class :isa(): check for not only a stash, but a class

If class Foo::Bar exists then the Foo stash exists, so checking
for that when Foo is the requested base class is meaningless.

Instead try to load the base class if it is not known as a class, not
just if it's not a stash, just as we do immediately following.

Fixes #21332

MANIFEST

@@ -5790,6 +5790,8 @@ t/lib/caller/Foo.pm			test Module for caller.t and t/op/hook/require.t
 t/lib/caller/Tricycle.pm		test Module for t/op/hook/require.t (cyclic)
 t/lib/CannotParse.pm			For test case in op/require_errors.t
 t/lib/charnames/alias			Tests of "use charnames" with aliases.
+t/lib/class/A.pm			Test class loading of hierarchical parent
+t/lib/class/A/B.pm			Test class loading of hierarchical parent
 t/lib/Cname.pm				Test charnames in regexes (op/pat.t)
 t/lib/common.pl				Helper for lib/{warnings,feature}.t
 t/lib/commonsense.t			See if configuration meets basic needs

class.c

@@ -508,7 +508,7 @@ apply_class_attribute_isa(pTHX_ HV *stash, SV *value)
         croak("Class already has a superclass, cannot add another");
 
     HV *superstash = gv_stashsv(superclassname, 0);
-    if(!superstash) {
+    if (!superstash || !HvSTASH_IS_CLASS(superstash)) {
         /* Try to `require` the module then attempt a second time */
         load_module(PERL_LOADMOD_NOIMPORT, newSVsv(superclassname), NULL, NULL);
         superstash = gv_stashsv(superclassname, 0);

t/class/inherit.t

@@ -96,4 +96,13 @@ no warnings 'experimental::class';
     is(Test5B->new->classname, "Test5B", '__CLASS__ yields correct class name for subclass');
 }
 
+{
+    # https://github.com/Perl/perl5/issues/21332
+    use lib 'lib/class';
+    ok(eval <<'EOS', "hierarchical base class loaded");
+use A::B;
+1;
+EOS
+}
+
 done_testing;

t/lib/class/A.pm

@@ -0,0 +1,4 @@
+no warnings 'experimental::class';
+use feature 'class';
+class A {}
+1;

t/lib/class/A/B.pm

@@ -0,0 +1,4 @@
+no warnings 'experimental::class';
+use feature 'class';
+class A::B :isa(A) {}
+1;