Filehandle method calls load IO::File on demand

When a method call on a filehandle would die because the method can not
be resolved and L<IO::File> has not been loaded, Perl now loads IO::File
via C<require> and attempts method resolution again:

  open my $fh, ">", $file;
  $fh->binmode(":raw");     # loads IO::File and succeeds

This also works for globs like STDOUT, STDERR and STDIN:

  STDOUT->autoflush(1);

Because this on-demand load only happens if method resolution fails, the
legacy approach of manually loading an IO::File parent class for partial
method support still works as expected:

  use IO::Handle;
  open my $fh, ">", $file;
  $fh->autoflush(1);        # IO::File not loaded

1 files changed, 13 insertions, 0 deletions

diff --git a/gv.c b/gv.c
index 5fd385bd11..4775bccf37 100644
--- a/gv.c
+++ b/gv.c
@@ -718,6 +718,19 @@ Perl_gv_fetchmethod_flags(pTHX_ HV *stash, const char *name, U32 flags)
 	    /* Right now this is exclusively for the benefit of S_method_common
 	       in pp_hot.c  */
 	    if (stash) {
+		/* If we can't find an IO::File method, it might be a call on
+		 * a filehandle. If IO:File has not been loaded, try to
+		 * require it first instead of croaking */
+		const char *stash_name = HvNAME_get(stash);
+		const char *io_file = "IO/File.pm";
+		if (stash_name && strEQ(stash_name,"IO::File")
+		    && ! hv_exists(GvHVn(PL_incgv),io_file, strlen(io_file))
+		) {
+		    require_pv(io_file);
+		    gv = gv_fetchmeth(stash, name, nend - name, 0);
+		    if (gv)
+			return gv;
+		}
 		Perl_croak(aTHX_
 			   "Can't locate object method \"%s\" via package \"%.*s\"",
 			   name, (int)HvNAMELEN_get(stash), HvNAME_get(stash));