Fix type_info's constructor by making it explicit again.

In recent changes type_info's private constructor was
accidentally made implicit. This patch fixes that.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292294 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/language.support/support.rtti/type.info/type_info.pass.cpp

@@ -10,11 +10,16 @@
 // test type_info
 
 #include <typeinfo>
+#include <string>
 #include <cstring>
 #include <cassert>
 
+bool test_constructor_explicit(std::type_info const&) { return false; }
+bool test_constructor_explicit(std::string const&) { return true; }
+
 int main()
 {
+  {
     const std::type_info& t1 = typeid(int);
     const std::type_info& t2 = typeid(int);
     assert(t1 == t2);
@@ -23,4 +28,13 @@ int main()
     assert(!t1.before(t2));
     assert(strcmp(t1.name(), t2.name()) == 0);
     assert(strcmp(t1.name(), t3.name()) != 0);
+  }
+  {
+    // type_info has a protected constructor taking a string literal. This
+    // constructor is not intended for users. However it still participates
+    // in overload resolution, so we need to ensure that it is marked explicit
+    // to avoid ambiguous conversions.
+    // See: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=216201
+    assert(test_constructor_explicit("abc"));
+  }
 }