[Mehdi Amini via llvm-commits]

joker-eph wrote:


> Implicit copies shouldn't matter, copying uninitialized data is fine for Valgrind (in fact, it happens all the time, e.g. when copying padding).

Mmmmm other sources disagree: https://github.com/llvm/llvm-project/pull/220561#issuecomment-5509928373

I asked an agent to help me figure out, it argues that:

>  implicit copy/assignment performs memberwise operations, including built-in assignment of the scalar callable. Copying an indeterminate scalar member is not equivalent to copying padding: under C++23 it is undefined behavior; current draft wording classifies the corresponding read as  erroneous behavior. 

Also MSAN complain in the (revised) unit-test without the initialization:

```
  Uninitialized bytes in __msan_check_mem_is_initialized at offset 8 inside [0x..., 16)
  ==...==WARNING: MemorySanitizer: use-of-uninitialized-value
      #0 ... in FunctionRefTest_DefaultConstructedCopy_Test::TestBody()
         llvm/unittests/ADT/FunctionRefTest.cpp:36:3

  SUMMARY: MemorySanitizer: use-of-uninitialized-value
  Exiting
```

See https://github.com/llvm/llvm-project/pull/220600

https://github.com/llvm/llvm-project/pull/220561