[libc++] Implement P0608R3 - A sane variant converting constructor

Summary:
Prefer user-defined conversions over narrowing conversions and conversions to bool.

References:
 http://wg21.link/p0608

Reviewers: EricWF, mpark, mclow.lists

Reviewed By: mclow.lists

Subscribers: zoecarver, ldionne, libcxx-commits, cfe-commits, christof

Differential Revision: https://reviews.llvm.org/D44865

llvm-svn: 363692

4 files changed, 207 insertions, 3 deletions

diff --git a/libcxx/test/std/utilities/variant/variant.variant/variant.assign/T.pass.cpp b/libcxx/test/std/utilities/variant/variant.variant/variant.assign/T.pass.cpp
index 02498b1acde..b2b53d6c6ea 100644
--- a/libcxx/test/std/utilities/variant/variant.variant/variant.assign/T.pass.cpp
+++ b/libcxx/test/std/utilities/variant/variant.variant/variant.assign/T.pass.cpp
@@ -22,6 +22,7 @@
 #include <string>
 #include <type_traits>
 #include <variant>
+#include <memory>
 
 #include "test_macros.h"
 #include "variant_test_helpers.hpp"
@@ -122,7 +123,7 @@ void test_T_assignment_noexcept() {
 
 void test_T_assignment_sfinae() {
   {
-    using V = std::variant<long, unsigned>;
+    using V = std::variant<long, long long>;
     static_assert(!std::is_assignable<V, int>::value, "ambiguous");
   }
   {
@@ -133,6 +134,31 @@ void test_T_assignment_sfinae() {
     using V = std::variant<std::string, void *>;
     static_assert(!std::is_assignable<V, int>::value, "no matching operator=");
   }
+  {
+    using V = std::variant<std::string, float>;
+    static_assert(!std::is_assignable<V, int>::value, "no matching operator=");
+  }
+  {
+    using V = std::variant<std::unique_ptr<int>, bool>;
+    static_assert(!std::is_assignable<V, std::unique_ptr<char>>::value,
+                  "no explicit bool in operator=");
+    struct X {
+      operator void*();
+    };
+    static_assert(!std::is_assignable<V, X>::value,
+                  "no boolean conversion in operator=");
+    static_assert(!std::is_assignable<V, std::false_type>::value,
+                  "no converted to bool in operator=");
+  }
+  {
+    struct X {};
+    struct Y {
+      operator X();
+    };
+    using V = std::variant<X>;
+    static_assert(std::is_assignable<V, Y>::value,
+                  "regression on user-defined conversions in operator=");
+  }
 #if !defined(TEST_VARIANT_HAS_NO_REFERENCES)
   {
     using V = std::variant<int, int &&>;
@@ -161,6 +187,37 @@ void test_T_assignment_basic() {
     assert(v.index() == 1);
     assert(std::get<1>(v) == 43);
   }
+  {
+    std::variant<unsigned, long> v;
+    v = 42;
+    assert(v.index() == 1);
+    assert(std::get<1>(v) == 42);
+    v = 43u;
+    assert(v.index() == 0);
+    assert(std::get<0>(v) == 43);
+  }
+  {
+    std::variant<std::string, bool> v = true;
+    v = "bar";
+    assert(v.index() == 0);
+    assert(std::get<0>(v) == "bar");
+  }
+  {
+    std::variant<bool, std::unique_ptr<int>> v;
+    v = nullptr;
+    assert(v.index() == 1);
+    assert(std::get<1>(v) == nullptr);
+  }
+  {
