Fix std::string assignment ambiguity from braced initializer lists.

When support for `basic_string_view` was added to string it also
added new assignment operators from `basic_string_view`. These caused
ambiguity when assigning from a braced initializer. This patch fixes
that regression by making the basic_string_view assignment operator
rank lower in overload resolution by making it a template.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292276 91177308-0d34-0410-b5e6-96231b3b80d8

include/string

@@ -818,6 +818,7 @@ public:
     operator __self_view() const _NOEXCEPT { return __self_view(data(), size()); }
 
     basic_string& operator=(const basic_string& __str);
+    template <class = void>
     _LIBCPP_INLINE_VISIBILITY
     basic_string& operator=(__self_view __sv)  {return assign(__sv);}
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES

test/std/strings/basic.string/string.cons/brace_assignment.pass.cpp

@@ -0,0 +1,36 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// <string>
+
+// basic_string<charT,traits,Allocator>&
+//   operator=(basic_string<charT,traits,Allocator>&& str);
+
+#include <string>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main()
+{
+  // Test that assignment from {} and {ptr, len} are allowed and are not
+  // ambiguous.
+  {
+    std::string s = "hello world";
+    s = {};
+    assert(s.empty());
+  }
+  {
+    std::string s = "hello world";
+    s = {"abc", 2};
+    assert(s == "ab");
+  }
+}

test/std/strings/basic.string/string.cons/string_view.pass.cpp

@@ -28,12 +28,23 @@ test(std::basic_string_view<charT> sv)
     typedef std::basic_string<charT, std::char_traits<charT>, test_allocator<charT> > S;
     typedef typename S::traits_type T;
     typedef typename S::allocator_type A;
+  {
     S s2(sv);
     LIBCPP_ASSERT(s2.__invariants());
     assert(s2.size() == sv.size());
     assert(T::compare(s2.data(), sv.data(), sv.size()) == 0);
     assert(s2.get_allocator() == A());
     assert(s2.capacity() >= s2.size());
+  }
+  {
+    S s2;
+    s2 = sv;
+    LIBCPP_ASSERT(s2.__invariants());
+    assert(s2.size() == sv.size());
+    assert(T::compare(s2.data(), sv.data(), sv.size()) == 0);
+    assert(s2.get_allocator() == A());
+    assert(s2.capacity() >= s2.size());
+  }
 }
 
 template <class charT, class A>
@@ -42,12 +53,23 @@ test(std::basic_string_view<charT> sv, const A& a)
 {
     typedef std::basic_string<charT, std::char_traits<charT>, A> S;
     typedef typename S::traits_type T;
+  {
     S s2(sv, a);
     LIBCPP_ASSERT(s2.__invariants());
     assert(s2.size() == sv.size());
     assert(T::compare(s2.data(), sv.data(), sv.size()) == 0);
     assert(s2.get_allocator() == a);
     assert(s2.capacity() >= s2.size());
+  }
+  {
+    S s2(a);
+    s2 = sv;
+    LIBCPP_ASSERT(s2.__invariants());
+    assert(s2.size() == sv.size());
+    assert(T::compare(s2.data(), sv.data(), sv.size()) == 0);
+    assert(s2.get_allocator() == a);
+    assert(s2.capacity() >= s2.size());
+  }
 }
 
 int main()