Process generic function type arguments in more places (function type tests,

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 1413348aeffc678ad1ff61b2a08b608f57fa8bd7..b848a0ba4434c2ca1984594d5373ff24510c8ff1 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -1783,7 +1783,7 @@ class RawClosure : public RawInstance {
     return reinterpret_cast<RawObject**>(&ptr()->instantiator_type_arguments_);
   }
 
-  // No instance fields should be declared before the following 3 fields whose
+  // No instance fields should be declared before the following 4 fields whose
   // offsets must be identical in Dart and C++.
 
   // These 4 fields are also declared in the Dart source of class _Closure.
@@ -1793,6 +1793,16 @@ class RawClosure : public RawInstance {
   RawContext* context_;
 
   RawObject** to() { return reinterpret_cast<RawObject**>(&ptr()->context_); }
+
+  // Note that instantiator_type_arguments_ and function_type_arguments_ are
+  // used to instantiate the signature of function_ when this closure is
+  // involved in a type test. In other words, these fields define the function
+  // type of this closure instance, but they are not used when invoking it.
+  // If this closure is generic, it can be invoked with function type arguments
+  // that will be processed in the prolog of the closure function_. For example,
+  // if the generic closure function_ has a generic parent function, the
+  // passed-in function type arguments get concatenated to the function type
+  // arguments of the parent that are found in the context_.
 };