+    std::variant<bool volatile, int> v = 42;
+    v = false;
+    assert(v.index() == 0);
+    assert(!std::get<0>(v));
+    bool lvt = true;
+    v = lvt;
+    assert(v.index() == 0);
+    assert(std::get<0>(v));
+  }
 #if !defined(TEST_VARIANT_HAS_NO_REFERENCES)
   {
     using V = std::variant<int &, int &&, long>;
diff --git a/libcxx/test/std/utilities/variant/variant.variant/variant.assign/conv.fail.cpp b/libcxx/test/std/utilities/variant/variant.variant/variant.assign/conv.fail.cpp
new file mode 100644
index 00000000000..d5f370d2720
--- /dev/null
+++ b/libcxx/test/std/utilities/variant/variant.variant/variant.assign/conv.fail.cpp
@@ -0,0 +1,52 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// <variant>
+
+// template <class ...Types> class variant;
+
+// template <class T>
+// variant& operator=(T&&) noexcept(see below);
+
+#include <variant>
+#include <string>
+#include <memory>
+
+int main(int, char**)
+{
+  std::variant<int, int> v1;
+  std::variant<long, long long> v2;
+  std::variant<char> v3;
+  v1 = 1; // expected-error {{no viable overloaded '='}}
+  v2 = 1; // expected-error {{no viable overloaded '='}}
+  v3 = 1; // expected-error {{no viable overloaded '='}}
+
+  std::variant<std::string, float> v4;
+  std::variant<std::string, double> v5;
+  std::variant<std::string, bool> v6;
+  v4 = 1; // expected-error {{no viable overloaded '='}}
+  v5 = 1; // expected-error {{no viable overloaded '='}}
+  v6 = 1; // expected-error {{no viable overloaded '='}}
+
+  std::variant<int, bool> v7;
+  std::variant<int, bool const> v8;
+  std::variant<int, bool volatile> v9;
+  v7 = "meow"; // expected-error {{no viable overloaded '='}}
+  v8 = "meow"; // expected-error {{no viable overloaded '='}}
+  v9 = "meow"; // expected-error {{no viable overloaded '='}}
+
+  std::variant<bool> v10;
+  std::variant<bool> v11;
+  std::variant<bool> v12;
+  v10 = std::true_type(); // expected-error {{no viable overloaded '='}}
+  v11 = std::unique_ptr<char>(); // expected-error {{no viable overloaded '='}}
+  v12 = nullptr; // expected-error {{no viable overloaded '='}}
+}
diff --git a/libcxx/test/std/utilities/variant/variant.variant/variant.ctor/T.pass.cpp b/libcxx/test/std/utilities/variant/variant.variant/variant.ctor/T.pass.cpp
index 73bd2c6283d..40fa20b4f5e 100644
--- a/libcxx/test/std/utilities/variant/variant.variant/variant.ctor/T.pass.cpp
+++ b/libcxx/test/std/utilities/variant/variant.variant/variant.ctor/T.pass.cpp
@@ -20,8 +20,8 @@
 #include <string>
 #include <type_traits>
 #include <variant>
+#include <memory>
 
-#include "test_convertible.hpp"
 #include "test_macros.h"
 #include "variant_test_helpers.hpp"
 
@@ -39,6 +39,8 @@ struct NoThrowT {
 
 struct AnyConstructible { template <typename T> AnyConstructible(T&&) {} };
 struct NoConstructible { NoConstructible() = delete; };
+template <class T>
+struct RValueConvertibleFrom { RValueConvertibleFrom(T&&) {} };
 
 void test_T_ctor_noexcept() {
   {
@@ -53,7 +55,7 @@ void test_T_ctor_noexcept() {
 
