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Overload resolution is strange with `constexpr` #26878

Open nico opened 8 years ago

nico commented 8 years ago
Bugzilla Link 26504
Version trunk
OS All
CC @DougGregor,@zygoloid,@rnk

Extended Description

Consider this:

Nicos-MBP:llvm-build thakis$ cat test.cc
template<class T> const T& min(const T& a, const T& b) {
  return a < b ? a : b;
}
struct A {
  A();
  operator int() const;
  operator unsigned() const;

  A foo() const {
    return min(*this, A());
  }
};
inline bool operator<(const A &a, const A &b) { return true; }
Nicos-MBP:llvm-build thakis$ bin/clang -c test.cc -std=c++11

This builds fine. Now make max() constexpr:

Nicos-MBP:llvm-build thakis$ bin/clang -c test.cc -std=c++11
test.cc:2:12: error: use of overloaded operator '<' is ambiguous (with operand types 'const A' and 'const A')
  return a < b ? a : b;
         ~ ^ ~
test.cc:10:12: note: in instantiation of function template specialization 'min<A>' requested here
    return min(*this, A());
           ^
test.cc:2:12: note: because of ambiguity in conversion of 'const A' to 'float'
  return a < b ? a : b;
           ^
test.cc:6:3: note: candidate function
  operator int() const;
  ^
test.cc:7:3: note: candidate function
  operator unsigned() const;
  ^
test.cc:2:12: note: because of ambiguity in conversion of 'const A' to 'float'
  return a < b ? a : b;
           ^
test.cc:6:3: note: candidate function
  operator int() const;
  ^
test.cc:7:3: note: candidate function
  operator unsigned() const;
  ^
test.cc:2:12: note: built-in candidate operator<(float, float)
  return a < b ? a : b;
           ^
test.cc:2:12: note: built-in candidate operator<(float, double)
test.cc:2:12: note: built-in candidate operator<(float, long double)
test.cc:2:12: note: built-in candidate operator<(float, int)
test.cc:2:12: note: built-in candidate operator<(float, long)
test.cc:2:12: note: built-in candidate operator<(float, long long)
test.cc:2:12: note: built-in candidate operator<(float, __int128)
test.cc:2:12: note: built-in candidate operator<(float, unsigned int)
test.cc:2:12: note: built-in candidate operator<(float, unsigned long)
test.cc:2:12: note: built-in candidate operator<(float, unsigned long long)
test.cc:2:12: note: built-in candidate operator<(float, unsigned __int128)
test.cc:2:12: note: built-in candidate operator<(double, float)
test.cc:2:12: note: built-in candidate operator<(double, double)
test.cc:2:12: note: built-in candidate operator<(double, long double)
test.cc:2:12: note: built-in candidate operator<(double, int)
test.cc:2:12: note: built-in candidate operator<(double, long)
test.cc:2:12: note: built-in candidate operator<(double, long long)
test.cc:2:12: note: built-in candidate operator<(double, __int128)
test.cc:2:12: note: built-in candidate operator<(double, unsigned int)
test.cc:2:12: note: built-in candidate operator<(double, unsigned long)
test.cc:2:12: note: built-in candidate operator<(double, unsigned long long)
test.cc:2:12: note: built-in candidate operator<(double, unsigned __int128)
test.cc:2:12: note: built-in candidate operator<(long double, float)
test.cc:2:12: note: built-in candidate operator<(long double, double)
test.cc:2:12: note: built-in candidate operator<(long double, long double)
test.cc:2:12: note: built-in candidate operator<(long double, int)
test.cc:2:12: note: built-in candidate operator<(long double, long)
test.cc:2:12: note: built-in candidate operator<(long double, long long)
test.cc:2:12: note: built-in candidate operator<(long double, __int128)
test.cc:2:12: note: built-in candidate operator<(long double, unsigned int)
test.cc:2:12: note: built-in candidate operator<(long double, unsigned long)
test.cc:2:12: note: built-in candidate operator<(long double, unsigned long long)
test.cc:2:12: note: built-in candidate operator<(long double, unsigned __int128)
test.cc:2:12: note: built-in candidate operator<(int, float)
test.cc:2:12: note: built-in candidate operator<(int, double)
test.cc:2:12: note: built-in candidate operator<(int, long double)
test.cc:2:12: note: built-in candidate operator<(int, int)
test.cc:2:12: note: built-in candidate operator<(int, long)
test.cc:2:12: note: built-in candidate operator<(int, long long)
test.cc:2:12: note: built-in candidate operator<(int, __int128)
test.cc:2:12: note: built-in candidate operator<(int, unsigned int)
test.cc:2:12: note: built-in candidate operator<(int, unsigned long)
test.cc:2:12: note: built-in candidate operator<(int, unsigned long long)
test.cc:2:12: note: built-in candidate operator<(int, unsigned __int128)
test.cc:2:12: note: built-in candidate operator<(long, float)
test.cc:2:12: note: built-in candidate operator<(long, double)
test.cc:2:12: note: built-in candidate operator<(long, long double)
test.cc:2:12: note: built-in candidate operator<(long, int)
test.cc:2:12: note: built-in candidate operator<(long, long)
test.cc:2:12: note: built-in candidate operator<(long, long long)
test.cc:2:12: note: built-in candidate operator<(long, __int128)
test.cc:2:12: note: built-in candidate operator<(long, unsigned int)
test.cc:2:12: note: built-in candidate operator<(long, unsigned long)
test.cc:2:12: note: built-in candidate operator<(long, unsigned long long)
test.cc:2:12: note: built-in candidate operator<(long, unsigned __int128)
test.cc:2:12: note: built-in candidate operator<(long long, float)
test.cc:2:12: note: built-in candidate operator<(long long, double)
test.cc:2:12: note: built-in candidate operator<(long long, long double)
