test/SemaCXX/cxx1z-constexpr-lambdas.cpp

// RUN: %clang_cc1 -std=c++1z -verify -fsyntax-only -fblocks %s -fcxx-exceptions
// RUN: %clang_cc1 -std=c++20 -verify -fsyntax-only -fblocks %s -fcxx-exceptions
// RUN: %clang_cc1 -std=c++1z -verify -fsyntax-only -fblocks -fdelayed-template-parsing %s -fcxx-exceptions
// RUN: %clang_cc1 -std=c++14 -verify -fsyntax-only -fblocks %s -DCPP14_AND_EARLIER -fcxx-exceptions


namespace test_lambda_is_literal {
#ifdef CPP14_AND_EARLIER
//expected-error@+4{{not a literal type}}
//expected-note@+2{{lambda closure types are non-literal types before C++17}}
#endif
auto L = [] { };
constexpr int foo(decltype(L) l) { return 0; }

}

#ifndef CPP14_AND_EARLIER
namespace test_constexpr_checking {

namespace ns1 {
  struct NonLit { ~NonLit(); };  //expected-note{{not literal}}
  auto L = [](NonLit NL) constexpr { }; //expected-error{{not a literal type}}
} // end ns1

namespace ns2 {
  auto L = [](int I) constexpr { if (I == 5) asm("non-constexpr");  };
#if __cpp_constexpr < 201907L
  //expected-warning@-2{{use of this statement in a constexpr function is a C++20 extension}}
#endif
} // end ns1

// This is not constexpr until C++20, as the requirements on constexpr
// functions don't permit try-catch blocks.
#if __cplusplus <= 201703L
// expected-error@#try-catch {{constant expression}}
// expected-note@#try-catch {{non-constexpr function 'operator()'}}
// expected-note@#try-catch {{declared here}}
#endif
constexpr int try_catch = [] { // #try-catch
  try { return 0; } catch (...) { return 1; }
}();

// These lambdas have constexpr operator() even though they can never produce a
// constant expression.
auto never_constant_1 = [] { // expected-note {{here}}
  volatile int n = 0;
  return n;
};
auto never_constant_2 = [] () -> int { // expected-note {{here}}
};
struct test_never_constant {
  #if __cplusplus >= 201703L
  // expected-error@+3 {{non-constexpr declaration of 'operator()' follows constexpr declaration}}
  // expected-error@+3 {{non-constexpr declaration of 'operator()' follows constexpr declaration}}
  #endif
  friend auto decltype(never_constant_1)::operator()() const;
  friend int decltype(never_constant_2)::operator()() const;
};

} // end ns test_constexpr_checking

namespace test_constexpr_call {

namespace ns1 {
  auto L = [](int I) { return I; };
  static_assert(L(3) == 3);
} // end ns1
namespace ns2 {
  auto L = [](auto a) { return a; };
  static_assert(L(3) == 3);
  static_assert(L(3.14) == 3.14);
}
namespace ns3 {
  auto L = [](auto a) { asm("non-constexpr"); return a; };
  constexpr int I =  //expected-error{{must be initialized by a constant expression}}
      L(3);
#if __cpp_constexpr < 201907L
//expected-note@-2{{non-constexpr function}}
//expected-note@-5{{declared here}}
#else
//expected-note@-7{{subexpression not valid in a constant expression}}
//expected-note@-6{{in call to}}
#endif
}

} // end ns test_constexpr_call

namespace test_captureless_lambda {
void f() {
  const char c = 'c';
  auto L = [] { return c; };
  constexpr char C = L();
}

void f(char c) { //expected-note{{declared here}}
  auto L = [] { return c; }; //expected-error{{cannot be implicitly captured}} expected-note{{lambda expression begins here}}
  int I = L();
}

}

namespace test_conversion_function_for_non_capturing_lambdas {

namespace ns1 {
auto L = [](int i) { return i; };
constexpr int (*fpi)(int) = L;
static_assert(fpi(3) == 3);
auto GL = [](auto a) { return a; };

constexpr char (*fp2)(char) = GL;
constexpr double (*fp3)(double) = GL;
constexpr const char* (*fp4)(const char*) = GL;
static_assert(fp2('3') == '3');
static_assert(fp3(3.14) == 3.14);
constexpr const char *Str = "abc";
static_assert(fp4(Str) == Str);

auto NCL = [](int i) { static int j; return j; }; //expected-note{{declared here}}
constexpr int (*fp5)(int) = NCL;
constexpr int I =  //expected-error{{must be initialized by a constant expression}}
                  fp5(5); //expected-note{{non-constexpr function}}

namespace test_dont_always_instantiate_constexpr_templates {

auto explicit_return_type = [](auto x) -> int { return x.get(); };
decltype(explicit_return_type(0)) c;  // OK

auto deduced_return_type = [](auto x) { return x.get(); }; //expected-error{{not a structure or union}}
decltype(deduced_return_type(0)) d;  //expected-note{{requested here}}


} // end ns test_dont_always_instantiate_constexpr_templates
} // end ns1

} // end ns test_conversion_function_for_non_capturing_lambdas

namespace test_lambda_is_cce {
namespace ns1_simple_lambda {

namespace ns0 {
constexpr int I = [](auto a) { return a; }(10);

static_assert(I == 10);
static_assert(10 == [](auto a) { return a; }(10));
static_assert(3.14 == [](auto a) { return a; }(3.14));

