[llvm-bugs] [Bug 43921] New: Rejects valid: reading of an inactive union member that shares common initial sequence with the active one

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43921

            Bug ID: 43921
           Summary: Rejects valid: reading of an inactive union member
                    that shares common initial sequence with the active
                    one
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Keywords: compile-fail
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: nok.raven at gmail.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

The wording is in [class.union]/1 and [class.mem]/25.

[class.mem]/25 contains an example:

struct T1 { int a, b; };
struct T2 { int c; double d; };
union U { T1 t1; T2 t2; };
int f() {
  U u = { { 1, 2 } };   // active member is t1
  return u.t2.c;        // OK, as if u.t1.a were nominated
}

Running this example in constexpr context, or just turning the `f` into a
constexpr function produces an error:

<source>:4:15: error: constexpr function never produces a constant expression
[-Winvalid-constexpr]
constexpr int f() {
              ^
<source>:6:10: note: read of member 't2' of union with active member 't1' is
not allowed in a constant expression
  return u.t2.c;        // OK, as if u.t1.a were nominated
         ^
1 errors generated.

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20191106/d96a5fd2/attachment.html>