Question

Taken from the C++0x FDIS (n3290):

If a lambda-expression does not include a lambda-declarator, it is as if the lambda-declarator were (). If a lambda-expression does not include a trailing-return-type, it is as if the trailing-return-type denotes the following type:

• if the compound-statement is of the form
  { attribute-specifier-seq_opt return expression ; }
  the type of the returned expression after lvalue-to-rvalue conversion (4.1), array-to-pointer conversion
  (4.2), and function-to-pointer conversion (4.3);
• otherwise, void.

Why doesn’t the standard allow the compiler to analyse the compound-statement and determine the return type based on the first found return statement?

I can’t see any reason to not allow this, but maybe I’m overlooking something.

Example:

int main(){
  // compiler: nope.jpg
  auto l = []{
    // one computation
    // another computation
    // yet another one!
    return something;
  }
}

Edit: Please no “because the standard says so” answers. 🙂

Answer 1

There’s no technical reason for this. IIRC it was already implemented by the GCC C++ maintainer and he said it’s trivial to implement.

The committee is very conservative about accepting features into the Standard so they went with this simple form of deduction and will later hopefully accept a more powerful form. See the reason for rejection on US 30 comment.

DR 975 is already marked “ready” so chances are good it will be accepted.