libstdc++: Fix outdated comment about std::string instantiations (PR 94854)

	PR libstdc++/94854
	* include/bits/basic_string.tcc: Update comment about explicit
	instantiations.

libstdc++-v3/ChangeLog

+2020-04-29  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/94854
+	* include/bits/basic_string.tcc: Update comment about explicit
+	instantiations.
+
 2020-04-28  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/91480

libstdc++-v3/include/bits/basic_string.tcc

@@ -1609,11 +1609,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // Inhibit implicit instantiations for required instantiations,
   // which are defined via explicit instantiations elsewhere.
 #if _GLIBCXX_EXTERN_TEMPLATE
-  // The explicit instantiations definitions in src/c++11/string-inst.cc
-  // are compiled as C++14, so the new C++17 members aren't instantiated.
-  // Until those definitions are compiled as C++17 suppress the declaration,
-  // so C++17 code will implicitly instantiate std::string and std::wstring
-  // as needed.
+  // The explicit instantiation definitions in src/c++11/string-inst.cc and
+  // src/c++17/string-inst.cc only instantiate the members required for C++17
+  // and earlier standards (so not C++20's starts_with and ends_with).
+  // Suppress the explicit instantiation declarations for C++20, so C++20
+  // code will implicitly instantiate std::string and std::wstring as needed.
 # if __cplusplus <= 201703L && _GLIBCXX_EXTERN_TEMPLATE > 0
   extern template class basic_string<char>;
 # elif ! _GLIBCXX_USE_CXX11_ABI