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  1. #include <utility>
  2.  
  3. namespace util {
  4. template< typename t >
  5. t traits_type_entry( t const & ); // Declared, never defined.
  6.  
  7. template< typename t >
  8. using traits_type = decltype( traits_type_entry( std::declval< t >() ) );
  9. }
  10.  
  11. class outer_scope { // Just for illustration
  12.     struct special;
  13.     struct special_traits {
  14.         typedef int value_type;
  15.         constexpr static int limit = 5;
  16.     };
  17.     friend special_traits traits_type_entry( special const & );
  18.  
  19.     struct unspecial {
  20.         typedef double baz_type;
  21.         int table[ util::traits_type< special >::limit ];
  22.     };
  23.  
  24.     struct special : special_traits {
  25.         void f() {
  26.              std::pair< typename util::traits_type< unspecial >::baz_type, value_type >();
  27.         }
  28.     };
  29. };
  30.  
Compilation error #stdin compilation error #stdout 0s 0KB
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stdin
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Standard input is empty
compilation info 
/usr/lib/gcc/i486-linux-gnu/4.8/../../../i386-linux-gnu/crt1.o: In function `_start':
(.text+0x18): undefined reference to `main'
collect2: error: ld returned 1 exit status
stdout
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Standard output is empty
https://ideone.com/QztQ6i
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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