Using lambda captured constexpr value as an array dimension

Question

GCC and Clang do compile the following code:

void Test()
{
   constexpr int Size = 3;
   auto Lambda = [Size]{ int Dim[Size]; };
}
         


        

Answer 1

C++11 [expr.prim.lambda]/12

If a lambda-expression odr-uses this or a variable with automatic storage duration from its reaching scope, that entity shall be captured by the lambda-expression.

and /17

Every id-expression that is an odr-use of an entity captured by copy is transformed into an access to the corresponding unnamed data member of the closure type. [ Note: An id-expression that is not an odr-use refers to the original entity, never to a member of the closure type. [...] — end note ]

So we don't even need to capture Size in the first example (where it is not static), since reading from that variable is not an odr-use since it can appear in a constant expression and the lvalue-to-rvalue conversion is immediately applied to it, [basic.def.odr]/2

A variable whose name appears as a potentially-evaluated expression is odr-used unless it is an object that satisfies the requirements for appearing in a constant expression and the lvalue-to-rvalue conversion is immediately applied.

(It is not clear to me if the array bounds require an l-t-r conversion, though.)

The same applies when capturing Size by reference, or when capturing Size explicitly (by copy) but not odr-using it: the use of the id-expression Size within the lambda accesses the constexpr variable declared in Test, not any captured member (for capture-by-copy: IFF the access does not constitute an odr-use).

C++14 [expr.prim.lamda]/12 adds some wording for polymorphic lambdas that is irrelevant here, and moves /17 to /18. The rules for odr-use are more complicated, but I'd argue it's not an odr-use for the same underlying reason (reading a compile-time constant).