report an error when making an object of an incomplete class

instead of asserting or crashing

Fixes #22159

class.c

@@ -140,6 +140,11 @@ XS(injected_constructor)
         Perl_warn(aTHX_ "Odd number of arguments passed to %" HvNAMEf_QUOTEDPREFIX " constructor",
                 HvNAMEfARG(stash));
 
+    if (!aux->xhv_class_initfields_cv) {
+        Perl_croak(aTHX_ "Cannot create an object of incomplete class %" HvNAMEf_QUOTEDPREFIX,
+                   HvNAMEfARG(stash));
+    }
+
     HV *params = NULL;
     {
         /* Set up params HV */

pod/perldiag.pod

@@ -731,6 +731,18 @@ be directly assigned to.
 an C<@ISA> array, and the array is not empty.  This is not permitted, as it
 would lead to a class with inconsistent inheritance.
 
+=item Cannot create an object of incomplete class "%s"
+
+(F) An attempt was made to create an object of a class where the start
+of the class definition has been seen, but the class has not been
+completed.
+
+This can happen for a failed eval, or if you attempt to create an
+object at compile time before the class is complete:
+
+  eval "class Foo {"; Foo->new; # error
+  class Bar { BEGIN { Bar->new } }; # error
+
 =item Cannot find encoding "%s"
 
 (S io) You tried to apply an encoding that did not exist to a filehandle,

t/lib/croak/class

@@ -137,3 +137,21 @@ class XXX {
 }
 EXPECT
 Cannot use __CLASS__ outside of a method or field initializer expression at - line 5.
+########
+# NAME try to create an object of incomplete class (error)
+use v5.36;
+use feature 'class';
+no warnings 'experimental::class';
+eval "class C {";
+C->new;
+EXPECT
+Cannot create an object of incomplete class "C" at - line 5.
+########
+# NAME try to create an object of incomplete class (compile-time)
+use v5.36;
+use feature 'class';
+no warnings 'experimental::class';
+class C { BEGIN { C->new; } };
+EXPECT
+Cannot create an object of incomplete class "C" at - line 4.
+BEGIN failed--compilation aborted at - line 4.