} //end ns0

namespace ns1 {
constexpr auto f(int i) {
  double d = 3.14;
  auto L = [=](auto a) {
    int Isz = sizeof(i);
    return sizeof(i) + sizeof(a) + sizeof(d);
  };
  int I = L("abc") + L(nullptr);
  return L;
}
constexpr auto L = f(3);
constexpr auto M =  L("abc") + L(nullptr);

static_assert(M == sizeof(int) * 2 + sizeof(double) * 2 + sizeof(nullptr) + sizeof(const char*));

} // end ns1

namespace ns2 {
constexpr auto f(int i) {
  auto L = [](auto a) { return a + a; };
  return L;
}
constexpr auto L = f(3);
constexpr int I = L(6);
static_assert(I == 12);
} // end ns2

namespace contained_lambdas_call_operator_is_not_constexpr {
constexpr auto f(int i) {
  double d = 3.14;
  auto L = [=](auto a) {
    int Isz = sizeof(i);
    asm("hello");
    return sizeof(i) + sizeof(a) + sizeof(d);
  };
  return L;
}

constexpr auto L = f(3);

constexpr auto M =  // expected-error{{must be initialized by}}
    L("abc");
#if __cpp_constexpr < 201907L
//expected-note@-2{{non-constexpr function}}
//expected-note@-14{{declared here}}
#else
//expected-note@-14{{subexpression not valid in a constant expression}}
//expected-note@-6{{in call to}}
#endif
} // end ns contained_lambdas_call_operator_is_not_constexpr


} // end ns1_simple_lambda

namespace test_captures_1 {
namespace ns1 {
constexpr auto f(int i) {
  struct S { int x; } s = { i * 2 };
  auto L = [=](auto a) {
    return i + s.x + a;
  };
  return L;
}
constexpr auto M = f(3);

static_assert(M(10) == 19);

} // end test_captures_1::ns1

namespace ns2 {

constexpr auto foo(int n) {
  auto L = [i = n] (auto N) mutable {
    if (!N(i)) throw "error";
    return [&i] {
      return ++i;
    };
  };
  auto M = L([n](int p) { return p == n; });
  M(); M();
  L([n](int p) { return p == n + 2; });

  return L;
}

constexpr auto L = foo(3);

} // end test_captures_1::ns2
namespace ns3 {

constexpr auto foo(int n) {
  auto L = [i = n] (auto N) mutable {
    if (!N(i)) throw "error";
    return [&i] {
      return [i]() mutable {
        return ++i;
      };
    };
  };
  auto M = L([n](int p) { return p == n; });
  M()(); M()();
  L([n](int p) { return p == n; });

  return L;
}

constexpr auto L = foo(3);
} // end test_captures_1::ns3

namespace ns2_capture_this_byval {
struct S {
  int s;
  constexpr S(int s) : s{s} { }
  constexpr auto f(S o) {
    return [*this,o] (auto a) { return s + o.s + a.s; };
  }
};

constexpr auto L = S{5}.f(S{10});
static_assert(L(S{100}) == 115);
} // end test_captures_1::ns2_capture_this_byval

namespace ns2_capture_this_byref {

struct S {
  int s;
  constexpr S(int s) : s{s} { }
  constexpr auto f() const {
    return [this] { return s; };
  }
};

constexpr S SObj{5};
constexpr auto L = SObj.f();
constexpr int I = L();
static_assert(I == 5);

} // end ns2_capture_this_byref

} // end test_captures_1

namespace test_capture_array {
namespace ns1 {
constexpr auto f(int I) {
  int arr[] = { I, I *2, I * 3 };
  auto L1 = [&] (auto a) { return arr[a]; };
  int r = L1(2);
  struct X { int x, y; };
  return [=](auto a) { return X{arr[a],r}; };
}
constexpr auto L = f(3);
static_assert(L(0).x == 3);
static_assert(L(0).y == 9);
static_assert(L(1).x == 6);
static_assert(L(1).y == 9);
} // end ns1

} // end test_capture_array
namespace ns1_test_lvalue_type {
  void f() {
    volatile int n;
    constexpr bool B = [&]{ return &n; }() == &n; // should be accepted
  }
} // end ns1_unimplemented

} // end ns test_lambda_is_cce

namespace PR36054 {
constexpr int fn() {
  int Capture = 42;
  return [=]() constexpr { return Capture; }();
}

static_assert(fn() == 42, "");

template <class T>
constexpr int tfn() {
  int Capture = 42;
  return [=]() constexpr { return Capture; }();
}

static_assert(tfn<int>() == 42, "");

constexpr int gfn() {
  int Capture = 42;
  return [=](auto P) constexpr { return Capture + P; }(58);
}

static_assert(gfn() == 100, "");

constexpr bool OtherCaptures() {
  int Capture = 42;
  constexpr auto Outer = [](auto P) constexpr { return 42 + P; };
  auto Inner = [&](auto O) constexpr { return O(58) + Capture; };
  return Inner(Outer) == 142;
}

static_assert(OtherCaptures(), "");
} // namespace PR36054

#endif // ndef CPP14_AND_EARLIER