dcl.spec/dcl.constexpr/p4.cpp

// RUN: %clang_cc1 -verify -std=c++11 -fcxx-exceptions -Werror=c++14-extensions -Werror=c++20-extensions %s
// RUN: %clang_cc1 -verify -std=c++14 -fcxx-exceptions -DCXX14 -Werror=c++20-extensions %s
// RUN: %clang_cc1 -verify -std=c++20 -fcxx-exceptions -DCXX14 -DCXX2A %s

namespace N {
  typedef char C;
}

namespace M {
  typedef double D;
}

struct NonLiteral { // expected-note 2{{no constexpr constructors}}
  NonLiteral() {}
  NonLiteral(int) {}
};
struct Literal {
  constexpr Literal() {}
  explicit Literal(int); // expected-note 2 {{here}}
  operator int() const { return 0; }
};

// In the definition of a constexpr constructor, each of the parameter types
// shall be a literal type.
struct S {
  constexpr S(int, N::C) {}
  constexpr S(int, NonLiteral, N::C) {} // expected-error {{constexpr constructor's 2nd parameter type 'NonLiteral' is not a literal type}}
  constexpr S(int, NonLiteral = 42) {} // expected-error {{constexpr constructor's 2nd parameter type 'NonLiteral' is not a literal type}}

  // In addition, either its function-body shall be = delete or = default
  constexpr S() = default;
  constexpr S(Literal) = delete;
};

// or it shall satisfy the following constraints:

// - the class shall not have any virtual base classes;
struct T : virtual S { // expected-note {{here}}
  constexpr T() {} // expected-error {{constexpr constructor not allowed in struct with virtual base class}}
};
namespace IndirectVBase {
  struct A {};
  struct B : virtual A {}; // expected-note {{here}}
  class C : public B {
  public:
    constexpr C() {} // expected-error {{constexpr constructor not allowed in class with virtual base class}}
  };
}

// - its function-body shall not be a function-try-block;
struct U {
  constexpr U()
    try
#ifndef CXX2A
  // expected-error@-2 {{function try block in constexpr constructor is a C++20 extension}}
#endif
    : u() {
#ifndef CXX14
  // expected-error@-2 {{use of this statement in a constexpr constructor is a C++14 extension}}
#endif
  } catch (...) {
    throw;
  }
  int u;
};

// - the compound-statememt of its function-body shall contain only
struct V {
  constexpr V() {
    //  - null statements,
    ;

    //  - static_assert-declarations,
    static_assert(true, "the impossible happened!");

    //  - typedef declarations and alias-declarations that do not define classes
    //    or enumerations,
    typedef int I;
    typedef struct S T;
    using J = int;
    using K = int[sizeof(I) + sizeof(J)];
    // Note, the standard requires we reject this.
    struct U;

    //  - using-declarations,
    using N::C;

    //  - and using-directives;
    using namespace N;
  }

  constexpr V(int(&)[1]) {
    for (int n = 0; n < 10; ++n)
      /**/;
#ifndef CXX14
    // expected-error@-3 {{statement not allowed in constexpr constructor}}
#endif
  }
  constexpr V(int(&)[2]) {
    constexpr int a = 0;
#ifndef CXX14
    // expected-error@-2 {{variable declaration in a constexpr constructor is a C++14 extension}}
#endif
  }
  constexpr V(int(&)[3]) {
    constexpr int ForwardDecl(int);
#ifndef CXX14
    // expected-error@-2 {{use of this statement in a constexpr constructor is a C++14 extension}}
#endif
  }
  constexpr V(int(&)[4]) {
    typedef struct { } S1;
#ifndef CXX14
    // expected-error@-2 {{type definition in a constexpr constructor is a C++14 extension}}
#endif
  }
  constexpr V(int(&)[5]) {
    using S2 = struct { };
#ifndef CXX14
    // expected-error@-2 {{type definition in a constexpr constructor is a C++14 extension}}
#endif
  }
  constexpr V(int(&)[6]) {
    struct S3 { };
#ifndef CXX14
    // expected-error@-2 {{type definition in a constexpr constructor is a C++14 extension}}
#endif
  }
  constexpr V(int(&)[7]) {
    return;
#ifndef CXX14
    // expected-error@-2 {{use of this statement in a constexpr constructor is a C++14 extension}}
#endif
  }
};

