This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-10-26

2211. Hiding by lambda captures and parameters

[Adopted at the February/March, 2017 meeting.]

Consider:

  #include <iostream>

  int main()
  {
    [x=2](int x) { std::cout << x << std::endl; }(3);
  }

What is the code supposed to print? There is implementation divergence.

Proposed resolution (February, 2017):

1. Add the following as a new paragraph after 7.5.6 [expr.prim.lambda] paragraph 11:

The identifier in a simple-capture is looked up using the usual rules for unqualified name lookup (6.5.3 [basic.lookup.unqual]); each such lookup shall find an entity. An entity that is designated by a simple-capture is said to be explicitly captured, and shall be *this (when the simple-capture is “this ” or “* this ”) or a variable with automatic storage duration declared in the reaching scope of the local lambda expression.

If an identifier in a simple-capture appears as the declarator-id of a parameter of the lambda-declarator's parameter-declaration-clause, the program is ill-formed. [Example:

  void f() {
    int x = 0;
    auto g = [x](int x) { return 0; }  // error: parameter and simple-capture have the same name
  }

—end example]

2. Change the example of 7.5.6 [expr.prim.lambda] paragraph 12 as follows:

  int x = 4;
  auto y = [&r = x, x = x+1]()->int {
                  r += 2;
                  return x+2;
               }(); // Updates ::x to 6, and initializes y to 7.
  auto z = [a = 42](int a) { return 1; } // error: parameter and local variable have the same name