[cfe] Fix off-by-one error in type variable target resolution

Change-Id: Ia0adbd27b9f3067d754851f081a25ac4a6126a7d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155128
Reviewed-by: Dmitry Stefantsov <[email protected]>

Author: johnniwinther

pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart

@@ -3175,7 +3175,7 @@ class TypeInferrerImpl implements TypeInferrer {
       DartType step1 = resolveOneStep(hare);
       if (step1 == null) return hare;
       DartType step2 = resolveOneStep(step1);
-      if (step2 == null) return hare;
+      if (step2 == null) return step1;
       hare = step2;
 
       // Tortoise takes one step

pkg/front_end/testcases/general/type_variable_bound_access.dart

@@ -22,6 +22,25 @@ class NumClass<T extends num, S extends T> {
   num method2() => field1 + field2.length;
 }
 
+class Class<X5 extends X4, X4 extends X3, X3 extends X2, X2 extends X1,
+    X1 extends X0, X0 extends int> {
+  X0 field0;
+  X1 field1;
+  X2 field2;
+  X3 field3;
+  X4 field4;
+  X5 field5;
+
+  method() {
+    field0.isEven;
+    field1.isEven;
+    field2.isEven;
+    field3.isEven;
+    field4.isEven;
+    field5.isEven;
+  }
+}
+
 main() {
   new DynamicClass<num, int>(0.5, 2).method();
   new NumClass<num, double>(2, 0.5).method1();

pkg/front_end/testcases/general/type_variable_bound_access.dart.outline.expect

@@ -40,5 +40,27 @@ class NumClass<T extends core::num* = core::num*, S extends self::NumClass::T* =
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
   abstract member-signature get runtimeType() → core::Type*;
 }
+class Class<X5 extends self::Class::X4* = core::int*, X4 extends self::Class::X3* = core::int*, X3 extends self::Class::X2* = core::int*, X2 extends self::Class::X1* = core::int*, X1 extends self::Class::X0* = core::int*, X0 extends core::int* = core::int*> extends core::Object {
+  generic-covariant-impl field self::Class::X0* field0;
+  generic-covariant-impl field self::Class::X1* field1;
+  generic-covariant-impl field self::Class::X2* field2;
+  generic-covariant-impl field self::Class::X3* field3;
+  generic-covariant-impl field self::Class::X4* field4;
+  generic-covariant-impl field self::Class::X5* field5;
+  synthetic constructor •() → self::Class<self::Class::X5*, self::Class::X4*, self::Class::X3*, self::Class::X2*, self::Class::X1*, self::Class::X0*>*
+    ;
+  method method() → dynamic
+    ;
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
 static method main() → dynamic
   ;

pkg/front_end/testcases/general/type_variable_bound_access.dart.strong.expect

@@ -53,6 +53,35 @@ Try correcting the name to the name of an existing getter, or defining a getter
   abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
   abstract member-signature get runtimeType() → core::Type*;
 }
+class Class<X5 extends self::Class::X4* = core::int*, X4 extends self::Class::X3* = core::int*, X3 extends self::Class::X2* = core::int*, X2 extends self::Class::X1* = core::int*, X1 extends self::Class::X0* = core::int*, X0 extends core::int* = core::int*> extends core::Object {
+  generic-covariant-impl field self::Class::X0* field0 = null;
+  generic-covariant-impl field self::Class::X1* field1 = null;
+  generic-covariant-impl field self::Class::X2* field2 = null;
+  generic-covariant-impl field self::Class::X3* field3 = null;
+  generic-covariant-impl field self::Class::X4* field4 = null;
+  generic-covariant-impl field self::Class::X5* field5 = null;
+  synthetic constructor •() → self::Class<self::Class::X5*, self::Class::X4*, self::Class::X3*, self::Class::X2*, self::Class::X1*, self::Class::X0*>*
+    : super core::Object::•()
+    ;
+  method method() → dynamic {
+    this.{self::Class::field0}.{core::int::isEven};
+    this.{self::Class::field1}.{core::int::isEven};
+    this.{self::Class::field2}.{core::int::isEven};
+    this.{self::Class::field3}.{core::int::isEven};
+    this.{self::Class::field4}.{core::int::isEven};
+    this.{self::Class::field5}.{core::int::isEven};
+  }
+  abstract member-signature get _identityHashCode() → core::int*;
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*;
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*;
+  abstract member-signature operator ==(dynamic other) → core::bool*;
+  abstract member-signature get hashCode() → core::int*;
+  abstract member-signature method toString() → core::String*;
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic;
+  abstract member-signature get runtimeType() → core::Type*;
+}
 static method main() → dynamic {
   new self::DynamicClass::•<core::num*, core::int*>(0.5, 2).{self::DynamicClass::method}();
   new self::NumClass::•<core::num*, core::double*>(2, 0.5).{self::NumClass::method1}();

pkg/front_end/testcases/general/type_variable_bound_access.dart.textual_outline.expect

@@ -13,4 +13,15 @@ class NumClass<T extends num, S extends T> {
   num method2() => field1 + field2.length;
 }
 
+class Class<X5 extends X4, X4 extends X3, X3 extends X2, X2 extends X1,
+    X1 extends X0, X0 extends int> {
+  X0 field0;
+  X1 field1;
+  X2 field2;
+  X3 field3;
+  X4 field4;
+  X5 field5;
+  method() {}
+}
+
 main() {}

pkg/front_end/testcases/general/type_variable_bound_access.dart.textual_outline_modelled.expect

@@ -1,3 +1,14 @@
+class Class<X5 extends X4, X4 extends X3, X3 extends X2, X2 extends X1,
+    X1 extends X0, X0 extends int> {
+  X0 field0;
+  X1 field1;
+  X2 field2;
+  X3 field3;
+  X4 field4;
+  X5 field5;
+  method() {}
+}
+
 class DynamicClass<T extends dynamic, S extends T> {
   DynamicClass(this.field1, this.field2);
   T field1;