Type cast failed in switch for enum with restricted storage

Question

enum class confirm {yes};

struct item
{
  confirm s:4; // (1) limiting storage size required
};

int main()
{
  item itm;

  itm.s = confirm::yes; // (2) OK

  switch (itm.s)
  {
    case confirm::yes: // (3) Failure, need static data cast here?
      break;
  }
}

produces error:

In function ‘int main()’:
error: could not convert ‘yes’ from ‘confirm’ to ‘int’
 case confirm::yes:
               ^

while compiling with g++ but compiled fine by clang++. Why assignment marked by (2) possible but case clause marked by (3) not?

Warning about too small storage is of offtopic

Would "this is a gcc bug" be a sufficient answer? (naturally with evidence that it should be allowed) — Yakk - Adam Nevraumont
– Yakk - Adam Nevraumont, Commented Aug 27, 2015 at 15:39
@dyp: thanks for tagging, but a question is specific for enum class —C++11 only; no such problem in previous standards — dyomas
– dyomas, Commented Aug 27, 2015 at 15:56
I had to choose between the C++11 tag and some other tags (5 tags max); my reasoning was this: [enum-class] implies C++11, so it is clear from the tags that this is >= C++11. The three [switch-statement], [bit-fields] and [enum-class] describe the issue in [g++]. I guess the [c++11] tag will become redundant in some time anyway, I see various questions already accepting C++11 answers w/o the [c++11] tag. — dyp
– dyp, Commented Aug 27, 2015 at 16:12
@dyp it looks like a bug to me and the latest gcc allows this with only a warning. — Shafik Yaghmour
– Shafik Yaghmour, Commented Sep 18, 2015 at 20:08

T.C. · Accepted Answer · 2015-09-18 21:08:01Z

This looks like a gcc bug, we can see this works in the latest gcc version:

From the draft C++11 standard section 6.4.2 [stmt.switch]:

The condition shall be of integral type, enumeration type, or of a class type for which a single non-explicit conversion function to integral or enumeration type exists (12.3). [...] Integral promotions are performed. Any statement within the switch statement can be labeled with one or more case labels as follows:
case constant-expression :
where the constant-expression shall be a converted constant expression (5.19) of the promoted type of the switch condition.

converted constant expression is covered in section 5.19 which says:

[...]A converted constant expression of type T is a literal constant expression, implicitly converted to type T, where the implicit conversion (if any) is permitted in a literal constant expression and the implicit conversion sequence contains only user-defined conversions, lvalue-to-rvalue conversions (4.1), integral promotions (4.5), and integral conversions (4.7) other than narrowing conversions (8.5.4). [ Note: such expressions may be used as case expressions (6.4.2), as enumerator initializers if the underlying type is fixed (7.2), and as integral or enumeration non-type template arguments (14.3). —end note ] [...]

Perhaps this is related to defect report 1767: Scoped enumeration in a switch statement. So perhaps it was forcing a promotion to int and then the comparison in the case would fail.

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