[PATCH] libstdc++: Test for tzdata.zi before fallback version files.

[Iain Sandoe via Gcc-patches]

I tested this (along with the other posted patches for zoneinfo) using
an installation built from the 2022g release on x86_64-darwin21.  Wider
testing will follow.  If we are in an installation without the tzdata.zi
then although we'll report the version OK, the available functionality
will be basic.

OK for trunk?
Iain

--- 8< ---

Several systems/distributions do not provide the raw tzdata.zi file in
their zoneinfo installation.  However, we might provide an alternate
installation path at configure time, so that we should check for the
tzdata.zi file first and then fall back to system-specific files like
+VERSION etc. on those systems.

Signed-off-by: Iain Sandoe <i...@sandoe.co.uk>

libstdc++-v3/ChangeLog:

        * src/c++20/tzdb.cc (remote_version): Look for the tzdata.zi
        file before falling back to system-specific ones on Darwin and
        BSD.
---
 libstdc++-v3/src/c++20/tzdb.cc | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index c4311d0902a..39e4466f932 100644
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -1061,16 +1061,11 @@ namespace std::chrono
   namespace
   {
     // Read the version number from a tzdata.zi file.
+    // Note that some systems do not have this file available by default
+    // but we can configure the library to point to an alternate installation.
     string
     remote_version(istream* zif)
     {
-#if defined __NetBSD__
-    if (string ver; ifstream(zoneinfo_dir() + "/TZDATA_VERSION") >> ver)
-      return ver;
-#elif defined __APPLE__
-    if (string ver; ifstream(zoneinfo_dir() + "/+VERSION") >> ver)
-      return ver;
-#else
       ifstream f;
       if (!zif)
        {
@@ -1083,6 +1078,14 @@ namespace std::chrono
       if (*zif >> hash >> label >> version)
        if (hash == '#' && label == "version")
          return version;
+#if defined __NetBSD__
+    if (string ver; ifstream(zoneinfo_dir() + "/TZDATA_VERSION") >> ver)
+      return ver;
+#elif defined __APPLE__
+    // The standard install on macOS has no tzdata.zi, but we can find the
+    // version from +VERSION.
+    if (string ver; ifstream(zoneinfo_dir() + "/+VERSION") >> ver)
+      return ver;
 #endif
       __throw_runtime_error("tzdb: no version found in tzdata.zi");
     }
-- 
2.37.1 (Apple Git-137.1)