Movatterモバイル変換

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

Additional note (February, 2012):

EWG requested that the adoption of this proposed resolution bepostponed to allow them to discuss it. The issue has thus beenreturned to "review" status pending EWG action.

Proposed resolution (August, 2011):

Change 7.5.6 [expr.prim.lambda] paragraph 4 as follows:

...If alambda-expression does not include atrailing-return-type, it is as if thetrailing-return-typedenotes the following type:

• if the compound-statement is of the form

  {attribute-specifier-seq_optreturnexpression;

  the type of the returned expression after lvalue-to-rvalueconversion (7.3.2 [conv.lval]), array-to-pointer conversion(7.3.3 [conv.array]), and function-to-pointer conversion(7.3.4 [conv.func]);

• otherwise,void.

• if there are noreturn statements in thecompound-statement, or allreturn statements returneither an expression of typevoid or noexpressionorbraced-init-list, the typevoid;

• otherwise, if allreturn statements return anexpression and the types of the returned expressions afterlvalue-to-rvalue conversion (7.3.2 [conv.lval]),array-to-pointer conversion (7.3.3 [conv.array]), andfunction-to-pointer conversion (7.3.4 [conv.func]) arethe same, that common type;

• otherwise, the program is ill-formed.

[Example:

  auto x1 = [](int i){ return i; }; // OK: return type isint  auto x2 = []{ return { 1, 2 }; }; // error: the return type isvoid (a                                    // braced-init-list is not an expression)  struct A { int fn1(); const int& fn2(); };  template <class T> void f () {    [](T t, bool b){      if (b)        return t.fn1();      else        return t.fn2();    };  }  template void f<A>();             // OK: lambda return type isint

—end example]

There does not appear to be any technical difficulty that wouldrequire the current restriction that the return type of a lambda canbe deduced only if the body of the lambda consists of a single returnstatement. In particular, multiple return statements could bepermitted if they all return the same type.