[dart2js] Assume isPotentialSubtype is always true.

The current implementation of this check is incorrect and produces false
negatives. Conservatively assuming that one type is always a potential
subtype of another is a simple fix to get this working.

The main downsides are that we may emit signatures and provide type
arguments to functions even when they're not needed because we no longer
statically deduce that a type check must fail. This may cause size
regressions.

I've added a TODO to add proper constraint solving per the local type
inference spec, but this is not currently a priority since no regression
is observed on large Google apps.

Change-Id: Ie5065596331c33a030e06e66481f258ef937e659
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155544
Reviewed-by: Stephen Adams <sra@google.com>
Commit-Queue: Mayank Patke <fishythefish@google.com>

Author: fishythefish

pkg/compiler/lib/src/elements/types.dart

@@ -1915,6 +1915,11 @@ abstract class DartTypes {
 
   bool _subtypeHelper(DartType s, DartType t,
       {bool allowPotentialSubtypes: false, bool assumeInstantiations: false}) {
+    assert(allowPotentialSubtypes || !assumeInstantiations);
+
+    // TODO(fishythefish): Add constraint solving for potential subtypes.
+    if (allowPotentialSubtypes) return true;
+
     /// Based on
     /// https://github.com/dart-lang/language/blob/master/resources/type-system/subtyping.md.
     /// See also [_isSubtype] in `dart:_rti`.
@@ -1927,10 +1932,6 @@ abstract class DartTypes {
           env.isAssumed(s, t)) return true;
 
       if (s is AnyType) return true;
-      if (allowPotentialSubtypes &&
-          (s is TypeVariableType || t is TypeVariableType)) return true;
-      if (assumeInstantiations &&
-          (s is FunctionTypeVariable || t is FunctionTypeVariable)) return true;
 
       // Right Top:
       if (isTopType(t)) return true;
@@ -2039,10 +2040,10 @@ abstract class DartTypes {
           List<FunctionTypeVariable> sTypeVariables = s.typeVariables;
           List<FunctionTypeVariable> tTypeVariables = t.typeVariables;
           int length = tTypeVariables.length;
-          if (length == sTypeVariables.length) {
-            env ??= _Assumptions();
-            env.assumePairs(sTypeVariables, tTypeVariables);
-          } else if (!assumeInstantiations || length > 0) return false;
+          if (length != sTypeVariables.length) return false;
+
+          env ??= _Assumptions();
+          env.assumePairs(sTypeVariables, tTypeVariables);
           try {
             for (int i = 0; i < length; i++) {
               DartType sBound = sTypeVariables[i].bound;

pkg/compiler/test/rti/data/async_local_typed.dart

@@ -13,7 +13,7 @@ main() async {
   // `dynamic Function(dynamic, Class<int>)`, is not a potential subtype and
   // therefore doesn't need its signature.
 
-  /*spec.needsSignature*/
+  /*needsSignature*/
   local(object, Class<int> stacktrace) => null;
 
   return local;

pkg/compiler/test/rti/data/closure_generic_unneeded.dart

@@ -5,10 +5,11 @@
 // @dart = 2.7
 
 /*spec.class: A:direct,explicit=[A.T*],needsArgs*/
+/*prod.class: A:needsArgs*/
 class A<T> {
   @pragma('dart2js:noInline')
   m() {
-    return (T t, String s) {};
+    return /*needsSignature*/(T t, String s) {};
   }
 }
 

pkg/compiler/test/rti/data/closure_unneeded.dart

@@ -5,10 +5,11 @@
 // @dart = 2.7
 
 /*spec.class: A:direct,explicit=[A.T*],needsArgs*/
+/*prod.class: A:needsArgs*/
 class A<T> {
   @pragma('dart2js:noInline')
   m() {
-    return (T t, String s) {};
+    return /*needsSignature*/(T t, String s) {};
   }
 }
 

pkg/compiler/test/rti/data/generic_bounds.dart

@@ -41,13 +41,13 @@ class Class4 {}
 method10<T extends Class4>() => null;
 
 main() {
-  /*spec.needsArgs,selectors=[Selector(call, call, arity=0, types=1)]*/
+  /*needsArgs,needsSignature,selectors=[Selector(call, call, arity=0, types=1)]*/
   method7<T extends Class1a>() => null;
 