 void test_T_ctor_sfinae() {
   {
-    using V = std::variant<long, unsigned>;
+    using V = std::variant<long, long long>;
     static_assert(!std::is_constructible<V, int>::value, "ambiguous");
   }
   {
@@ -66,6 +68,32 @@ void test_T_ctor_sfinae() {
                   "no matching constructor");
   }
   {
+    using V = std::variant<std::string, float>;
+    static_assert(!std::is_constructible<V, int>::value,
+                  "no matching constructor");
+  }
+  {
+    using V = std::variant<std::unique_ptr<int>, bool>;
+    static_assert(!std::is_constructible<V, std::unique_ptr<char>>::value,
+                  "no explicit bool in constructor");
+    struct X {
+      operator void*();
+    };
+    static_assert(!std::is_constructible<V, X>::value,
+                  "no boolean conversion in constructor");
+    static_assert(!std::is_constructible<V, std::false_type>::value,
+                  "no converted to bool in constructor");
+  }
+  {
+    struct X {};
+    struct Y {
+      operator X();
+    };
+    using V = std::variant<X>;
+    static_assert(std::is_constructible<V, Y>::value,
+                  "regression on user-defined conversions in constructor");
+  }
+  {
     using V = std::variant<AnyConstructible, NoConstructible>;
     static_assert(
         !std::is_constructible<V, std::in_place_type_t<NoConstructible>>::value,
@@ -99,6 +127,34 @@ void test_T_ctor_basic() {
     static_assert(v.index() == 1, "");
     static_assert(std::get<1>(v) == 42, "");
   }
+  {
+    constexpr std::variant<unsigned, long> v(42);
+    static_assert(v.index() == 1, "");
+    static_assert(std::get<1>(v) == 42, "");
+  }
+  {
+    std::variant<std::string, bool const> v = "foo";
+    assert(v.index() == 0);
+    assert(std::get<0>(v) == "foo");
+  }
+  {
+    std::variant<bool volatile, std::unique_ptr<int>> v = nullptr;
+    assert(v.index() == 1);
+    assert(std::get<1>(v) == nullptr);
+  }
+  {
+    std::variant<bool volatile const, int> v = true;
+    assert(v.index() == 0);
+    assert(std::get<0>(v));
+  }
+  {
+    std::variant<RValueConvertibleFrom<int>> v1 = 42;
+    assert(v1.index() == 0);
+
+    int x = 42;
+    std::variant<RValueConvertibleFrom<int>, AnyConstructible> v2 = x;
+    assert(v2.index() == 1);
+  }
 #if !defined(TEST_VARIANT_HAS_NO_REFERENCES)
   {
     using V = std::variant<const int &, int &&, long>;
diff --git a/libcxx/test/std/utilities/variant/variant.variant/variant.ctor/conv.fail.cpp b/libcxx/test/std/utilities/variant/variant.variant/variant.ctor/conv.fail.cpp
new file mode 100644
index 00000000000..76b42ac226d
--- /dev/null
+++ b/libcxx/test/std/utilities/variant/variant.variant/variant.ctor/conv.fail.cpp
@@ -0,0 +1,39 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// <variant>
+
+// template <class ...Types> class variant;
+
+// template <class T> constexpr variant(T&&) noexcept(see below);
+
+#include <variant>
+#include <string>
+#include <memory>
+
+int main(int, char**)
+{
+  std::variant<int, int> v1 = 1; // expected-error {{no viable conversion}}
+  std::variant<long, long long> v2 = 1; // expected-error {{no viable conversion}}
+  std::variant<char> v3 = 1; // expected-error {{no viable conversion}}
+
+  std::variant<std::string, float> v4 = 1; // expected-error {{no viable conversion}}
+  std::variant<std::string, double> v5 = 1; // expected-error {{no viable conversion}}
+  std::variant<std::string, bool> v6 = 1; // expected-error {{no viable conversion}}
+
+  std::variant<int, bool> v7 = "meow"; // expected-error {{no viable conversion}}
+  std::variant<int, bool const> v8 = "meow"; // expected-error {{no viable conversion}}
+  std::variant<int, bool volatile> v9 = "meow"; // expected-error {{no viable conversion}}
+
+  std::variant<bool> v10 = std::true_type(); // expected-error {{no viable conversion}}
+  std::variant<bool> v11 = std::unique_ptr<char>(); // expected-error {{no viable conversion}}
+  std::variant<bool> v12 = nullptr; // expected-error {{no viable conversion}}
+}