Implementations of abstract methods can have supertype parameter types

Summary:
Currently the case if `force_hh` is on. Make it true for any subclasses in Hack, rather than PHP.

Going to put up a separate diff RE this only applying to abstract functions.

refs #8256

Reviewed By: swtaarrs

Differential Revision: D8868379

fbshipit-source-id: 83f749139c610e6b0546695b6090f0970bf302b5

hphp/runtime/vm/type-constraint.cpp

@@ -194,7 +194,8 @@ bool TypeConstraint::compat(const TypeConstraint& other) const {
     const Class* cls = Unit::lookupClass(m_typeName);
     const Class* otherCls = Unit::lookupClass(other.m_typeName);
 
-    return cls && otherCls && cls == otherCls;
+    return cls && otherCls && (isHHType() ? otherCls->classof(cls)
+                                          : cls == otherCls);
   }
 
   return false;

hphp/test/slow/class_method/abstract_variance.php

@@ -0,0 +1,22 @@
+<?hh // strict
+
+class ParamBase {
+}
+
+class ParamSub extends ParamBase {
+}
+
+abstract class ImplBase {
+  abstract public function doStuff(ParamSub $_): void;
+}
+
+class ImplSub extends ImplBase {
+  public function doStuff(ParamBase $_): void {
+  }
+}
+
+function main(): void {
+  (new ImplSub())->doStuff(new ParamSub());
+}
+
+main();