title: Constexpr string_view comparison
body: <p>I have <a href="https://godbolt.org/#g:!((g:!((g:!((h:codeEditor,i:(j:1,lang:c%2B%2B,source:&#39;%23include+%3Ciostream%3E%0A%23include+%3Cstring_view%3E%0A%0Aint+main(int+argc,+char+**argv)+%7B%0A%09const+constexpr+auto+a+%3D+%22z%22%3B%0A%09const+constexpr+std::string_view+test(%22z%22,1)%3B%0A%09const+constexpr+std::string_view+test2(a,1)%3B%0A%09if+constexpr(test+%3D%3D+test2)+%7B%0A%09%09return+5%3B%0A%09%7D%0A++++else%7B%0A%09++++return+2%3B%0A++++%7D%0A%7D&#39;),l:&#39;5&#39;,n:&#39;0&#39;,o:&#39;C%2B%2B+source+%231&#39;,t:&#39;0&#39;)),k:53.99056071826385,l:&#39;4&#39;,n:&#39;0&#39;,o:&#39;&#39;,s:0,t:&#39;0&#39;),(g:!((h:compiler,i:(compiler:g73,filters:(b:&#39;0&#39;,binary:&#39;1&#39;,commentOnly:&#39;0&#39;,demangle:&#39;0&#39;,directives:&#39;0&#39;,execute:&#39;1&#39;,intel:&#39;0&#39;,trim:&#39;0&#39;),lang:c%2B%2B,libs:!(),options:&#39;-std%3Dc%2B%2B17+-O2+&#39;,source:1),l:&#39;5&#39;,n:&#39;0&#39;,o:&#39;x86-64+gcc+7.3+(Editor+%231,+Compiler+%231)+C%2B%2B&#39;,t:&#39;0&#39;)),k:22.26856861236029,l:&#39;4&#39;,n:&#39;0&#39;,o:&#39;&#39;,s:0,t:&#39;0&#39;),(g:!((h:compiler,i:(compiler:cl19_64,filters:(b:&#39;0&#39;,binary:&#39;1&#39;,commentOnly:&#39;0&#39;,demangle:&#39;0&#39;,directives:&#39;0&#39;,execute:&#39;1&#39;,intel:&#39;0&#39;,trim:&#39;0&#39;),lang:c%2B%2B,libs:!(),options:&#39;/std:c%2B%2Blatest&#39;,source:1),l:&#39;5&#39;,n:&#39;0&#39;,o:&#39;x86-64+MSVC+19+2017+RTW+(Editor+%231,+Compiler+%232)+C%2B%2B&#39;,t:&#39;0&#39;)),k:23.74087066937585,l:&#39;4&#39;,n:&#39;0&#39;,o:&#39;&#39;,s:0,t:&#39;0&#39;)),l:&#39;2&#39;,n:&#39;0&#39;,o:&#39;&#39;,t:&#39;0&#39;)),version:4" rel="nofollow noreferrer">a small program</a> that compiles on <code>GCC</code> but not on <code>MSVC</code>which compiler isn't following the standard for <code>constexpr string_view</code> comparison?</p>
  
  <pre><code>#include &lt;iostream&gt;
  #include &lt;string_view&gt;
  
  int main(int argc, char **argv) {
      const constexpr auto a = "z";
      const constexpr std::string_view test("z",1);
      const constexpr std::string_view test2(a,1);
      if constexpr(test == test2) {
          return 5;
      }
      else{
          return 2;
      }
  }
  </code></pre>