Add reproducer for Clang < 15 bug with string_view & C++11 char types

Author: Flamefire

test/sv_conversion_test.cpp

@@ -29,6 +29,27 @@ boost::core::string_view f( boost::core::string_view const& str )
     return str;
 }
 
+template<typename Char>
+boost::core::basic_string_view<Char> f( boost::core::basic_string_view<Char> const& str )
+{
+    return str;
+}
+
+template<typename Char>
+void subtest()
+{
+    std::basic_string<Char> s1( 3, Char('1') );
+    s1[1] = Char('2');
+    s1[2] = Char('3');
+
+    std::basic_string<Char> s2 = f<Char>( s1 );
+#if defined(BOOST_CLANG) && (BOOST_CLANG_VERSION >= 150000)
+    // leads to undefined symbols in Clang < 15: https://github.com/llvm/llvm-project/issues/55560
+    std::basic_string<Char> s2( (f<Char>( s1 )) );
+#endif
+    BOOST_TEST( s1 == s2 );
+}
+
 int main()
 {
     {
@@ -62,5 +83,15 @@ int main()
 
 #endif
 
+#if !defined(BOOST_NO_CXX11_CHAR16_T)
+    subtest<char16_t>();
+#endif
+#if !defined(BOOST_NO_CXX11_CHAR32_T)
+    subtest<char32_t>();
+#endif
+#if defined(__cpp_char8_t) && __cpp_char8_t >= 201811L
+    subtest<char8_t>();
+#endif
+
     return boost::report_errors();
 }