[Bug c++/54348] New: wrong error reported for type mismatch in conditional expression : "error: no match for ternary 'operator?:' in 'false ?"

[Bug c++/54348] New: wrong error reported for type mismatch in conditional expression : "error: no match for ternary 'operator?:' in 'false ?"

[jason.vas.dias at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54348

             Bug #: 54348
           Summary: wrong error reported for type mismatch in conditional
                    expression : "error: no match for ternary 'operator?:'
                    in 'false ?"
    Classification: Unclassified
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jason.vas.dias@gmail.com


Having  made the simple mistake of returning an object of a different type
in the '? ( ... )' and ': ( ... )' clauses of a ternary expression ,
I'd expect and it would be helpful if g++ would emit the "C" error
  "error: type mismatch in conditional expression" and not
  "error: no match for ternary 'operator?:' in 'false ? ..."
This is extremely confusing, as it suggests that the ternary expression
 somehow contains an unbalanced number of parentheses or something.

This code triggers the issue:

<code>
#include <string>
#include <list>
using namespace std;
void f()
{
      struct strct { string name, items ;};
      list <string> myItems;
      string myName("");
      string as      ( (   (&(((strct*)0)  -> items)) 
            == (&(((strct*)0) -> name))
               ) ? myItems 
                 : myName
             )
             ;
}
</code>

Compilation with gcc-4.6.0 & gcc-4.6.3 returns this error:

$ g++ -c gxx_bug.cpp
gxx_bug.cpp: In function 'void f()':
gxx_bug.cpp:12:14: error: no match for ternary 'operator?:' in 'false ? myItems
: myName'

whereas changing 'list <string> myItems' to 'string myItems' allows compilation
to succeed.

Shouldn't g++ be complaining about initializing a string with a list<string>
rather than this cryptic "no match for ternary 'operator?:'" here ?

[Bug c++/54348] confusing error reported for type mismatch in conditional expression : "error: no match for ternary 'operator?:' in 'false ?"

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54348

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-08-21
            Summary|wrong error reported for    |confusing error reported
                   |type mismatch in            |for type mismatch in
                   |conditional expression :    |conditional expression :
                   |"error: no match for        |"error: no match for
                   |ternary 'operator?:' in     |ternary 'operator?:' in
                   |'false ?"                   |'false ?"
     Ever Confirmed|0                           |1

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-08-21 19:44:55 UTC ---
In mainline the diagnostics is better because we output the types. But I agree
that given that the conditional operator cannot be overloaded the error message
could be more clear.

[Bug c++/54348] confusing error reported for type mismatch in conditional expression : "error: no match for ternary 'operator?:' in 'false ?"

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54348

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-08-21 19:51:53 UTC ---
(In reply to comment #0)
> Shouldn't g++ be complaining about initializing a string with a list<string>
> rather than this cryptic "no match for ternary 'operator?:'" here ?

No, not really.

The object being initialized by the result of the condition expression is
irrelevant, the conditional expression isn't valid whether or not you're using
it to initialize another object.

In this reduced version it wouldn't make sense to refer to initializing any
object with any other:

struct A {} a;
struct B {} b;

void f()
{
    false ? a : b;
}

[Bug c++/54348] confusing error reported for type mismatch in conditional expression : "error: no match for ternary 'operator?:' in 'false ?"

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54348

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-08-21 20:08:02 UTC ---
Indeed.

About my own reply, I'm not sure, the wording here is pretty subtle, we already
handle separately the ambiguous overloading case. ICC refers explicitly to the
types being incompatible, maybe focusing on the types is more clear when the
operator at issue cannot be overloaded.

[Bug c++/54348] confusing error reported for type mismatch in conditional expression : "error: no match for ternary 'operator?:' in 'false ?"

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54348

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-08-21 20:24:15 UTC ---
I think clang's "incompatible operand types" is simple and fairly clear

[Bug c++/54348] confusing error reported for type mismatch in conditional expression : "error: no match for ternary 'operator?:' in 'false ?"

[jason.vas.dias at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54348

--- Comment #5 from Jason Vas Dias <jason.vas.dias at gmail dot com> 2012-08-21 20:27:36 UTC ---
Oops, I was interrupted adding this comment to my initial comment - will
respond
to subsequent commment next :

Incidentally, I found this issue while developing a C++-98 replacement for
C-99 designated initializers for specific structs with generated macros :
<code>
#include <string>
#include <list>
using namespace std;

struct strct 
{
    string name, items ;
    strct (string n, string i) : name(n), items(i){}
};

#define tok(t) t

#define the_struct_strct_member( member, a0, a1, a2, a3 )            \
    ( (&(((struct strct*)0) -> member ) == &(((struct strct*)0) -> a0)) \
       ? a1                                \
       :( ( &(((struct strct*)0) -> member) ==  &(((struct strct*)0) -> a2) ) \
          ? a3                                \
          : NULL                            \
        )                                \
    )

#define struct_strct( a0, a1, a2, a3 )                    \
    (    the_struct_strct_member( tok(name)  , a0, a1, a2, a3 ),    \
     the_struct_strct_member( tok(items) , a0, a1, a2, a3 )            \
    )

void f()
{

      string myItems;
      string myName("");

      strct s  struct_strct( items, myItems, name, myName ) ;
}
</code>

This works!

