* CopSTASH_set(..., NULL) does not work with older threaded perls

  (5.16.x and older)

  Workaround by using the old code before commit
  8eb3ec5 code in this case. Detect
  problem with a new C test file copstashset.c.

Author: eserte

config/copstashset.c

@@ -0,0 +1,7 @@
+#include <EXTERN.h>
+#include <perl.h>
+#include <XSUB.h>
+
+int main() {
+    CopSTASH_set(PL_curcop, NULL);
+}

myConfig

@@ -225,6 +225,11 @@ if (try_compile("config/svtrv.c",  ["-I$Config{'archlibexp'}/CORE"], undef, ["-c
   $define{'HAS_REAL_SVT_RV'} = 1; # changed in perl 5.11.0, SVt_RV is since then defined as SVt_IV 
  }
 
+if (try_compile("config/copstashset.c",  ["-I$Config{'archlibexp'}/CORE"], undef, ["-c"]))
+ {
+  $define{'CAN_COPSTASH_SET_NULL'} = 1; # this seems to work since 5.17.x, and fails with older threaded perls
+ }
+
 if (!$IsWin32)
  {
   if (!try_compile("config/tod.c"))

tkGlue.c

@@ -4154,7 +4154,15 @@ int type;
    SV *x = NULL;
    int prefix = '?';
    name = SvPV(sv,na);
+#ifdef CAN_COPSTASH_SET_NULL
    CopSTASH_set(PL_curcop, NULL);
+#else
+#  ifdef USE_ITHREADS
+   CopSTASHPV(PL_curcop) = NULL;
+#  else
+   CopSTASH(PL_curcop) = NULL;
+#  endif
+#endif
    switch (type)
     {
      case TK_CONFIG_SCALARVAR: