g++ won't allow generalized capture of const object by reference in lambda?

Question

This is rejected by g++ (4.9.3 and 5.2.0), but is accepted by clang 3.5.0:

int main() { 
    const int ci = 0;
    auto lambda = [ &cap = ci ]() { };
}
<         


        

Answer 1

This looks similar to gcc bug: [C++14] lambda init-capture fails for const references which says:

This code fails to compile:

int main() {
    int x = 0;
    auto l = [&rx = static_cast(x)]() {};
}

The error message is:

test.cpp:3:14: error: binding 'const int' to reference of type 'int&' discards qualifiers

auto l = [&rx = static_cast(x)]() {

But according to [expr.prim.lambda]/11 rx should be captured as auto &rx = static_cast(x), that is as const int&.

the bug report references [expr.prim.lambda]/11 which says:

An init-capture behaves as if it declares and explicitly captures a variable of the form “auto init-capture ;” whose declarative region is the lambda-expression’s compound-statement, except that[...]