test.cc:2:12: note: built-in candidate operator<(long long, int)
test.cc:2:12: note: built-in candidate operator<(long long, long)
test.cc:2:12: note: built-in candidate operator<(long long, long long)
test.cc:2:12: note: built-in candidate operator<(long long, __int128)
test.cc:2:12: note: built-in candidate operator<(long long, unsigned int)
test.cc:2:12: note: built-in candidate operator<(long long, unsigned long)
test.cc:2:12: note: built-in candidate operator<(long long, unsigned long long)
test.cc:2:12: note: built-in candidate operator<(long long, unsigned __int128)
test.cc:2:12: note: built-in candidate operator<(__int128, float)
test.cc:2:12: note: built-in candidate operator<(__int128, double)
test.cc:2:12: note: built-in candidate operator<(__int128, long double)
test.cc:2:12: note: built-in candidate operator<(__int128, int)
test.cc:2:12: note: built-in candidate operator<(__int128, long)
test.cc:2:12: note: built-in candidate operator<(__int128, long long)
test.cc:2:12: note: built-in candidate operator<(__int128, __int128)
test.cc:2:12: note: built-in candidate operator<(__int128, unsigned int)
test.cc:2:12: note: built-in candidate operator<(__int128, unsigned long)
test.cc:2:12: note: built-in candidate operator<(__int128, unsigned long long)
test.cc:2:12: note: built-in candidate operator<(__int128, unsigned __int128)
test.cc:2:12: note: built-in candidate operator<(unsigned int, float)
test.cc:2:12: note: built-in candidate operator<(unsigned int, double)
test.cc:2:12: note: built-in candidate operator<(unsigned int, long double)
test.cc:2:12: note: built-in candidate operator<(unsigned int, int)
test.cc:2:12: note: built-in candidate operator<(unsigned int, long)
test.cc:2:12: note: built-in candidate operator<(unsigned int, long long)
test.cc:2:12: note: built-in candidate operator<(unsigned int, __int128)
test.cc:2:12: note: built-in candidate operator<(unsigned int, unsigned int)
test.cc:2:12: note: built-in candidate operator<(unsigned int, unsigned long)
test.cc:2:12: note: built-in candidate operator<(unsigned int, unsigned long long)
test.cc:2:12: note: built-in candidate operator<(unsigned int, unsigned __int128)
test.cc:2:12: note: built-in candidate operator<(unsigned long, float)
test.cc:2:12: note: built-in candidate operator<(unsigned long, double)
test.cc:2:12: note: built-in candidate operator<(unsigned long, long double)
test.cc:2:12: note: built-in candidate operator<(unsigned long, int)
test.cc:2:12: note: built-in candidate operator<(unsigned long, long)
test.cc:2:12: note: built-in candidate operator<(unsigned long, long long)
test.cc:2:12: note: built-in candidate operator<(unsigned long, __int128)
test.cc:2:12: note: built-in candidate operator<(unsigned long, unsigned int)
test.cc:2:12: note: built-in candidate operator<(unsigned long, unsigned long)
test.cc:2:12: note: built-in candidate operator<(unsigned long, unsigned long long)
test.cc:2:12: note: built-in candidate operator<(unsigned long, unsigned __int128)
test.cc:2:12: note: built-in candidate operator<(unsigned long long, float)
test.cc:2:12: note: built-in candidate operator<(unsigned long long, double)
test.cc:2:12: note: built-in candidate operator<(unsigned long long, long double)
test.cc:2:12: note: built-in candidate operator<(unsigned long long, int)
test.cc:2:12: note: built-in candidate operator<(unsigned long long, long)
test.cc:2:12: note: built-in candidate operator<(unsigned long long, long long)
test.cc:2:12: note: built-in candidate operator<(unsigned long long, __int128)
test.cc:2:12: note: built-in candidate operator<(unsigned long long, unsigned int)
test.cc:2:12: note: built-in candidate operator<(unsigned long long, unsigned long)
test.cc:2:12: note: built-in candidate operator<(unsigned long long, unsigned long long)
test.cc:2:12: note: built-in candidate operator<(unsigned long long, unsigned __int128)
test.cc:2:12: note: built-in candidate operator<(unsigned __int128, float)
test.cc:2:12: note: built-in candidate operator<(unsigned __int128, double)
test.cc:2:12: note: built-in candidate operator<(unsigned __int128, long double)
test.cc:2:12: note: built-in candidate operator<(unsigned __int128, int)
test.cc:2:12: note: built-in candidate operator<(unsigned __int128, long)
test.cc:2:12: note: built-in candidate operator<(unsigned __int128, long long)
test.cc:2:12: note: built-in candidate operator<(unsigned __int128, __int128)
test.cc:2:12: note: built-in candidate operator<(unsigned __int128, unsigned int)
test.cc:2:12: note: built-in candidate operator<(unsigned __int128, unsigned long)
test.cc:2:12: note: built-in candidate operator<(unsigned __int128, unsigned long long)
test.cc:2:12: note: built-in candidate operator<(unsigned __int128, unsigned __int128)
1 error generated.

cl.exe builds this fine, and that's how it implements std::min().

a) Who is right?

b) If it's clang, we need to figure out what cl.exe does and accept that as an extension.

(Also, the diagnostic is not very good.)

ec04fc15-fa35-46f2-80e1-5d271f2ef708 commented 8 years ago

Both compilers are right; there are two points of instantiation for min, and we pick the first one if it's constexpr.

nico commented 8 years ago

(The constexpr version compiles if I add

  struct A;
  inline bool operator<(const A&, const A&);

between the definition of min() and struct A, so I can probably use that as a workaround for now. But it'd be good to know if this is intentional, and if so, to get a better diagnostic.)