[Bug c++/52288] New: Trouble with operator?: and lambdas

[Bug c++/52288] New: Trouble with operator?: and lambdas

[igodard at pacbell dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52288

             Bug #: 52288
           Summary: Trouble with operator?: and lambdas
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: igodard@pacbell.net


This code:

int main(int argc, char** argv) {
    bool b;
    void* p = b ? [argc](int i){ return i; } :
        [argc](int i){ return i; };
return 0; }

gets you this:

s3:~/ootbc/personal/ivan$ g++ --std=c++0x foo.cc
foo.cc: In function âint main(int, char**)â:
foo.cc:5:28: error: no match for ternary âoperator?:â in âb ? {argc} : {argc}â

which is a poor. Meanwhile this code:

int main(int argc, char** argv) {
    bool b;
    void* p = b ? &[argc](int i){ return i; } :
        &[argc](int i){ return i; };
return 0; }

gets you this:

s3:~/ootbc/personal/ivan$ g++ --std=c++0x foo.cc
foo.cc: In function âint main(int, char**)â:
foo.cc:4:42: error: taking address of temporary [-fpermissive]
foo.cc:5:29: error: taking address of temporary [-fpermissive]
foo.cc:5:29: error: conditional expression between distinct pointer types
âmain(int, char**)::<lambda(int)>*â and âmain(int, char**)::<lambda(int)>*â
lacks a cast

which is even worse.

[Bug c++/52288] Trouble with operator?: and lambdas

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52288

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
           Severity|normal                      |enhancement

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-02-16 23:49:39 UTC ---
(In reply to comment #0)
> This code:
> 
> int main(int argc, char** argv) {
>     bool b;
>     void* p = b ? [argc](int i){ return i; } :
>         [argc](int i){ return i; };
> return 0; }
> 
> gets you this:
> 
> s3:~/ootbc/personal/ivan$ g++ --std=c++0x foo.cc
> foo.cc: In function âint main(int, char**)â:
> foo.cc:5:28: error: no match for ternary âoperator?:â in âb ? {argc} : {argc}â
> 
> which is a poor.

What do you suggest instead?

> Meanwhile this code:
> 
> int main(int argc, char** argv) {
>     bool b;
>     void* p = b ? &[argc](int i){ return i; } :
>         &[argc](int i){ return i; };
> return 0; }
> 
> gets you this:
> 
> s3:~/ootbc/personal/ivan$ g++ --std=c++0x foo.cc
> foo.cc: In function âint main(int, char**)â:
> foo.cc:4:42: error: taking address of temporary [-fpermissive]
> foo.cc:5:29: error: taking address of temporary [-fpermissive]
> foo.cc:5:29: error: conditional expression between distinct pointer types
> âmain(int, char**)::<lambda(int)>*â and âmain(int, char**)::<lambda(int)>*â
> lacks a cast
> 
> which is even worse.

Why?  It's entirely accurate, you can't take the address of a lambda, and the
two types are different and incompatible so can't be used as the second and
third operands of a conditional expression.

[Bug c++/52288] Trouble with operator?: and lambdas

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52288

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-02-16 23:53:15 UTC ---
(In reply to comment #1)
> > foo.cc:5:28: error: no match for ternary âoperator?:â in âb ? {argc} : {argc}â
> > 
> > which is a poor.
> 
> What do you suggest instead?

Sorry, I missed the {argc} part.  Trunk gives a better error:

l.cc:4:34: error: no match for ternary 'operator?:' in 'b ? main(int,
char**)::<lambda(int)>{argc} : main(int, char**)::<lambda(int)>{argc}'

[Bug c++/52288] Trouble with operator?: and lambdas

[igodard at pacbell dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52288

--- Comment #3 from Ivan Godard <igodard at pacbell dot net> 2012-02-17 00:00:48 UTC ---
As lambdas get used more you will get this kind of report a lot. I suggest you
head off a lot of nuisance by having any lambda/lambda type comparisons emit a
"Note: each lambda is it own unique type".

[Bug c++/52288] Trouble with operator?: and lambdas

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52288

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-02-17 00:18:21 UTC ---
I would expect as lambdas get used more people will understand they produce a
unique closure type

[Bug c++/52288] Trouble with operator?: and lambdas

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52288

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu.org

--- Comment #5 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-02-17 00:36:59 UTC ---
(In reply to comment #3)
> As lambdas get used more you will get this kind of report a lot. I suggest you
> head off a lot of nuisance by having any lambda/lambda type comparisons emit a
> "Note: each lambda is it own unique type".

For the little that it is worth, I agree: both error messages are plain awful.

As programmers start using more complex lambdas, you should expect the
pretty-printing of expressions to reach heavy-template code levels of
awfulness.

Can't we just say?

invalid operands to ternary operator ('lambda' and 'lambda')

[Bug c++/52288] Trouble with operator?: and lambdas

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52288

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #6 from Jason Merrill <jason at gcc dot gnu.org> ---
G++ will now say

wa.C:3:17: error: operands to ?: have different types ‘main(int,
char**)::<lambda(int)>’ and ‘main(int, char**)::<lambda(int)>’

I think that addresses the ?: part of this issue.  I'll leave it open for now
in case we want to keep it as a question about how to name lambdas in error
messages.  It seems to me that we might want to omit the function scope if
we're currently in the same function.  Do we want to give them numbers to
distinguish them?
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Subject: [Bug c++/51317] [C++0x] [DR 587] Wrong value category of conditional expression where lvalue operands differ only in cv-qualification
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idQ317

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2014-05-09
                 CC|                            |jason at gcc dot gnu.org
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed on trunk.

[Bug c++/52288] operator?: and lambdas error message could be better

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52288

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-08-06
            Summary|Trouble with operator?: and |operator?: and lambdas
                   |lambdas                     |error message could be
                   |                            |better

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Jason Merrill from comment #6)
> G++ will now say
> 
> wa.C:3:17: error: operands to ?: have different types ‘main(int,
> char**)::<lambda(int)>’ and ‘main(int, char**)::<lambda(int)>’
> 
> I think that addresses the ?: part of this issue.  I'll leave it open for
> now in case we want to keep it as a question about how to name lambdas in
> error messages.  It seems to me that we might want to omit the function
> scope if we're currently in the same function.  Do we want to give them
> numbers to distinguish them?

This is how clang does their names of the lambdas in error message:
<source>:4:17: error: incompatible operand types ('(lambda at <source>:4:19)'
and '(lambda at <source>:5:9)')
    void* p = b ? [argc](int i){ return i; } :
                ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~

Maybe we can do the same.

[Bug c++/52288] operator?: and lambdas error message could be better

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52288

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
If we don't do the same as Clang, I definitely like the suggestion to drop the
function name if within the same scope. For a function template with many
template arguments and function arguments it can get very hard to read and hard
to find the ::<lambda(...)> part.