[cfe-dev] No warning when lambda returns reference to temporary

[Csaba Raduly via cfe-dev]

Hi Matthew,

When you don't specify a return type for the lambda, the compiler
deduces 'int', not 'int&'.

auto deduced     = []()         { int i = 123; return i; };
auto spelled_out = []() -> int& { int i = 123; return i; };

int d = deduced;
int s = spelled_out;

This doesn't compile of course, but the error messages are the useful info:

$ g++-8 -std=c++11 is_it_ref.cpp
is_it_ref.cpp: In lambda function:
is_it_ref.cpp:2:39: warning: reference to local variable ‘i’ returned
[-Wreturn-local-addr]
 auto spelled_out = []() -> int& { int i = 123; return i; };
                                       ^
is_it_ref.cpp: At global scope:
is_it_ref.cpp:4:9: error: invalid user-defined conversion from
‘<lambda()>’ to ‘int’ [-fpermissive]
 int d = deduced;
         ^~~~~~~
is_it_ref.cpp:1:23: note: candidate is: ‘<lambda()>::operator int
(*)()() const’ <near match>
 auto deduced     = []()         { int i = 123; return i; };
                       ^
is_it_ref.cpp:1:23: note:   no known conversion from ‘int (*)()’ to ‘int’
is_it_ref.cpp:5:9: error: invalid user-defined conversion from
‘<lambda()>’ to ‘int’ [-fpermissive]
 int s = spelled_out;
         ^~~~~~~~~~~
is_it_ref.cpp:2:31: note: candidate is: ‘<lambda()>::operator int&
(*)()() const’ <near match>
 auto spelled_out = []() -> int& { int i = 123; return i; };
                               ^
is_it_ref.cpp:2:31: note:   no known conversion from ‘int& (*)()’ to ‘int’


On Thu, Jul 13, 2017 at 6:55 AM, Matthew Urquhart via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> Hiya,
>
> I ran into this (presumably) undefined behaviour today, and wonder if it's a
> bug or expected. My app was coded similarly and ran correctly on macOS/LLVM
> version 8.1.0 (clang-802.0.38) but not Ubuntu14.04/gcc-4.7.3-12ubuntu1
>
> In the case of this code, it's invalid since the lambda returns a reference
> to a temp object. My bad, I'll fix it. Seems the compiler could've warned me
> though :)
>
> Compiler warns when explicitly specifying the return type of the lambda
> using `->` operator, but not when implicit. Is this expected or a bug?
>
> Thanks in advance,
> Matt
>
> // Minimal test case:
>
> #include <functional>
>
> #include <iostream>
>
>
> void print_stuff(std::function<const int&()> f) {
>
>     std::cout << "stuff is: " << f() << std::endl;
>
> }
>
>
> int main(int argc, char *argv[]) {
>
>     print_stuff([]() {
>
>         int i = 123;
>
>         return i;
>
>     });
>
> }
>
>
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>



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