llvm-project/clang/test/SemaTemplate/constexpr-instantiate.cpp

// RUN: %clang_cc1 -std=c++11 -verify %s

namespace UseBeforeDefinition {
  struct A {
    template<typename T> static constexpr T get() { return T(); }
    // ok, not a constant expression.
    int n = get<int>();
  };

  // ok, constant expression.
  constexpr int j = A::get<int>();

  template<typename T> constexpr int consume(T);
  // ok, not a constant expression.
  const int k = consume(0); // expected-note {{here}}

  template<typename T> constexpr int consume(T) { return 0; }
  // ok, constant expression.
  constexpr int l = consume(0);

  constexpr int m = k; // expected-error {{constant expression}} expected-note {{initializer of 'k'}}
}

namespace IntegralConst {
  template<typename T> constexpr T f(T n) { return n; }
  enum E {
    v = f(0), w = f(1) // ok
  };
  static_assert(w == 1, "");

  char arr[f('x')]; // ok
  static_assert(sizeof(arr) == 'x', "");
}

namespace ConvertedConst {
  template<typename T> constexpr T f(T n) { return n; }
  int f() {
    switch (f()) {
      case f(4): return 0;
    }
    return 1;
  }
}

namespace OverloadResolution {
  template<typename T> constexpr T f(T t) { return t; }

  template<int n> struct S { };

  template<typename T> auto g(T t) -> S<f(sizeof(T))> &;
  char &f(...);

  template<typename T> auto h(T t[f(sizeof(T))]) -> decltype(&*t) {
    return t;
  }

  S<4> &k = g(0);
  int *p, *q = h(p);
}

namespace DataMember {
  template<typename T> struct S { static const int k; };
  const int n = S<int>::k; // expected-note {{here}}
  template<typename T> const int S<T>::k = 0;
  constexpr int m = S<int>::k; // ok
  constexpr int o = n; // expected-error {{constant expression}} expected-note {{initializer of 'n'}}
}

namespace Reference {
  const int k = 5;
  template<typename T> struct S {
    static volatile int &r;
  };
  template<typename T> volatile int &S<T>::r = const_cast<volatile int&>(k);
  constexpr int n = const_cast<int&>(S<int>::r);
  static_assert(n == 5, "");
}

namespace Unevaluated {
  // We follow g++ in treating any reference to a constexpr function template
  // specialization as requiring an instantiation, even if it occurs in an
  // unevaluated context.
  //
  // We go slightly further than g++, and also trigger the implicit definition
  // of a defaulted special member in the same circumstances. This seems scary,
  // since a lot of classes have constexpr special members in C++11, but the
  // only observable impact should be the implicit instantiation of constexpr
  // special member templates (defaulted special members should only be
  // generated if they are well-formed, and non-constexpr special members in a
  // base or member cause the class's special member to not be constexpr).
  //
  // FIXME: None of this is required by the C++ standard. The rules in this
  //        area are poorly specified, so this is subject to change.
  namespace NotConstexpr {
    template<typename T> struct S {
      S() : n(0) {}
      S(const S&) : n(T::error) {}
      int n;
    };
    struct U : S<int> {};
    decltype(U(U())) u; // ok, don't instantiate S<int>::S() because it wasn't declared constexpr
  }
  namespace Constexpr {
    template<typename T> struct S {
      constexpr S() : n(0) {}
      constexpr S(const S&) : n(T::error) {} // expected-error {{has no members}}
      int n;
    };
    struct U : S<int> {}; // expected-note {{instantiation}}
    decltype(U(U())) u; // expected-note {{here}}
  }

  namespace PR11851_Comment0 {
    template<int x> constexpr int f() { return x; }
    template<int i> void ovf(int (&x)[f<i>()]);
    void f() { int x[10]; ovf<10>(x); }
  }

  namespace PR11851_Comment1 {
    template<typename T>
    constexpr bool Integral() {
      return true;
    }
    template<typename T, bool Int = Integral<T>()>
    struct safe_make_unsigned {
      typedef T type;
    };
    template<typename T>
    using Make_unsigned = typename safe_make_unsigned<T>::type;
    template <typename T>
    struct get_distance_type {
      using type = int;
    };
    template<typename R>
    auto size(R) -> Make_unsigned<typename get_distance_type<R>::type>;
    auto check() -> decltype(size(0));
  }

  namespace PR11851_Comment6 {
    template<int> struct foo {};
    template<class> constexpr int bar() { return 0; }
    template<class T> foo<bar<T>()> foobar();
    auto foobar_ = foobar<int>();
  }

  namespace PR11851_Comment9 {
    struct S1 {
      constexpr S1() {}
      constexpr operator int() const { return 0; }
    };
    int k1 = sizeof(short{S1(S1())});

    struct S2 {
      constexpr S2() {}
      constexpr operator int() const { return 123456; }
    };
    int k2 = sizeof(short{S2(S2())}); // expected-error {{cannot be narrowed}} expected-note {{insert an explicit cast to silence this issue}}
  }

  namespace PR12288 {
    template <typename> constexpr bool foo() { return true; }
    template <bool> struct bar {};
    template <typename T> bar<foo<T>()> baz() { return bar<foo<T>()>(); }
    int main() { baz<int>(); }
  }

  namespace PR13423 {
    template<bool, typename> struct enable_if {};
    template<typename T> struct enable_if<true, T> { using type = T; };

    template<typename T> struct F {
      template<typename U>
      static constexpr bool f() { return sizeof(T) < U::size; }

      template<typename U>
      static typename enable_if<f<U>(), void>::type g() {} // expected-note {{disabled by 'enable_if'}}
    };

    struct U { static constexpr int size = 2; };

    void h() { F<char>::g<U>(); }
    void i() { F<int>::g<U>(); } // expected-error {{no matching function}}
  }

  namespace PR14203 {
    struct duration { constexpr duration() {} };

    template <typename>
    void sleep_for() {
      constexpr duration max = duration();
    }
  }
}

namespace NoInstantiationWhenSelectingOverload {
  // Check that we don't instantiate conversion functions when we're checking
  // for the existence of an implicit conversion sequence, only when a function
  // is actually chosen by overload resolution.
  struct S {
    template<typename T> constexpr S(T) : n(T::error) {} // expected-error {{no members}}
    int n;
  };

  int f(S);
  int f(int);

  void g() { f(0); }
  void h() { (void)sizeof(f(0)); }
  void i() { (void)sizeof(f("oops")); } // expected-note {{instantiation of}}
}