[Bug c++/84544] New: Missing warning when returning a reference to internal variable inside a lambda

[jpakkane at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84544

            Bug ID: 84544
           Summary: Missing warning when returning a reference to internal
                    variable inside a lambda
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jpakkane at gmail dot com
  Target Milestone: ---

Suppose you have a C++14 method like this:

auto f1() {
    int x = 3;
    auto l = [&]() -> int& { return x; };
    return l;
}

Here the returned lambda contains a reference to the stack allocated integer
that goes out of scope. Gcc does not give a warning for this. If you call
function f1 like this:

int& f2() {
    auto l = f1();
    return l();
}

Then Gcc does print the following warning (but only with -O2 or higher):

<source>: In function 'int& f2()':
<source>:9:14: warning: function returns address of local variable
[-Wreturn-local-addr]
     return l();
              ^
<source>:2:9: note: declared here
     int x = 3;
         ^
Compiler returned: 0

It would be useful if gcc generated a warning for f1 already.