go/types: move NewTypeParam off of Checker

This aligns with the API proposal.

Change-Id: I9967a317196392ffa5ddbe5391d7aba5f6e7bad2
Reviewed-on: https://go-review.googlesource.com/c/go/+/347561
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/internal/gcimporter/iimport.go

@@ -355,7 +355,7 @@ func (r *importReader) obj(name string) {
 		}
 		name0, sub := parseSubscript(name)
 		tn := types.NewTypeName(pos, r.currPkg, name0, nil)
-		t := (*types.Checker)(nil).NewTypeParam(tn, nil)
+		t := types.NewTypeParam(tn, nil)
 		if sub == 0 {
 			errorf("missing subscript")
 		}

src/go/types/builtins.go

@@ -830,7 +830,7 @@ func (check *Checker) applyTypeFunc(f func(Type) Type, x Type) Type {
 		// type param is placed in the current package so export/import
 		// works as expected.
 		tpar := NewTypeName(token.NoPos, check.pkg, "<type parameter>", nil)
-		ptyp := check.NewTypeParam(tpar, NewInterfaceType(nil, []Type{NewUnion(terms)})) // assigns type to tpar as a side-effect
+		ptyp := check.newTypeParam(tpar, NewInterfaceType(nil, []Type{NewUnion(terms)})) // assigns type to tpar as a side-effect
 		ptyp.index = tp.index
 
 		return ptyp

src/go/types/decl.go

@@ -677,7 +677,7 @@ func (check *Checker) collectTypeParams(list *ast.FieldList) *TParamList {
 func (check *Checker) declareTypeParams(tparams []*TypeParam, names []*ast.Ident) []*TypeParam {
 	for _, name := range names {
 		tname := NewTypeName(name.Pos(), check.pkg, name.Name, nil)
-		tpar := check.NewTypeParam(tname, &emptyInterface)       // assigns type to tpar as a side-effect
+		tpar := check.newTypeParam(tname, &emptyInterface)       // assigns type to tpar as a side-effect
 		check.declare(check.scope, name, tname, check.scope.pos) // TODO(gri) check scope position
 		tparams = append(tparams, tpar)
 	}

src/go/types/typeparam.go

@@ -32,15 +32,19 @@ type TypeParam struct {
 // or Signature type by calling SetTParams. Setting a type parameter on more
 // than one type will result in a panic.
 //
-// The bound argument can be nil, and set later via SetBound.
-func (check *Checker) NewTypeParam(obj *TypeName, bound Type) *TypeParam {
+// The bound argument can be nil, and set later via SetConstraint.
+func NewTypeParam(obj *TypeName, constraint Type) *TypeParam {
+	return (*Checker)(nil).newTypeParam(obj, constraint)
+}
+
+func (check *Checker) newTypeParam(obj *TypeName, constraint Type) *TypeParam {
 	// Always increment lastID, even if it is not used.
 	id := nextID()
 	if check != nil {
 		check.nextID++
 		id = check.nextID
 	}
-	typ := &TypeParam{check: check, id: id, obj: obj, index: -1, bound: bound}
+	typ := &TypeParam{check: check, id: id, obj: obj, index: -1, bound: constraint}
 	if obj.typ == nil {
 		obj.typ = typ
 	}