On VMS, %ENV in scalar context must call prime_env_iter()

Otherwise it will return wrong results, unless other code happens to
have iterated over %ENV. This bug has probably existed forever.

pod/perldelta.pod

@@ -371,9 +371,18 @@ L</Modules and Pragmata> section.
 
 =over 4
 
-=item XXX-some-platform
-
-XXX
+=item C<keys %ENV> on VMS returns consistent results
+
+On VMS entries in the C<%ENV> hash are loaded from the OS environment on
+first access, hence the first iteration of C<%ENV> requires the entire
+environment to be scanned to find all possible keys. This initialisation had
+always been done correctly for full iteration, but previously was not
+happening for C<%ENV> in scalar context, meaning that C<scalar %ENV> would
+return 0 if called before any other C<%ENV> access, or would only return the
+count of keys accessed if there had been no iteration.
+
+These bugs are now fixed - C<%ENV> and C<keys %ENV> in scalar context now
+return the correct result - the count of all keys in the environment.
 
 =back
 

pp_hot.c

@@ -1813,6 +1813,19 @@ S_padhv_rv2hv_common(pTHX_ HV *hv, U8 gimme, bool is_keys, bool has_targ)
         }
     }
     else {
+#if defined(DYNAMIC_ENV_FETCH) && defined(VMS)
+        /* maybe nothing set up %ENV for iteration yet...
+           do this always (not just if HvUSEDKEYS(hv) is currently 0) because
+           we ought to give a *consistent* answer to "how many keys?"
+           whether we ask this op in scalar context, or get the list of all
+           keys then check its length, and whether we do either with or without
+           an %ENV lookup first. prime_env_iter() returns quickly if nothing
+           needs doing. */
+        if (SvRMAGICAL((const SV *)hv)
+            && mg_find((const SV *)hv, PERL_MAGIC_env)) {
+            prime_env_iter();
+        }
+#endif
         i = HvUSEDKEYS(hv);
         if (is_bool) {
             sv = i ? &PL_sv_yes : &PL_sv_zero;

t/op/each.t

@@ -286,6 +286,15 @@ for my $k (qw(each keys values)) {
     ok(!$warned, "no warnings 'internal' silences each() after insert warnings");
 }
 
+fresh_perl_like('$a = keys %ENV; $b = () = keys %ENV; $c = keys %ENV; print qq=$a,$b,$c=',
+                qr/^([1-9][0-9]*),\1,\1$/,
+                undef,
+                'keys %ENV in scalar context triggers prime_env_iter if needed');
+fresh_perl_like('$a = $ENV{PATH}; $a = $ENV{q=DCL$PATH=}; $a = keys %ENV; $b = () = keys %ENV; $c = keys %ENV; print qq=$a,$b,$c=',
+                qr/^([1-9][0-9]*),\1,\1$/,
+                undef,
+                '%ENV lookup, and keys %ENV in scalar context remain consistent');
+
 use feature 'refaliasing';
 no warnings 'experimental::refaliasing';
 $a = 7;