// - every non-static data member and base class sub-object shall be initialized
struct W {
  int n;
  constexpr W() {}
#ifndef CXX2A
  // expected-error@-2 {{constexpr constructor that does not initialize all members}}
  // expected-note@-4 {{member not initialized by constructor}}
#endif
};
struct AnonMembers {
  int a; // expected-note 0-1{{member not initialized by constructor}}
  union { // expected-note 0-2{{member not initialized by constructor}}
    char b;
    struct {
      double c;
      long d; // expected-note 0-1{{member not initialized by constructor}}
    };
    union {
      char e;
      void *f;
    };
  };
  struct { // expected-note 0-1{{member not initialized by constructor}}
    long long g;
    struct {
      int h; // expected-note 0-1{{member not initialized by constructor}}
      double i; // expected-note 0-1{{member not initialized by constructor}}
    };
    union { // expected-note 0-2{{member not initialized by constructor}}
      char *j;
      AnonMembers *k;
    };
  };

  constexpr AnonMembers(int(&)[1]) : a(), b(), g(), h(), i(), j() {} // ok
  // missing d, i, j/k union
  constexpr AnonMembers(int(&)[2]) : a(), c(), g(), h() {}
#ifndef CXX2A
  // expected-error@-2 {{constexpr constructor that does not initialize all members}}
#endif
  constexpr AnonMembers(int(&)[3]) : a(), e(), g(), h(), i(), k() {} // ok
  // missing h, j/k union
  constexpr AnonMembers(int(&)[4]) : a(), c(), d(), g(), i() {}
#ifndef CXX2A
  // expected-error@-2 {{constexpr constructor that does not initialize all members}}
#endif
  // missing b/c/d/e/f union
  constexpr AnonMembers(int(&)[5]) : a(), g(), h(), i(), k() {}
#ifndef CXX2A
  // expected-error@-2 {{constexpr constructor that does not initialize all members}}
#endif
  // missing a, b/c/d/e/f union, g/h/i/j/k struct
  constexpr AnonMembers(int(&)[6]) {}
#ifndef CXX2A
  // expected-error@-2 {{constexpr constructor that does not initialize all members}}
#endif
};

union Empty {
  constexpr Empty() {} // ok
} constexpr empty1;

struct EmptyVariant {
  union {}; // expected-warning {{does not declare anything}}
  struct {}; // expected-warning {{does not declare anything}}
  constexpr EmptyVariant() {} // ok
} constexpr empty2;

template<typename T> using Int = int;
template<typename T>
struct TemplateInit {
  T a;
  int b; // desired-note {{not initialized}}
  Int<T> c; // desired-note {{not initialized}}
  struct {
    T d;
    int e; // desired-note {{not initialized}}
    Int<T> f; // desired-note {{not initialized}}
  };
  struct {
    Literal l;
    Literal m;
    Literal n[3];
  };
  union { // desired-note {{not initialized}}
    T g;
    T h;
  };
  // FIXME: This is ill-formed (no diagnostic required). We should diagnose it.
  constexpr TemplateInit() {} // desired-error {{must initialize all members}}
};
template<typename T> struct TemplateInit2 {
  Literal l;
  constexpr TemplateInit2() {} // ok
};

template<typename T> struct weak_ptr {
  constexpr weak_ptr() : p(0) {}
  T *p;
};
template<typename T> struct enable_shared_from_this {
  weak_ptr<T> weak_this;
  constexpr enable_shared_from_this() {} // ok
};
constexpr int f(enable_shared_from_this<int>);

