[Bug c/64509] New: _Generic throws error in unselected generic association

[maurits.de.jong at ericsson dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64509

            Bug ID: 64509
           Summary: _Generic throws error in unselected generic
                    association
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: maurits.de.jong at ericsson dot com

Created attachment 34387
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34387&action=edit
The offending source separately

I would have expected this to work (basically insert a conversion function
call if the type doesn't match):


-----
typedef struct {
        int x;
} mystruct;

extern int __f(int);
extern int __stoi(mystruct);

void call(void)
{
        mystruct S = {1};

        double z = _Generic((S), mystruct: __f(__stoi(S)), default: __f(S));
}
-----

but the typechecker is not happy about __f(S), which is in an association 
not relevant for the controlling expression:

The standard says:

  None of the expressions from any other generic association of the generic
  selection is evaluated.

Of course, evaluation is different from typechecking, but since the construct
is intended to conditionalise based on types, wouldn't it make sense to only
typecheck the selected association?




bash$ ./bin/gcc -std=c11 -v generic.c
Using built-in specs.
COLLECT_GCC=./bin/gcc
COLLECT_LTO_WRAPPER=/space/radmmj/gcc492/gcc-4.9.2/libexec/gcc/x86_64-unknown-linux-gnu/4.9.2/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --prefix=/space/radmmj/gcc492/gcc-4.9.2 :
(reconfigured) ../configure --prefix=/space/radmmj/gcc492/gcc-4.9.2
--enable-languages=c
Thread model: posix
gcc version 4.9.2 (GCC) 
COLLECT_GCC_OPTIONS='-std=c11' '-v' '-mtune=generic' '-march=x86-64'
 /space/radmmj/gcc492/gcc-4.9.2/libexec/gcc/x86_64-unknown-linux-gnu/4.9.2/cc1
-quiet -v -imultiarch x86_64-linux-gnu generic.c -quiet -dumpbase generic.c
-mtune=generic -march=x86-64 -auxbase generic -std=c11 -version -o
/tmp/ccEshK6s.s
GNU C (GCC) version 4.9.2 (x86_64-unknown-linux-gnu)
        compiled by GNU C version 4.9.2, GMP version 4.3.2, MPFR version 2.4.2,
MPC version 0.8.1
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/space/radmmj/gcc492/gcc-4.9.2/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../x86_64-unknown-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /space/radmmj/gcc492/gcc-4.9.2/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/include
 /usr/local/include
 /space/radmmj/gcc492/gcc-4.9.2/include

/space/radmmj/gcc492/gcc-4.9.2/lib/gcc/x86_64-unknown-linux-gnu/4.9.2/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
GNU C (GCC) version 4.9.2 (x86_64-unknown-linux-gnu)
        compiled by GNU C version 4.9.2, GMP version 4.3.2, MPFR version 2.4.2,
MPC version 0.8.1
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 1bfa4fb39c457521c17e15d89fa44581
generic.c: In function ‘call’:
generic.c:12:69: error: incompatible type for argument 1 of ‘__f’
         double z = _Generic((S), mystruct: __f(__stoi(S)), default: __f(S));
                                                                     ^
generic.c:5:12: note: expected ‘int’ but argument is of type ‘mystruct’
 extern int __f(int);
            ^