Compiler warning: lambda return type cannot be deduced

Question

Consider this example:

#include 
#include 

int main()
{
    std::string str = \"abcde4fghijk4l5mnopqrs6t8uvwxyz\";
    std:         


        

Answer 1

As @ildjarn says in his comment, your code is simply ill-formed according to the standard.

§5.1.2 [expr.prim.lambda] p4

[...] 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.

[...]

That's it, basically if the code inside the curly brackets (called compund-statement in the standard) is anything but return some_expr;, the standard says the return type is undeducible and you get a void return type.