Partially address bug #78647

Produce a sensible error message for the case where inheritance
should fail. There is still a remaining issue that we sometimes
fail inheritance while we should not.

Zend/tests/type_declarations/variance/class_order_autoload_error7.phpt

@@ -0,0 +1,24 @@
+--TEST--
+Bug #78647: Outstanding dependency obligation
+--FILE--
+<?php
+
+spl_autoload_register(function ($class) {
+    if ($class == 'A') {
+        class A {
+            function m(): B {}
+        }
+    } elseif ($class == 'B') {
+        class B extends A {
+            function m(): X {}
+        }
+    } else {
+        class C extends B {}
+    }
+});
+
+new B;
+
+?>
+--EXPECTF--
+Fatal error: Could not check compatibility between B::m(): X and A::m(): B, because class X is not available in %s on line %d

Zend/zend_inheritance.c

@@ -2548,7 +2548,8 @@ static void report_variance_errors(zend_class_entry *ce) {
 		} else if (obligation->type == OBLIGATION_PROPERTY_COMPATIBILITY) {
 			emit_incompatible_property_error(obligation->child_prop, obligation->parent_prop);
 		} else {
-			zend_error_noreturn(E_CORE_ERROR, "Bug #78647");
+			/* Report variance errors of the dependency, which prevent it from being linked. */
+			report_variance_errors(obligation->dependency_ce);
 		}
 	} ZEND_HASH_FOREACH_END();