c++ - When can we omit the return type in a C++11 lambda?

Question

As far as I know, in standard C++11 (not C++14), when omitting the return type of a lambda, its return type is deduced to be:

1. The type of the returned expression, whenever the lambda consists of nothing but a single return statement with an expression, or
2. void in all other cases.

Consider now this code:

#include <iostream>

auto closure = [](int x)
{
    x++;
    return x;
};

int main()
{
    int y = closure(10);
    std::cout << y << std::endl;
}

This should fall under case 2., however the code compiles as if were C++14 with auto type deduction, in both g++4.9.2, g++5 and clang++, with -pedantic -Wall -Wextra -std=c++11. What's going on here? Am I interpreting the standard wrong?

Answer 1

Your code is being accepted without any warnings because the original C++11 restriction is considered a defect in the standard, which allows implementations to fix the behavior. See CWG DR975, DR1048 and N3638.

975. Restrictions on return type deduction for lambdas

[Moved to DR status at the April, 2013 meeting as part of paper N3638.]

There does not appear to be any technical difficulty that would require the current restriction that the return type of a lambda can be deduced only if the body of the lambda consists of a single return statement. In particular, multiple return statements could be permitted if they all return the same type.

1048. auto deduction and lambda return type deduction.

...

Notes from the November, 2014 meeting:

CWG agreed that the change embodied in paper N3638 should be considered to have been a DR against C++11.

In summary, DR975 proposed modifying the rules for return type deduction for lambda expressions to allow multiple return statements.

DR1048 identifies a discrepancy where the rules for deducing the return type for normal functions using the placeholder type auto differs slightly from the rules proposed in DR975. Specifically, return type deduction for normal functions would discard top-level cv-qualifiers in all cases, where as those for lambda expressions would preserve cv-qualifiers for class types.

N3638 resolves this issue, amongst others.

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I doubt there's any way to revert to the original behavior short of finding a compiler version that shipped with C++11 lambda support prior to the implementation of the DR above.