[all-commits] [llvm/llvm-project] c92f50: Correct the behavior of va_arg checking in C++

[Aaron Ballman via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: c92f505346b80fd053ef191bbc66810c9d564b0c
      https://github.com/llvm/llvm-project/commit/c92f505346b80fd053ef191bbc66810c9d564b0c
  Author: Aaron Ballman <aaron at aaronballman.com>
  Date:   2021-06-09 (Wed, 09 Jun 2021)

  Changed paths:
    M clang/lib/Sema/SemaExpr.cpp
    M clang/test/SemaCXX/varargs.cpp

  Log Message:
  -----------
  Correct the behavior of va_arg checking in C++

Clang checks whether the type given to va_arg will automatically cause
undefined behavior, but this check was issuing false positives for
enumerations in C++. The issue turned out to be because
typesAreCompatible() in C++ checks whether the types are *the same*, so
this uses custom logic if the type compatibility check fails.

This issue was found by a user on code like:

typedef enum {
  CURLINFO_NONE,
  CURLINFO_EFFECTIVE_URL,
  CURLINFO_LASTONE = 60
} CURLINFO;

...

__builtin_va_arg(list, CURLINFO); // false positive warning

Given that C++ defers to C for the rules around va_arg, the behavior
should be the same in both C and C++ and not diagnose because int and
CURLINFO are "compatible enough" types for va_arg.