go/types, types2: document nil scope for imported and instantiated Func objects

Also, don't set the scope anymore when instantiating (substituting)
a signature.

Per discussion with rfindley.

Change-Id: I560d4571c7ff14b0df3e15fece634cb5f9f94a99
Reviewed-on: https://go-review.googlesource.com/c/go/+/363435
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/object.go

@@ -389,6 +389,8 @@ func (obj *Func) FullName() string {
 }
 
 // Scope returns the scope of the function's body block.
+// The result is nil for imported or instantiated functions and methods
+// (but there is also no mechanism to get to an instantiated function).
 func (obj *Func) Scope() *Scope { return obj.typ.(*Signature).scope }
 
 // hasPtrRecv reports whether the receiver is of the form *T for the given method obj.

src/cmd/compile/internal/types2/signature.go

@@ -18,7 +18,7 @@ type Signature struct {
 	// We then unpack the *Signature and use the scope for the literal body.
 	rparams  *TypeParamList // receiver type parameters from left to right, or nil
 	tparams  *TypeParamList // type parameters from left to right, or nil
-	scope    *Scope         // function scope, present for package-local signatures
+	scope    *Scope         // function scope for package-local and non-instantiated signatures; nil otherwise
 	recv     *Var           // nil if not a method
 	params   *Tuple         // (incoming) parameters from left to right; or nil
 	results  *Tuple         // (outgoing) results from left to right; or nil

src/cmd/compile/internal/types2/subst.go

@@ -115,8 +115,8 @@ func (subst *subster) typ(typ Type) Type {
 			return &Signature{
 				rparams: t.rparams,
 				// TODO(gri) why can't we nil out tparams here, rather than in instantiate?
-				tparams:  t.tparams,
-				scope:    t.scope,
+				tparams: t.tparams,
+				// instantiated signatures have a nil scope
 				recv:     recv,
 				params:   params,
 				results:  results,

src/go/types/object.go

@@ -343,6 +343,8 @@ func (obj *Func) FullName() string {
 }
 
 // Scope returns the scope of the function's body block.
+// The result is nil for imported or instantiated functions and methods
+// (but there is also no mechanism to get to an instantiated function).
 func (obj *Func) Scope() *Scope { return obj.typ.(*Signature).scope }
 
 // hasPtrRecv reports whether the receiver is of the form *T for the given method obj.

src/go/types/signature.go

@@ -21,7 +21,7 @@ type Signature struct {
 	// We then unpack the *Signature and use the scope for the literal body.
 	rparams  *TypeParamList // receiver type parameters from left to right, or nil
 	tparams  *TypeParamList // type parameters from left to right, or nil
-	scope    *Scope         // function scope, present for package-local signatures
+	scope    *Scope         // function scope for package-local and non-instantiated signatures; nil otherwise
 	recv     *Var           // nil if not a method
 	params   *Tuple         // (incoming) parameters from left to right; or nil
 	results  *Tuple         // (outgoing) results from left to right; or nil

src/go/types/subst.go

@@ -115,8 +115,8 @@ func (subst *subster) typ(typ Type) Type {
 			return &Signature{
 				rparams: t.rparams,
 				// TODO(rFindley) why can't we nil out tparams here, rather than in instantiate?
-				tparams:  t.tparams,
-				scope:    t.scope,
+				tparams: t.tparams,
+				// instantiated signatures have a nil scope
 				recv:     recv,
 				params:   params,
 				results:  results,