[dev.typeparams] go/types: use the TParams API consistently

Even internally to the type checker, we should use the TParams and
RParams methods instead of accessing fields directly, as TParams may be
lazily expanded, and in the future we may want to pack receiver and
function type parameters into a single field on Signature.

We should also not differentiate a nil *TParamList from an empty
*TParamList.

Change-Id: I85c616e6c708a89b6a5eb1e69fe0b014276eda90
Reviewed-on: https://go-review.googlesource.com/c/go/+/336251
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: findleyr

src/go/types/call.go

@@ -61,7 +61,7 @@ func (check *Checker) funcInst(x *operand, ix *typeparams.IndexExpr) {
 
 	// instantiate function signature
 	res := check.Instantiate(x.Pos(), sig, targs, poslist, true).(*Signature)
-	assert(res.tparams == nil) // signature is not generic anymore
+	assert(res.TParams().Len() == 0) // signature is not generic anymore
 	if inferred {
 		check.recordInferred(ix.Orig, targs, res)
 	}
@@ -334,7 +334,7 @@ func (check *Checker) arguments(call *ast.CallExpr, sig *Signature, targs []Type
 
 		// compute result signature
 		rsig = check.Instantiate(call.Pos(), sig, targs, nil, true).(*Signature)
-		assert(rsig.tparams == nil) // signature is not generic anymore
+		assert(rsig.TParams().Len() == 0) // signature is not generic anymore
 		check.recordInferred(call, targs, rsig)
 
 		// Optimization: Only if the parameter list was adjusted do we

src/go/types/decl.go

@@ -625,7 +625,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *Named) {
 	named.underlying = under(named)
 
 	// If the RHS is a type parameter, it must be from this type declaration.
-	if tpar, _ := named.underlying.(*TypeParam); tpar != nil && tparamIndex(named.tparams.list(), tpar) < 0 {
+	if tpar, _ := named.underlying.(*TypeParam); tpar != nil && tparamIndex(named.TParams().list(), tpar) < 0 {
 		check.errorf(tdecl.Type, _Todo, "cannot use function type parameter %s as RHS in type declaration", tpar)
 		named.underlying = Typ[Invalid]
 	}

src/go/types/subst.go

@@ -193,7 +193,7 @@ func (subst *subster) typ(typ Type) Type {
 			}
 		}
 
-		if t.TParams() == nil {
+		if t.TParams().Len() == 0 {
 			dump(">>> %s is not parameterized", t)
 			return t // type is not parameterized
 		}

src/go/types/typestring.go

@@ -276,7 +276,7 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 			buf.WriteByte('[')
 			writeTypeList(buf, t.targs, qf, visited)
 			buf.WriteByte(']')
-		} else if t.TParams() != nil {
+		} else if t.TParams().Len() != 0 {
 			// parameterized type
 			writeTParamList(buf, t.TParams().list(), qf, visited)
 		}
@@ -424,7 +424,7 @@ func WriteSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier) {
 }
 
 func writeSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier, visited []Type) {
-	if sig.tparams != nil {
+	if sig.TParams().Len() != 0 {
 		writeTParamList(buf, sig.TParams().list(), qf, visited)
 	}