Allow non-abstract type constants from interfaces to be overriden

Summary:
The runtime allows this, and it seems to be the desired behavior for now.
Without this diff, HHVM could throw a static-analysis error.

Reviewed By: vassilmladenov

Differential Revision: D38561777

fbshipit-source-id: 10f60e60732c014fe5897afd87bc5b9e918203c0

hphp/hhbbc/index.cpp

@@ -2515,9 +2515,10 @@ bool build_class_constants(IndexData& index,
 
       if ((cns->cls->attrs & AttrInterface ||
            (RO::EvalTraitConstantInterfaceBehavior && (cns->cls->attrs & AttrTrait))) &&
-          existing->isAbstract) {
-        // because existing has val, this covers the case where it is
-        // abstract with default allow incoming to win
+          (existing->isAbstract || cns->kind == ConstModifiers::Kind::Type)) {
+        // Because existing has val, this covers the case where it is abstract
+        // with default allow incoming to win.  Also, type constants from
+        // interfaces may be overridden even if they're not abstract.
       } else {
         // A constant from an interface or from an included enum collides
         // with an existing constant.

hphp/test/slow/constants/trait_interface_equivalence.php.expectf

@@ -1 +1 @@
-Fatal error: C cannot inherit the type constant T from T2, because it was previously inherited from I1 in %s/test/slow/constants/trait_interface_equivalence.php on line 13
+Fatal error: C cannot inherit the type constant T from %rT2|C%r, because it was previously inherited from I1 in %s/test/slow/constants/trait_interface_equivalence.php on line 13

hphp/test/slow/constants/trait_unsoundness_example.php.expectf

@@ -1 +1 @@
-Fatal error: B cannot inherit the type constant T from T, because it was previously inherited from A in %s/hphp/test/slow/constants/trait_unsoundness_example.php on line 11
+Fatal error: B cannot inherit the type constant T from %rT|B%r, because it was previously inherited from A in %s/hphp/test/slow/constants/trait_unsoundness_example.php on line 11

hphp/test/slow/hhbbc/type-const-override2.php

@@ -0,0 +1,19 @@
+<?hh
+
+interface I1 {
+  abstract const type T1;
+}
+
+interface I2 extends I1 {
+  const type T1 = int;
+}
+
+interface I3 extends I2 {
+  const type T1 = int;
+}
+
+abstract class C implements I3 {}
+
+<<__EntryPoint>> function main() {
+  var_dump(C::class);
+}

hphp/test/slow/hhbbc/type-const-override2.php.expect

@@ -0,0 +1 @@
+string(1) "C"