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Bug report: 14.0.2 delegating ctor overload resolution failure via "using" alias


I am surprised that the following code produces an error:

#include <cstddef>

template<typename T, T... Is>
struct Bar { };

template<size_t... Is>
using Baz = Bar<size_t, Is...>;

struct Foo {
  // the following two lines give error:
  // no instance of constructor "Foo::Foo" matches the argument list
  Foo() : Foo(Baz<4, 2>()) { }  // delegating ctor using an alias
  Foo(int) : Foo(Bar<size_t, 4, 2>()) { }  // delagating ctor

  template<size_t... Is>
  Foo(Baz<Is...>) { }

  // however, the overload below which doesn't use the Baz alias works fine
  // template<size_t... Is>
  // Foo(Bar<size_t, Is...>) { }

  template<size_t... Is>
  void NoAlias(Bar<size_t, Is...>) { }

  template<size_t... Is>
  void Alias(Baz<Is...>) { }

template<typename T, T... Is>
void foo(Bar<T, Is...>) { }

template<size_t... Is>
void bar(Bar<size_t, Is...>) { }

int main(int argc, char* argv[]) {
  // In addition, all of these work fine
  foo(Bar<size_t, 4, 2>());
  foo(Baz<4, 2>());
  bar(Bar<size_t, 4, 2>());
  bar(Baz<4, 2>());
  Foo direct_noalias((Bar<size_t, 4, 2>()));
  Foo direct_alias((Baz<4, 2>()));
  direct_alias.NoAlias(Bar<size_t, 4, 2>());
  direct_alias.NoAlias(Baz<4, 2>());

  // However, these four give error, even though the constructor call chain
  // mimics the bar(Baz<4, 2>()) / bar(Bar<size_t, 4, 2>()) call
  Foo delegating_noalias(42);
  Foo delegating_alias;
  // and the only difference in NoAlias and Alias is (not) using the alias
  // for the member function parameter
  direct_alias.Alias(Bar<size_t, 4, 2>());
  direct_alias.Alias(Baz<4, 2>());

  return 0;

For the record, it compiles fine under both GCC 4.8 and Clang 3.4.2.

I discovered this issue by accident while implementing compile-time integer sequence according to the C++14 draft:

so it will manifest more in the future as people do more template metaprogramming involving integer sequences. (For example, the index_sequence alias would be unusable in member functions or constructors arguments.)

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2 Replies

Might be the same problem I described here: where I also reimplement the C++14 integer_sequence.

I had trouble with template type-deduction through the alias.

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Hi Leo,

What version and update number of the compiler are you using? This compiles fine for me with the latest development versions of 14.0 and 15.0.

FYI if it is the same issue as Daniel V. posted a link to then that was fixed as of March 26th 2014.


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