go/types: provide accessor to type arguments for instantiated types

Plus a couple of minor unrelated comment updates.

Change-Id: I38554c9ae9143172ea65da6c270fba10772c194b

src/go/types/NOTES

@@ -4,7 +4,6 @@ so we have a better track record. I only switched to this file in Nov 2019, henc
 ----------------------------------------------------------------------------------------------------
 TODO
 
-- type assertions on/against parameterized types
 - use Underlying() to return a type parameter's bound? investigate!
 - debug (and error msg) printing of generic instantiated types needs some work
 - improve error messages!

src/go/types/lookup.go

@@ -390,8 +390,8 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 // method required by V and whether it is missing or just has the wrong type.
 // The receiver may be nil if assertableTo is invoked through an exported API call
 // (such as AssertableTo), i.e., when all methods have been type-checked.
-// If strict (or the global constant forceStrict) is set, assertions are strict;
-// i.e., assertions that are guaranteed to fail are not permitted.
+// If strict (or the global constant forceStrict) is set, assertions that
+// are known to fail are not permitted.
 func (check *Checker) assertableTo(V *Interface, T Type, strict bool) (method, wrongType *Func) {
 	// no static check is required if T is an interface
 	// spec: "If T is an interface type, x.(T) asserts that the

src/go/types/type.go

@@ -512,9 +512,16 @@ func NewNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
 // Obj returns the type name for the named type t.
 func (t *Named) Obj() *TypeName { return t.obj }
 
+// TODO(gri) Come up with a better representation and API to distinguish
+//           between parameterized instantiated and non-instantiated types.
+
 // TParams returns the type parameters of the named type t, or nil.
+// The result is non-nil for an (originally) parameterized type even if it is instantiated.
 func (t *Named) TParams() []*TypeName { return t.tparams }
 
+// TArgs returns the type arguments after instantiation of the named type t, or nil if not instantiated.
+func (t *Named) TArgs() []Type { return t.targs }
+
 // NumMethods returns the number of explicit methods whose receiver is named type t.
 func (t *Named) NumMethods() int { return len(t.methods) }