flat_map: Readd constructors and assignment operator overloads.

https://chromium-review.googlesource.com/c/chromium/src/+/705955 ("base::flat_map
was missing defaults for duplication handling") removed them in favor of
inheriting them from flat_tree.

This can cause problems with GCC due to
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84782, and the build has been
failing with it since
https://chromium-review.googlesource.com/1000393 ("Mojo C++ Bindings:
unordered_map => flat_map") with a message like this:

    gen/mojo/public/interfaces/bindings/tests/test_unions.mojom.cc: In member function ‘void mojo::test::ObjectUnion::set_f_map_int8(const base::flat_map<std::__cxx11::basic_string<char>, signed char>&)’:
    gen/mojo/public/interfaces/bindings/tests/test_unions.mojom.cc:455:30: error: use of deleted function ‘base::flat_map<std::__cxx11::basic_string<char>, signed char>::flat_map(const base::flat_map<std::__cxx11::basic_string<char>, signed char>&)’
             std::move(f_map_int8));
                                  ^
    In file included from ../../mojo/public/cpp/bindings/clone_traits.h:11:0,
                     from gen/mojo/public/interfaces/bindings/tests/test_unions.mojom.h:22,
                     from gen/mojo/public/interfaces/bindings/tests/test_unions.mojom.cc:15:
    ../../base/containers/flat_map.h:152:7: note: ‘base::flat_map<std::__cxx11::basic_string<char>, signed char>::flat_map(const base::flat_map<std::__cxx11::basic_string<char>, signed char>&)’ is implicitly declared as deleted because ‘base::flat_map<std::__cxx11::basic_string<char>, signed char>’ declares a move constructor or move assignment operator
     class flat_map : public ::base::internal::flat_tree<
           ^~~~~~~~

Work around it by replacing the inheritance statements with actual
constructors and operator=() overloads again. This change does not
completely revert https://chromium-review.googlesource.com/1000393, as the
constructors maintain their new signature (so that they look like
flat_tree's).

Bug: 819294, 837221
Change-Id: I8f37a2c8aa269b3c1025cdc009518a7709e95947
Reviewed-on: https://chromium-review.googlesource.com/1038103
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556168}

base/containers/flat_map.h

@@ -171,14 +171,37 @@ class flat_map : public ::base::internal::flat_tree<
   // --------------------------------------------------------------------------
   // Lifetime and assignments.
   //
-  // Note: we explicitly bring operator= in because otherwise
-  //   flat_map<...> x;
-  //   x = {...};
-  // Would first create a flat_map and then move assign it. This most likely
-  // would be optimized away but still affects our debug builds.
+  // Note: we could do away with these constructors, destructor and assignment
+  // operator overloads by inheriting |tree|'s, but this breaks the GCC build
+  // due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84782 (see
+  // https://crbug.com/837221).
+
+  flat_map() = default;
+  explicit flat_map(const Compare& comp);
+
+  template <class InputIterator>
+  flat_map(InputIterator first,
+           InputIterator last,
+           FlatContainerDupes dupe_handling = KEEP_FIRST_OF_DUPES,
+           const Compare& comp = Compare());
+
+  flat_map(const flat_map&) = default;
+  flat_map(flat_map&&) noexcept = default;
+
+  flat_map(std::vector<value_type> items,
+           FlatContainerDupes dupe_handling = KEEP_FIRST_OF_DUPES,
+           const Compare& comp = Compare());
+
+  flat_map(std::initializer_list<value_type> ilist,
+           FlatContainerDupes dupe_handling = KEEP_FIRST_OF_DUPES,
+           const Compare& comp = Compare());
 
-  using tree::tree;
-  using tree::operator=;
+  ~flat_map() = default;
+
+  flat_map& operator=(const flat_map&) = default;
+  flat_map& operator=(flat_map&&) = default;
+  // Takes the first if there are duplicates in the initializer list.
+  flat_map& operator=(std::initializer_list<value_type> ilist);
 
   // --------------------------------------------------------------------------
   // Map-specific insert operations.
@@ -215,6 +238,49 @@ class flat_map : public ::base::internal::flat_tree<
   friend void swap(flat_map& lhs, flat_map& rhs) noexcept { lhs.swap(rhs); }
 };
 
+// ----------------------------------------------------------------------------
+// Lifetime.
+
+template <class Key, class Mapped, class Compare>
+flat_map<Key, Mapped, Compare>::flat_map(const Compare& comp) : tree(comp) {}
+
+template <class Key, class Mapped, class Compare>
+template <class InputIterator>
+flat_map<Key, Mapped, Compare>::flat_map(InputIterator first,
+                                         InputIterator last,
+                                         FlatContainerDupes dupe_handling,
+                                         const Compare& comp)
+    : tree(first, last, dupe_handling, comp) {}
+
+template <class Key, class Mapped, class Compare>
+flat_map<Key, Mapped, Compare>::flat_map(std::vector<value_type> items,
+                                         FlatContainerDupes dupe_handling,
+                                         const Compare& comp)
+    : tree(std::move(items), dupe_handling, comp) {}
+
+template <class Key, class Mapped, class Compare>
+flat_map<Key, Mapped, Compare>::flat_map(
+    std::initializer_list<value_type> ilist,
+    FlatContainerDupes dupe_handling,
+    const Compare& comp)
+    : flat_map(std::begin(ilist), std::end(ilist), dupe_handling, comp) {}
+
+// ----------------------------------------------------------------------------
+// Assignments.
+
+template <class Key, class Mapped, class Compare>
+auto flat_map<Key, Mapped, Compare>::operator=(
+    std::initializer_list<value_type> ilist) -> flat_map& {
+  // When https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84782 gets fixed, we
+  // need to remember to inherit tree::operator= to prevent
+  //   flat_map<...> x;
+  //   x = {...};
+  // from first creating a flat_map and then move assigning it. This most
+  // likely would be optimized away but still affects our debug builds.
+  tree::operator=(ilist);
+  return *this;
+}
+
 // ----------------------------------------------------------------------------
 // Insert operations.