[Voted into the WP at the July, 2009 meeting as part of N2927.]

Proposed resolution (July, 2009)

See document PL22.16/09-0117 = WG21 N2927.

How does name binding work in nested lambda-expressions? For example,

    void f1() {
      float v;
      []() { return [v]() { return v; } }
    }

    void f2() {
      float v;
      [v]() { return [v]() { return v; } }
    }

According to 7.5.5 [expr.prim.lambda] paragraph 3,

A name in the lambda-capture shall be in scope in the context of the lambda expression, and shall be this or shall refer to a local variable or reference with automatic storage duration.

One possible interpretation is that the lambda expression in f1 is ill-formed because v is used in the compound-statement of the outer lambda expression but does not appear in its effective capture set. However, the appearance of v in the inner lambda-capture is not a “use” in the sense of 6.3 [basic.def.odr] paragraph 2, because a lambda-capture is not an expression, and it's not clear whether the reference in the inner lambda expression's return expression should be considered a use of the automatic variable or of the member of the inner lambda expression's closure object.

Similarly, the lambda expression in f2 could be deemed to be ill-formed because the reference to v in the inner lambda expression's lambda-capture would refer to the field of the outer lambda-expression's closure object, not to a local automatic variable; however, it's not clear whether the inner lambda expression should be evaluated in situ or as part of the generated operator() member of the outer lambda expression's closure object.