Fix assorted bugs related to not having a UNIVERSAL::import

Since perl 5.0 the methods "import" and "unimport" have been
special cased in gv.c (unimport was removed for a while) to
not produce errors if they are called. This is partly
because

    use Foo;

is defined to be

    BEGIN {
        require Foo;
        Foo->import();
    }

which would blow up if there is no import function defined in
Foo, for instance if it were defining a class and not a package
which exports modules.

This special case can be broken by simple code like

    \&UNIVERSAL::isa

which will create a stub function which then blows up when it is
used. Notably the module "autouse" which is shipped with perl will
trigger this behavior.

A related issue is that if you ask for a function to be exported
from a module that does not have support for exporting there is no
error, eg:

    use File::Spec qw(catfile);

will silently succeed without exporting a catfile function. This is
exacerbated on case insensitive file systems when the module name
is case-mismatched, the use succeeds but the export does not, leading
to confusion, eg:

    use LIst::Util qw(sum);

will load List::Util but will not export the sum function.

This patch defines UNIVERSAL::import() and UNIVERSAL::unimport()
functions. This prevents the "reference to \&UNIVERSAL::import" bug.
The function is defined to be a no-op unless arguments are passed into
the functions, in which case a warning is thrown indicating
that there is likely a problem. The error is modelled after the
error produced by calling a non-existent method or function:

    ./perl -Ilib -le'BEGIN{ my $import_sub= \&UNIVERSAL::import;}
        use File::Spec qw(catfile);'
    Attempt to call UNIVERSAL::import() with arguments via package File::Spec
        (Perhaps you forgot to load "File::Spec"?) at -e line 1.
    BEGIN failed--compilation aborted at -e line 1.

This fixes Issue #19416, Issue #19417, Issue #19418. See also Issue #19410 for
discussion, however this patch does not fix that case (it may not be
fixable.)

gv.c

@@ -1216,13 +1216,7 @@ Perl_gv_fetchmethod_pvn_flags(pTHX_ HV *stash, const char *name, const STRLEN le
 
     gv = gv_fetchmeth_pvn(stash, name, name_end - name, 0, flags);
     if (!gv) {
-        /* This is the special case that exempts Foo->import and
-           Foo->unimport from being an error even if there's no
-          import/unimport subroutine */
-        if (strEQ(name,"import") || strEQ(name,"unimport")) {
-            gv = (GV*)sv_2mortal((SV*)newCONSTSUB_flags(NULL,
-                                                NULL, 0, 0, NULL));
-        } else if (autoload)
+        if (autoload)
             gv = gv_autoload_pvn(
                 ostash, name, name_end - name, GV_AUTOLOAD_ISMETHOD|flags
             );

lib/UNIVERSAL.pm

@@ -1,19 +1,9 @@
 package UNIVERSAL;
 
-our $VERSION = '1.14';
-
-# UNIVERSAL should not contain any extra subs/methods beyond those
-# that it exists to define. The existence of import() below is a historical
-# accident that can't be fixed without breaking code.
-
-# Make sure that even though the import method is called, it doesn't do
-# anything unless called on UNIVERSAL.
-sub import {
-    return unless $_[0] eq __PACKAGE__;
-    return unless @_ > 1;
-    require Carp;
-    Carp::croak("UNIVERSAL does not export anything");
-}
+our $VERSION = '1.15';
+
+# UNIVERSAL.pm should not contain any methods/subs, they
+# are all defined in universal.c
 
 1;
 __END__

t/op/universal.t

@@ -139,9 +139,9 @@ like $@, qr/^Invalid version format/;
 
 my $subs = join ' ', sort grep { defined &{"UNIVERSAL::$_"} } keys %UNIVERSAL::;
 if ('a' lt 'A') {
-    is $subs, "can isa DOES VERSION";
+    is $subs, "can import isa unimport DOES VERSION";
 } else {
-    is $subs, "DOES VERSION can isa";
+    is $subs, "DOES VERSION can import isa unimport";
 }
 
 ok $a->isa("UNIVERSAL");
@@ -160,11 +160,10 @@ eval "use UNIVERSAL";
 ok $a->isa("UNIVERSAL");
 
 my $sub2 = join ' ', sort grep { defined &{"UNIVERSAL::$_"} } keys %UNIVERSAL::;
-# XXX import being here is really a bug
 if ('a' lt 'A') {
-    is $sub2, "can import isa DOES VERSION";
+    is $sub2, "can import isa unimport DOES VERSION";
 } else {
-    is $sub2, "DOES VERSION can import isa";
+    is $sub2, "DOES VERSION can import isa unimport";
 }
 
 eval 'sub UNIVERSAL::sleep {}';
@@ -198,10 +197,12 @@ ok $x->isa('UNIVERSAL');
 ok $x->isa('UNIVERSAL');
 
 
-# Check that the "historical accident" of UNIVERSAL having an import()
-# method doesn't effect anyone else.
 eval { Some::Package->import("bar") };
-is $@, '';
+my $err= $@;
+$err=~s!t/op!op!;
+is $err, "Attempt to call undefined import with arguments via"
+       . " package Some::Package (Perhaps you forgot to load"
+       . " \"Some::Package\"?) at op/universal.t line 200.\n";
 
 
 # This segfaulted in a blead.

universal.c

@@ -433,6 +433,29 @@ XS(XS_UNIVERSAL_isa)
     }
 }
 
+XS(XS_UNIVERSAL_import_unimport); /* prototype to pass -Wmissing-prototypes */
+XS(XS_UNIVERSAL_import_unimport)
+{
+    dXSARGS;
+    dXSI32;
+
+    if (items > 1) {
+        char *class_pv= SvPV_nolen(ST(0));
+        if (strEQ(class_pv,"UNIVERSAL"))
+            Perl_croak(aTHX_ "UNIVERSAL does not export anything");
+        /* _charnames is special - ignore it for now as the code that
+         * depends on it has its own "no import" logic that produces better
+         * warnings than this does. */
+        if (strNE(class_pv,"_charnames"))
+            Perl_croak(aTHX_
+                "Attempt to call undefined %s method with arguments via package %" SVf
+                " (Perhaps you forgot to load \"%" SVf "\"?)",
+                ix ? "unimport" : "import", SVfARG(ST(0)), SVfARG(ST(0)));
+    }
+    XSRETURN_EMPTY;
+}
+
+
 XS(XS_UNIVERSAL_can); /* prototype to pass -Wmissing-prototypes */
 XS(XS_UNIVERSAL_can)
 {
@@ -1252,6 +1275,8 @@ static const struct xsub_details these_details[] = {
     {"UNIVERSAL::isa", XS_UNIVERSAL_isa, NULL, 0 },
     {"UNIVERSAL::can", XS_UNIVERSAL_can, NULL, 0 },
     {"UNIVERSAL::DOES", XS_UNIVERSAL_DOES, NULL, 0 },
+    {"UNIVERSAL::import", XS_UNIVERSAL_import_unimport, NULL, 0},
+    {"UNIVERSAL::unimport", XS_UNIVERSAL_import_unimport, NULL, 1},
 #define VXS_XSUB_DETAILS
 #include "vxs.inc"
 #undef VXS_XSUB_DETAILS