-  /*spec.needsArgs,selectors=[Selector(call, call, arity=0, types=1)]*/
+  /*needsArgs,needsSignature,selectors=[Selector(call, call, arity=0, types=1)]*/
   method8<T extends Class2a<num>>() => null;
 
-  method9<T>() => null;
+  /*needsArgs,needsSignature,selectors=[Selector(call, call, arity=0, types=1)]*/method9<T>() => null;
 
   dynamic f1 = method1;
   dynamic f2 = method2;

pkg/compiler/test/rti/data/instantiation3.dart

@@ -15,7 +15,7 @@ class B<S> {
   F<S> c;
 
   method() {
-    return () {
+    return /*spec.needsSignature*/() {
       c = f;
     };
   }

pkg/compiler/test/rti/data/instantiation4.dart

@@ -15,7 +15,7 @@ class B<S> {
   F<S> c;
 
   method() {
-    return () {
+    return /*spec.needsSignature*/() {
       c = f;
     };
   }

pkg/compiler/test/rti/data/local_function_map_literal.dart

@@ -13,7 +13,7 @@ import 'package:expect/expect.dart';
 /*spec.member: method:implicit=[method.T],indirect,needsArgs*/
 /*prod.member: method:needsArgs*/
 method<T>() {
-  return () => <T, int>{};
+  return /*spec.needsSignature*/() => <T, int>{};
 }
 
 @pragma('dart2js:noInline')

pkg/compiler/test/rti/data/local_function_signature2.dart

@@ -8,17 +8,17 @@ import 'package:expect/expect.dart';
 
 class Class1 {
   method1() {
-    num local<T>(num n) => null;
+    /*needsArgs,needsSignature*/num local<T>(num n) => null;
     return local;
   }
 
   method2() {
-    num local<T>(int n) => null;
+    /*needsArgs,needsSignature*/num local<T>(int n) => null;
     return local;
   }
 
   method3() {
-    int local<T>(num n) => null;
+    /*needsArgs,needsSignature*/int local<T>(num n) => null;
     return local;
   }
 }
@@ -43,9 +43,10 @@ class Class3 {
 }
 
 class Class4 {
+  /*prod.member: Class4.method6:needsArgs,selectors=[Selector(call, method6, arity=0, types=1)]*/
   /*spec.member: Class4.method6:direct,explicit=[method6.T*],needsArgs,selectors=[Selector(call, method6, arity=0, types=1)]*/
   method6<T>() {
-    num local(num n, T t) => null;
+    /*needsSignature*/num local(num n, T t) => null;
     return local;
   }
 }
@@ -66,25 +67,26 @@ method8<T>() {
 }
 
 /*spec.member: method9:direct,explicit=[method9.T*],needsArgs*/
+/*prod.member: method9:needsArgs*/
 method9<T>() {
-  num local(num n, T t) => null;
+  /*needsSignature*/num local(num n, T t) => null;
   return local;
 }
 
 method10() {
-  /*spec.direct,explicit=[local.T*],needsArgs*/
-  num local<T>(T n) => null;
+  /*spec.direct,explicit=[local.T*],needsArgs,needsSignature*/
+  /*prod.needsArgs,needsSignature*/num local<T>(T n) => null;
   return local;
 }
 
 method11() {
-  T local<T>(num n) => null;
+  /*needsArgs,needsSignature*/T local<T>(num n) => null;
   return local;
 }
 
 method12() {
-  /*spec.direct,explicit=[local.T*],needsArgs*/
-  num local<T>(num n, T t) => null;
+  /*spec.direct,explicit=[local.T*],needsArgs,needsSignature*/
+  /*prod.needsArgs,needsSignature*/num local<T>(num n, T t) => null;
   return local;
 }
 

pkg/compiler/test/rti/data/local_function_signatures.dart

@@ -14,12 +14,12 @@ class Class1 {
   }
 
   method2() {
-    num local(int n) => null;
+    /*needsSignature*/num local(int n) => null;
     return local;
   }
 
   method3() {
-    Object local(num n) => null;
+    /*needsSignature*/Object local(num n) => null;
     return local;
   }
 }
@@ -44,9 +44,10 @@ class Class3<T> {
 }
 
 /*spec.class: Class4:direct,explicit=[Class4.T*],needsArgs*/
+/*prod.class: Class4:needsArgs*/
 class Class4<T> {
   method6() {
-    num local(num n, T t) => null;
+    /*needsSignature*/num local(num n, T t) => null;
     return local;
   }
 }