Fix find of an empty substring at the end of a string_view

include/nonstd/string_view.hpp

@@ -873,7 +873,8 @@ class basic_string_view
     nssv_constexpr14 size_type find( basic_string_view v, size_type pos = 0 ) const nssv_noexcept  // (1)
     {
         return assert( v.size() == 0 || v.data() != nssv_nullptr )
-            , pos >= size()
+            , v.empty() && pos <= size() ? pos
+            : pos >= size()
             ? npos : to_pos(
 #if nssv_CPP11_OR_GREATER && ! nssv_CPP17_OR_GREATER
                 detail::search( substr(pos), v )

test/string-view.t.cpp

@@ -584,6 +584,65 @@ CASE( "string_view: Allows to search for a C-string substring, starting at posit
     EXPECT( sv.find("world", 7 ) == string_view::npos );
 }
 
+CASE( "string_view: Finds empty substrings through size() via find()" )
+{
+    string_view views[] = { string_view(), string_view(""), string_view("abc") };
+
+    for ( size_type i = 0; i < sizeof( views ) / sizeof( views[0] ); ++i )
+    {
+        string_view sv = views[i];
+        size_type positions[] = { 0, sv.size() / 2, sv.size(), sv.size() + 1, string_view::npos };
+
+        for ( size_type j = 0; j < sizeof( positions ) / sizeof( positions[0] ); ++j )
+        {
+            size_type pos = positions[j];
+            size_type expected = pos <= sv.size() ? pos : string_view::npos;
+
+            EXPECT( sv.find( string_view(), pos ) == expected );
+            EXPECT( sv.find( string_view(""), pos ) == expected );
+            EXPECT( sv.find( "", pos ) == expected );
+            EXPECT( sv.find( "ignored", pos, 0 ) == expected );
+        }
+    }
+}
+
+CASE( "string_view: Does not find nonempty substrings at or past size() via find()" )
+{
+    string_view sv("abc");
+
+    EXPECT( sv.find( string_view("c"), sv.size() ) == string_view::npos );
+    EXPECT( sv.find( "c", sv.size() ) == string_view::npos );
+    EXPECT( sv.find( "c", sv.size(), 1 ) == string_view::npos );
+    EXPECT( sv.find( 'c', sv.size() ) == string_view::npos );
+    EXPECT( sv.find( "c", sv.size() + 1 ) == string_view::npos );
+    EXPECT( sv.find( "c", string_view::npos ) == string_view::npos );
+}
+
+CASE( "string_view: Finds empty wide substrings at size() via find()" )
+{
+    nonstd::wstring_view sv(L"abc", 3);
+
+    EXPECT( nonstd::wstring_view().find( nonstd::wstring_view() ) == size_type( 0 ) );
+    EXPECT( sv.find( nonstd::wstring_view(), sv.size() ) == sv.size() );
+    EXPECT( sv.find( L"", sv.size() ) == sv.size() );
+    EXPECT( sv.find( L"ignored", sv.size(), 0 ) == sv.size() );
+    EXPECT( sv.find( L"", sv.size() + 1 ) == nonstd::wstring_view::npos );
+}
+
+CASE( "string_view: Allows to constexpr-find empty substrings via find() (C++14)" )
+{
+#if nssv_STD_SV_OR( nssv_HAVE_CONSTEXPR_14 )
+    static_assert( string_view().find( string_view() ) == 0, "" );
+    static_assert( string_view( "abc" ).find( string_view(), 3 ) == 3, "" );
+    static_assert( string_view( "abc" ).find( "", 3 ) == 3, "" );
+    static_assert( string_view( "abc" ).find( "ignored", 3, 0 ) == 3, "" );
+    static_assert( string_view( "abc" ).find( "", 4 ) == string_view::npos, "" );
+    static_assert( string_view( "abc" ).find( "", string_view::npos ) == string_view::npos, "" );
+#else
+    EXPECT( !!"C++14 constexpr is not available (no C++14)" );
+#endif
+}
+
 CASE( "string_view: Allows to search backwards for a string_view substring, starting at position pos (default: npos) via rfind(), (1)" )
 {
     char hello[] = "hello world";