[Bug c++/54348] confusing error reported for type mismatch in conditional expression : "error: no match for ternary 'operator?:' in 'false ?"

[jason.vas.dias at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54348

--- Comment #6 from Jason Vas Dias <jason.vas.dias at gmail dot com> 2012-08-21 20:29:53 UTC ---
(In reply to comment #1)
> In mainline the diagnostics is better because we output the types. But I agree
> that given that the conditional operator cannot be overloaded the error message
> could be more clear.

yes, that is all I'm saying

[Bug c++/54348] confusing error reported for type mismatch in conditional expression : "error: no match for ternary 'operator?:' in 'false ?"

[jason.vas.dias at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54348

--- Comment #7 from Jason Vas Dias <jason.vas.dias at gmail dot com> 2012-08-21 20:34:06 UTC ---
(In reply to comment #2)
> (In reply to comment #0)
> > Shouldn't g++ be complaining about initializing a string with a list<string>
> > rather than this cryptic "no match for ternary 'operator?:'" here ?
> 
> No, not really.
> 
> The object being initialized by the result of the condition expression is
> irrelevant, the conditional expression isn't valid whether or not you're using
> it to initialize another object.
> 
> In this reduced version it wouldn't make sense to refer to initializing any
> object with any other:
> 
> struct A {} a;
> struct B {} b;
> 
> void f()
> {
>     false ? a : b;
> }

but this is not the same. surely ? the above is not a valid statement -
I was saying:

      some_type some_object = false ? a : b;

[Bug c++/54348] confusing error reported for type mismatch in conditional expression : "error: no match for ternary 'operator?:' in 'false ?"

[jason.vas.dias at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54348

--- Comment #8 from Jason Vas Dias <jason.vas.dias at gmail dot com> 2012-08-21 20:52:12 UTC ---
All I'm suggesting is that g++ should try to find the most basic error, 
which is that different type objects are returned as the result of a
conditional expression, and not  "no match for ternary 'operator?:'" -
what does this mean, it was searching namespace std:: for string::operator::?:
?
then this succeeded, and it found it could not apply it because the types
were different - shouldn't it complain about the root cause, that the types
were different, rather than the symptom of not being able to satisfy
operator std::string::?:() ?

[Bug c++/54348] confusing error reported for type mismatch in conditional expression : "error: no match for ternary 'operator?:' in 'false ?"

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54348

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-08-22 07:37:18 UTC ---
(In reply to comment #5)
>       strct s  struct_strct( items, myItems, name, myName ) ;
> }
> </code>
> 
> This works!

Only because NULL can be converted to std::string - but it would crash at
runtime if that branch was taken.


(In reply to comment #7)
> > void f()
> > {
> >     false ? a : b;
> > }
> 
> but this is not the same. surely ? the above is not a valid statement -

It's exactly the same. It's not valid because the types are incompatible, just
like your example. e.g. this is fine:

  false ? a : a;

> I was saying:
> 
>       some_type some_object = false ? a : b;

As I said previously, the fact the conditional expression is used as an
initializer is irrelevant to the error.


(In reply to comment #8)
> All I'm suggesting is that g++ should try to find the most basic error, 

Exactly. And the error is in the conditional expression, it has nothing to do
with the object being initialized.


> which is that different type objects are returned as the result of a
> conditional expression, and not  "no match for ternary 'operator?:'" -
> what does this mean, it was searching namespace std:: for string::operator::?:
> ?
> then this succeeded, and it found it could not apply it because the types
> were different - shouldn't it complain about the root cause, that the types
> were different, rather than the symptom of not being able to satisfy
> operator std::string::?:() ?

It *is* complaining that the types are different (just not very clearly.)
The error has nothing to do with the type being initialized.

Here's a simpler form of your original which might convince you of that:

#include <string>
#include <list>
using namespace std;
void f()
{
      list <string> myItems;
      string myName;
      false ? myItems : myName;
}

This still gives the same error. Understand now?

[Bug c++/54348] confusing error reported for type mismatch in conditional expression : "error: no match for ternary 'operator?:' in 'false ?"

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54348

--- Comment #10 from Jason Merrill <jason at gcc dot gnu.org> ---
Author: jason
Date: Fri May  9 18:15:57 2014
New Revision: 210282

URL: http://gcc.gnu.org/viewcvs?rev=210282&root=gcc&view=rev
Log:
    PR c++/32019
    * call.c (build_conditional_expr_1): Improve ambiguity diagnostic.

    PR c++/54348
    * call.c (build_conditional_expr_1): If overload resolution finds
    no match, just say "different types".

Added:
    trunk/gcc/testsuite/g++.dg/expr/cond10.C
    trunk/gcc/testsuite/g++.dg/expr/cond11.C
    trunk/gcc/testsuite/g++.dg/expr/cond13.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/call.c
    trunk/gcc/testsuite/g++.dg/cpp0x/explicit3.C
    trunk/gcc/testsuite/g++.dg/parse/crash41.C

[Bug c++/54348] confusing error reported for type mismatch in conditional expression : "error: no match for ternary 'operator?:' in 'false ?"

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54348

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |jason at gcc dot gnu.org
         Resolution|---                         |FIXED
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org
   Target Milestone|---                         |4.10.0

--- Comment #11 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed to just say "different types".