builtin: make inf/nan conditional depending on float support

builtin.c

@@ -96,8 +96,12 @@ XS(XS_builtin_inf)
     dXSARGS;
     if(items)
         croak_xs_usage(cv, "");
+#ifdef DOUBLE_HAS_INF
     EXTEND(SP, 1);
     XSRETURN_NV(NV_INF);
+#else
+    Perl_croak_nocontext("builtin::inf not implemented");
+#endif
 }
 
 XS(XS_builtin_nan);
@@ -106,8 +110,12 @@ XS(XS_builtin_nan)
     dXSARGS;
     if(items)
         croak_xs_usage(cv, "");
+#ifdef DOUBLE_HAS_NAN
     EXTEND(SP, 1);
     XSRETURN_NV(NV_NAN);
+#else
+    Perl_croak_nocontext("builtin::nan not implemented");
+#endif
 }
 
 enum {
@@ -135,8 +143,16 @@ static OP *ck_builtin_const(pTHX_ OP *entersubop, GV *namegv, SV *ckobj)
     switch(builtin->ckval) {
         case BUILTIN_CONST_FALSE: constval = &PL_sv_no; break;
         case BUILTIN_CONST_TRUE:  constval = &PL_sv_yes; break;
+#ifdef DOUBLE_HAS_INF
         case BUILTIN_CONST_INF:   constval = newSVnv(NV_INF); break;
+#else
+        case BUILTIN_CONST_INF:   return entersubop;
+#endif
+#ifdef DOUBLE_HAS_NAN
         case BUILTIN_CONST_NAN:   constval = newSVnv(NV_NAN); break;
+#else
+        case BUILTIN_CONST_NAN:   return entersubop;
+#endif
         default:
             DIE(aTHX_ "panic: unrecognised builtin_const value %" IVdf,
                       builtin->ckval);

lib/builtin.pm

@@ -1,4 +1,4 @@
-package builtin 0.017;
+package builtin 0.018;
 
 use v5.40;
 
@@ -167,7 +167,8 @@ Available starting with Perl 5.36.
 
 This function is currently B<experimental>.
 
-Returns the floating-point infinity value.
+Returns the floating-point infinity value. If the underlying numeric C type
+does not support such a value, it throws a runtime error instead.
 
 Available starting with Perl 5.40.
 
@@ -177,7 +178,8 @@ Available starting with Perl 5.40.
 
 This function is currently B<experimental>.
 
-Returns the floating-point "Not-a-Number" value.
+Returns the floating-point "Not-a-Number" value. If the underlying numeric C
+type does not support such a value, it throws a runtime error instead.
 
 Available starting with Perl 5.40.
 

lib/builtin.t

@@ -8,6 +8,7 @@ BEGIN {
 
 use v5.36;
 no warnings 'experimental::builtin';
+use Config;
 
 package FetchStoreCounter {
     sub TIESCALAR($class, @args) { bless \@args, $class }
@@ -55,19 +56,31 @@ package FetchStoreCounter {
 {
     use builtin qw( inf nan );
 
-    ok(inf, 'inf is true');
-    ok(inf > 1E10, 'inf is bigger than 1E10');
-    ok(inf == inf, 'inf is equal to inf');
-    ok(inf == inf + 1, 'inf is equal to inf + 1');
+    if ($Config{d_double_has_inf}) {
+        ok(inf, 'inf is true');
+        ok(inf > 1E10, 'inf is bigger than 1E10');
+        ok(inf == inf, 'inf is equal to inf');
+        ok(inf == inf + 1, 'inf is equal to inf + 1');
 
-    # Invoke the real XSUB
-    my $inf = ( \&builtin::inf )->();
-    ok($inf == $inf + 1, 'inf returned by real xsub');
+        # Invoke the real XSUB
+        my $inf = ( \&builtin::inf )->();
+        ok($inf == $inf + 1, 'inf returned by real xsub');
+    } else {
+        is(eval { inf }, undef, 'inf throws');
+        my $e = $@;
+        like($e, qr/^builtin::inf not implemented at/, 'inf fails with correct error');
+    }
 
-    ok(nan != nan, 'NaN is not equal to NaN');
+    if ($Config{d_double_has_nan}) {
+        ok(nan != nan, 'NaN is not equal to NaN');
 
-    my $nan = ( \&builtin::nan )->();
-    ok($nan != $nan, 'NaN returned by real xsub');
+        my $nan = ( \&builtin::nan )->();
+        ok($nan != $nan, 'NaN returned by real xsub');
+    } else {
+        is(eval { nan }, undef, 'nan throws');
+        my $e = $@;
+        like($e, qr/^builtin::nan not implemented at/, 'nan fails with correct error');
+    }
 }
 
 # weakrefs

pod/perldelta.pod

@@ -129,6 +129,14 @@ L<XXX> has been upgraded from version A.xx to B.yy.
 
 XXX If there was something important to note about this change, include that here.
 
+=item *
+
+L<builtin> has been upgraded from version 0.017 to 0.018.
+
+On platforms that don't support Inf/NaN values in floating-point numbers (such
+as VAX), C<builtin::inf> and C<builtin::nan> now throw a runtime error (rather
+than breaking the perl build). [GH #22882]
+
 =back
 
 =head2 Removed Modules and Pragmata

pod/perldiag.pod

@@ -675,6 +675,16 @@ is currently being compiled. Since this method is used to introduce new
 lexical subroutines into the scope currently being compiled, this is not
 going to have any effect.
 
+=item builtin::inf not implemented
+
+(F) Your machine doesn't support infinity as a numeric value (probably because
+it's a VAX).
+
+=item builtin::nan not implemented
+
+(F) Your machine doesn't support NaN ("not a number") as a numeric value
+(probably because it's a VAX).
+
 =item Builtin version bundle "%s" is not supported by Perl
 
 (F) You attempted to C<use builtin :ver> for a version number that is either