Migration: add void type support to DecoratedType.forImplicitType.

Change-Id: Ifae57627d13a4913d86ab8a4789615a23ec66c1c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/107441
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

Author: stereotype441

pkg/nnbd_migration/lib/src/decorated_type.dart

@@ -136,7 +136,7 @@ class DecoratedType {
   /// nodes everywhere that don't correspond to any source location.  These
   /// nodes can later be unioned with other nodes.
   factory DecoratedType.forImplicitType(DartType type, NullabilityGraph graph) {
-    if (type.isDynamic) {
+    if (type.isDynamic || type.isVoid) {
       return DecoratedType(type, graph.always);
     } else if (type is InterfaceType) {
       return DecoratedType(type, NullabilityNode.forInferredType(),

pkg/nnbd_migration/test/node_builder_test.dart

@@ -421,6 +421,24 @@ void f(C x) {}
     expect(decoratedArgReturnType.typeArguments, isEmpty);
   }
 
+  test_interfaceType_generic_instantiate_to_function_type_void() async {
+    await analyze('''
+class C<T extends void Function()> {}
+void f(C x) {}
+''');
+    var decoratedCType = decoratedTypeAnnotation('C x');
+    expect(decoratedFunctionType('f').positionalParameters[0],
+        same(decoratedCType));
+    expect(decoratedCType.node, TypeMatcher<NullabilityNodeMutable>());
+    expect(decoratedCType.typeArguments, hasLength(1));
+    var decoratedArgType = decoratedCType.typeArguments[0];
+    expect(decoratedArgType.node, TypeMatcher<NullabilityNodeMutable>());
+    expect(decoratedArgType.typeArguments, isEmpty);
+    var decoratedArgReturnType = decoratedArgType.returnType;
+    expect(decoratedArgReturnType.node, same(always));
+    expect(decoratedArgReturnType.typeArguments, isEmpty);
+  }
+
   test_interfaceType_generic_instantiate_to_generic_type() async {
     await analyze('''
 class C<T> {}