Substitute lower or upper bounds of type params appearing only co- or contravariantly

Summary:
In some cases a polymorphic function type can be written as an equivalent monomorphic type. Typically we warn users about these cases ('redundant generics') when written in function declarations but the current typing of function pointers / meth_caller will produce redundant generics when it quantifies over class level  generics which only appear co- or contravariantly in the resulting function type.

Although the types are correct, we prefer to generate minimally-polymorphic function types because (1) typing is more performant and (2) they are easier for users to understand.

This diff adds an additional pass to the generated function types which ensure any generics occurring only co- or contravariantly *and* not appearing as the rhs of class refinement are 'inlined' i.e. replaced by the intersection of their upper bounds or union of their lower bounds

Reviewed By: mheiber

Differential Revision: D78662593

fbshipit-source-id: 4cbc2f30f09806b85018dba781bcce2e2afe1bb6

Author: Michael Thomas

hphp/hack/src/typing/typing_extract_method.ml

@@ -1,3 +1,3 @@
 --config allowed_files_for_ignore_readonly=lambda_good_co.php,lambda_good_contra.php,lambda_bad_co.php,lambda_bad_contra.php,mock_return.php,mock_implementation.php
---config enable_experimental_stx_features='{"polymorphic_function_hints": "Unstable","function_references":"Unstable","polymorphic_lambda": "Unstable"}'
+--config enable_experimental_stx_features='{"polymorphic_function_hints": "Unstable","function_references":"Unstable","polymorphic_lambda": "Unstable","union_intersection_type_hints": "Unstable"}'
 --config poly_function_pointers=true

hphp/hack/test/typecheck/first_class_polymorphism/HH_FLAGS

@@ -0,0 +1,16 @@
+<?hh
+<<file: __EnableUnstableFeatures('polymorphic_function_hints')>>
+<<file: __EnableUnstableFeatures('union_intersection_type_hints')>>
+
+class AAA {}
+interface III {}
+
+abstract class Contrav<-T super AAA super III> {
+  public function foo(): void {}
+  public function bar(): this{ throw new Exception(); }
+}
+
+function refIt(): void {
+  $f = meth_caller(Contrav::class, 'foo');
+  hh_expect<HH\FunctionRef<(readonly function(Contrav<(AAA | III)>): void)>>($f);
+}

hphp/hack/test/typecheck/first_class_polymorphism/polymorphism_optimisation_contrav.php

@@ -0,0 +1 @@
+No errors

hphp/hack/test/typecheck/first_class_polymorphism/polymorphism_optimisation_contrav.php.exp

@@ -0,0 +1,16 @@
+<?hh
+<<file: __EnableUnstableFeatures('polymorphic_function_hints')>>
+<<file: __EnableUnstableFeatures('union_intersection_type_hints')>>
+
+class AAA {}
+interface III {}
+
+abstract class Cov<+T as AAA as III> {
+  public function foo(): void {}
+  public function bar(): this{ throw new Exception(); }
+}
+
+function refIt(): void {
+  $g = meth_caller(Cov::class, 'foo');
+  hh_expect<HH\FunctionRef<(readonly function(Cov<(AAA & III)>): void)>>($g);
+}

hphp/hack/test/typecheck/first_class_polymorphism/polymorphism_optimisation_cov.php

@@ -0,0 +1 @@
+No errors

hphp/hack/test/typecheck/first_class_polymorphism/polymorphism_optimisation_cov.php.exp

@@ -0,0 +1,16 @@
+<?hh
+<<file: __EnableUnstableFeatures('polymorphic_function_hints')>>
+<<file: __EnableUnstableFeatures('union_intersection_type_hints')>>
+
+class AAA implements III {}
+interface III {}
+
+abstract class Inv<T super AAA as III> {
+  public function foo(): void {}
+  public function bar(): this{ throw new Exception(); }
+}
+
+function refIt(): void {
+  $h = meth_caller(Inv::class, 'foo');
+  hh_expect<HH\FunctionRef<(readonly function<T super AAA as III>(Inv<T>): void)>>($h);
+}

hphp/hack/test/typecheck/first_class_polymorphism/polymorphism_optimisation_inv.php

@@ -0,0 +1 @@
+No errors

hphp/hack/test/typecheck/first_class_polymorphism/polymorphism_optimisation_inv.php.exp

@@ -0,0 +1,16 @@
+<?hh
+<<file: __EnableUnstableFeatures('polymorphic_function_hints')>>
+<<file: __EnableUnstableFeatures('union_intersection_type_hints')>>
+
+class AAA {}
+interface III {}
+
+abstract class WithRfmt<+T as arraykey> {
+  abstract const type TC  as arraykey;
+  public function foo(this::TC $_) : void {}
+}
+
+function refIt(): void {
+  $i = meth_caller(WithRfmt::class, 'foo');
+  hh_expect<HH\FunctionRef<(readonly function<TC0 as arraykey>(WithRfmt<arraykey> with { type TC = TC0 }, TC0): void)>>($i);
+}

hphp/hack/test/typecheck/first_class_polymorphism/polymorphism_optimisation_with_rfmt.php

@@ -0,0 +1 @@
+No errors