[llvm-bugs] [Bug 50342] New: Missing diagnostic for comma in constant expression

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https://bugs.llvm.org/show_bug.cgi?id=50342

            Bug ID: 50342
           Summary: Missing diagnostic for comma in constant expression
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: john.brawn at arm.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

In the C11 (and earlier) standard it says in section 6.6 about constant
expressions:

  Constant expressions shall not contain assignment, increment, decrement,
function-call,
  or comma operators, except when they are contained within a subexpression
that is not
  evaluated.

Which means we should be giving a diagnostic for code like this

  static const int i = 0 ? 2 : ((void)3, 4);

There's a comment in clang/lib/Sema/SemaDecl.cpp about this:

  // FIXME: Need strict checking.  In C89, we need to check for
  // any assignment, increment, decrement, function-calls, or
  // commas outside of a sizeof.  In C99, it's the same list,
  // except that the aforementioned are allowed in unevaluated
  // expressions.  Everything else falls under the
  // "may accept other forms of constant expressions" exception.

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