[gcc r12-8214] libstdc++: Work around modules ICE in <charconv> [PR105297]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:1e6c0e69af8da436e1d1d2d23d8c38410d78ecf2

commit r12-8214-g1e6c0e69af8da436e1d1d2d23d8c38410d78ecf2
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Apr 21 08:34:59 2022 -0400

    libstdc++: Work around modules ICE in <charconv> [PR105297]
    
    This makes the initializer for __table in __from_chars_alnum_to_val
    dependent in an artificial way, which works around the reported modules
    testsuite ICE by preventing the compiler from evaluating the initializer
    parse time.
    
    Compared to the alternative workaround of using a non-local class type
    for __table, this workaround has the advantage of slightly speeding up
    compilation of <charconv>, since now the table won't get built (via
    constexpr evaluation) until the integer std::from_chars overload is
    instantiated.
    
            PR c++/105297
            PR c++/105322
    
    libstdc++-v3/ChangeLog:
    
            * include/std/charconv (__from_chars_alnum_to_val): Make
            initializer for __table dependent in an artificial way.

Diff:
---
 libstdc++-v3/include/std/charconv | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/charconv b/libstdc++-v3/include/std/charconv
index f1ace406017..561234cb2fc 100644
--- a/libstdc++-v3/include/std/charconv
+++ b/libstdc++-v3/include/std/charconv
@@ -445,7 +445,9 @@ namespace __detail
 	return __c - '0';
       else
 	{
-	  static constexpr auto __table = __from_chars_alnum_to_val_table();
+	  // This initializer is deliberately made dependent in order to work
+	  // around modules bug PR105322.
+	  static constexpr auto __table = (_DecOnly, __from_chars_alnum_to_val_table());
 	  return __table.__data[__c];
 	}
     }