// - every constructor involved in initializing non-static data members and base
//   class sub-objects shall be a constexpr constructor.
// This will no longer be the case once we support P2448R2
struct ConstexprBaseMemberCtors : Literal {
  Literal l;

  constexpr ConstexprBaseMemberCtors() : Literal(), l() {} // ok
  constexpr ConstexprBaseMemberCtors(char) : // expected-error {{constexpr constructor never produces a constant expression}}
    Literal(0), // expected-note {{non-constexpr constructor}}
    l() {}
  constexpr ConstexprBaseMemberCtors(double) : Literal(), // expected-error {{constexpr constructor never produces a constant expression}}
    l(0) // expected-note {{non-constexpr constructor}}
  {}
};

// - every assignment-expression that is an initializer-clause appearing
//   directly or indirectly within a brace-or-equal-initializer for a non-static
//   data member that is not named by a mem-initializer-id shall be a constant
//   expression; and
//
// Note, we deliberately do not implement this bullet, so that we can allow the
// following example. (See N3308).
struct X {
  int a = 0;
  int b = 2 * a + 1; // ok, not a constant expression.

  constexpr X() {}
  constexpr X(int c) : a(c) {} // ok, b initialized by 2 * c + 1
};

union XU1 { int a; constexpr XU1() = default; };
#ifndef CXX2A
// expected-error@-2{{cannot be marked constexpr}}
#endif
union XU2 { int a = 1; constexpr XU2() = default; };

struct XU3 {
  union {
    int a;
  };
  constexpr XU3() = default;
#ifndef CXX2A
  // expected-error@-2{{cannot be marked constexpr}}
#endif
};
struct XU4 {
  union {
    int a = 1;
  };
  constexpr XU4() = default;
};

static_assert(XU2().a == 1, "");
static_assert(XU4().a == 1, "");

//  - every implicit conversion used in converting a constructor argument to the
//    corresponding parameter type and converting a full-expression to the
//    corresponding member type shall be one of those allowed in a constant
//    expression.
//
// We implement the proposed resolution of DR1364 and ignore this bullet.
// However, we implement the intent of this wording as part of the p5 check that
// the function must be able to produce a constant expression.
int kGlobal; // expected-note {{here}}
struct Z {
  constexpr Z(int a) : n(a) {}
  constexpr Z() : n(kGlobal) {} // expected-error {{constexpr constructor never produces a constant expression}} expected-note {{read of non-const}}
  int n;
};


namespace StdExample {
  struct Length {
    explicit constexpr Length(int i = 0) : val(i) { }
  private:
      int val;
  };
}

namespace CtorLookup {
  // Ensure that we look up which constructor will actually be used.
  struct A {
    constexpr A(const A&) {}
    A(A&) {}
    constexpr A(int = 0);
  };

  struct B : A {
    B() = default;
    constexpr B(const B&);
    constexpr B(B&);
  };
  constexpr B::B(const B&) = default;
  constexpr B::B(B&) = default; // expected-error {{cannot be marked constexpr}}

  struct C {
    A a;
    C() = default;
    constexpr C(const C&);
    constexpr C(C&);
  };
  constexpr C::C(const C&) = default;
  constexpr C::C(C&) = default; // expected-error {{cannot be marked constexpr}}
}

namespace PR14503 {
  template<typename> struct V {
    union {
      int n;
      struct {
        int x,
            y; // expected-note {{subobject declared here}}
      };
    };
    constexpr V() : x(0) {}
  };

  // The constructor is still 'constexpr' here, but the result is not intended
  // to be a constant expression. The standard is not clear on how this should
  // work.
  constexpr V<int> v; // expected-error {{constant expression}} expected-note {{subobject 'y' is not initialized}}

  constexpr int k = V<int>().x; // FIXME: ok?
}