[cfe] Handle callable type variables.

Change-Id: Ibd963206bb81fef16f3cb410fc352e636b0596e0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/146580
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

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

@@ -1694,6 +1694,7 @@ class TypeInferrerImpl implements TypeInferrer {
   }
 
   FunctionType _getFunctionType(DartType calleeType) {
+    calleeType = resolveTypeParameter(calleeType);
     if (calleeType is FunctionType) {
       return calleeType;
     }
@@ -1701,6 +1702,7 @@ class TypeInferrerImpl implements TypeInferrer {
   }
 
   FunctionType getFunctionTypeForImplicitCall(DartType calleeType) {
+    calleeType = resolveTypeParameter(calleeType);
     if (calleeType is FunctionType) {
       return calleeType;
     } else if (calleeType is InterfaceType) {

pkg/front_end/testcases/general/callable_type_variable.dart

@@ -0,0 +1,33 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class Class1<T extends Function> {
+  T field;
+
+  Class1(this.field);
+
+  method() {
+    var v1 = field(); // ok
+    var v2 = field(0); // ok
+    var v3 = field.call; // ok
+    var v4 = field.call(); // ok
+    var v5 = field.call(0); // ok
+  }
+}
+
+class Class2<T extends String Function(int)> {
+  T field;
+
+  Class2(this.field);
+
+  method() {
+    var v1 = field(); // error
+    var v2 = field(0); // ok
+    var v3 = field.call; // ok
+    var v4 = field.call(); // error
+    var v5 = field.call(0); // ok
+  }
+}
+
+main() {}

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

@@ -0,0 +1,20 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Class1<T extends core::Function* = core::Function*> extends core::Object {
+  generic-covariant-impl field self::Class1::T* field;
+  constructor •(self::Class1::T* field) → self::Class1<self::Class1::T*>*
+    ;
+  method method() → dynamic
+    ;
+}
+class Class2<T extends (core::int*) →* core::String* = (core::int*) →* core::String*> extends core::Object {
+  generic-covariant-impl field self::Class2::T* field;
+  constructor •(self::Class2::T* field) → self::Class2<self::Class2::T*>*
+    ;
+  method method() → dynamic
+    ;
+}
+static method main() → dynamic
+  ;

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

@@ -0,0 +1,46 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/callable_type_variable.dart:25:19: Error: Too few positional arguments: 1 required, 0 given.
+//     var v1 = field(); // error
+//                   ^
+//
+// pkg/front_end/testcases/general/callable_type_variable.dart:28:24: Error: Too few positional arguments: 1 required, 0 given.
+//     var v4 = field.call(); // error
+//                        ^
+//
+import self as self;
+import "dart:core" as core;
+
+class Class1<T extends core::Function* = core::Function*> extends core::Object {
+  generic-covariant-impl field self::Class1::T* field;
+  constructor •(self::Class1::T* field) → self::Class1<self::Class1::T*>*
+    : self::Class1::field = field, super core::Object::•()
+    ;
+  method method() → dynamic {
+    dynamic v1 = this.{self::Class1::field}.call();
+    dynamic v2 = let final core::int* #t1 = 0 in this.{self::Class1::field}.call(#t1);
+    self::Class1::T* v3 = this.{self::Class1::field}.call;
+    dynamic v4 = this.{self::Class1::field}.call();
+    dynamic v5 = this.{self::Class1::field}.call(0);
+  }
+}
+class Class2<T extends (core::int*) →* core::String* = (core::int*) →* core::String*> extends core::Object {
+  generic-covariant-impl field self::Class2::T* field;
+  constructor •(self::Class2::T* field) → self::Class2<self::Class2::T*>*
+    : self::Class2::field = field, super core::Object::•()
+    ;
+  method method() → dynamic {
+    invalid-type v1 = let final<BottomType> #t2 = invalid-expression "pkg/front_end/testcases/general/callable_type_variable.dart:25:19: Error: Too few positional arguments: 1 required, 0 given.
+    var v1 = field(); // error
+                  ^" in this.{self::Class2::field}.call();
+    core::String* v2 = let final core::int* #t3 = 0 in this.{self::Class2::field}.call(#t3);
+    self::Class2::T* v3 = this.{self::Class2::field}.call;
+    invalid-type v4 = let final<BottomType> #t4 = invalid-expression "pkg/front_end/testcases/general/callable_type_variable.dart:28:24: Error: Too few positional arguments: 1 required, 0 given.
+    var v4 = field.call(); // error
+                       ^" in this.{self::Class2::field}.call();
+    core::String* v5 = this.{self::Class2::field}.call(0);
+  }
+}
+static method main() → dynamic {}

pkg/front_end/testcases/strong.status

@@ -20,6 +20,7 @@ general/bug21938: TypeCheckError
 general/bug30695: TypeCheckError
 general/bug31124: RuntimeError # Test has no main method (and we shouldn't add one).
 general/call: TypeCheckError
+general/callable_type_variable: TypeCheckError
 general/candidate_found: TypeCheckError
 general/cascade: RuntimeError
 general/constructor_initializer_invalid: RuntimeError # Fails execution after recovery

pkg/front_end/testcases/text_serialization.status

@@ -20,6 +20,7 @@ general/bug21938: TypeCheckError
 general/bug30695: TypeCheckError
 general/bug31124: RuntimeError
 general/call: TypeCheckError
+general/callable_type_variable: TypeCheckError
 general/candidate_found: TypeCheckError
 general/cascade: RuntimeError
 general/constructor_initializer_invalid: RuntimeError