[gcc/devel/gccgo] libstdc++: Fix outdated comment about std::string instantiations (PR 94854)

[Ian Lance Taylor <ian@gcc.gnu.org>]

https://gcc.gnu.org/g:8f1591763fd50b143af0dc1770741f326a97583a

commit 8f1591763fd50b143af0dc1770741f326a97583a
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Apr 29 18:57:34 2020 +0100

    libstdc++: Fix outdated comment about std::string instantiations (PR 94854)
    
            PR libstdc++/94854
            * include/bits/basic_string.tcc: Update comment about explicit
            instantiations.

Diff:
---
 libstdc++-v3/ChangeLog                     |  6 ++++++
 libstdc++-v3/include/bits/basic_string.tcc | 10 +++++-----
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index f8cf71e057f..2032e90abeb 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,9 @@
+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
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index e78bb2c14b3..e370f439390 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/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