cmd/compile/internal/types2: move NewTypeParam off of Checker

This is a port of CL 347561.

A comment was corrected both in types2 and go/types, and the
compiler adjusted for the updated NewTypeParameter function.

Change-Id: I4381f0dd8e43228e1d037c5d997d421b7838f905
Reviewed-on: https://go-review.googlesource.com/c/go/+/348574
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/importer/iimport.go

@@ -365,7 +365,7 @@ func (r *importReader) obj(name string) {
 		}
 		name0, sub := parseSubscript(name)
 		tn := types2.NewTypeName(pos, r.currPkg, name0, nil)
-		t := (*types2.Checker)(nil).NewTypeParam(tn, nil)
+		t := types2.NewTypeParam(tn, nil)
 		if sub == 0 {
 			errorf("missing subscript")
 		}

src/cmd/compile/internal/noder/reader2.go

@@ -483,7 +483,7 @@ func (r *reader2) typeParamNames() []*types2.TypeParam {
 		pkg, name := r.localIdent()
 
 		tname := types2.NewTypeName(pos, pkg, name, nil)
-		r.dict.tparams[i] = r.p.check.NewTypeParam(tname, nil)
+		r.dict.tparams[i] = types2.NewTypeParam(tname, nil)
 	}
 
 	for i, bound := range r.dict.bounds {

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

@@ -826,7 +826,7 @@ 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(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/cmd/compile/internal/types2/decl.go

@@ -648,7 +648,7 @@ func (check *Checker) declareTypeParam(name *syntax.Name) *TypeParam {
 	//           constraints to make sure we don't rely on them if they
 	//           are not properly set yet.
 	tname := NewTypeName(name.Pos(), check.pkg, name.Value, nil)
-	tpar := check.NewTypeParam(tname, Typ[Invalid])          // assigns type to tname as a side-effect
+	tpar := check.newTypeParam(tname, Typ[Invalid])          // assigns type to tname as a side-effect
 	check.declare(check.scope, name, tname, check.scope.pos) // TODO(gri) check scope position
 	return tpar
 }

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

@@ -32,15 +32,19 @@ func (t *TypeParam) Obj() *TypeName { return t.obj }
 // 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.
+// The constraint argument can be nil, and set later via SetConstraint.
-func (check *Checker) NewTypeParam(obj *TypeName, bound Type) *TypeParam {
+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
 	}

src/go/types/typeparam.go

@@ -32,7 +32,7 @@ type TypeParam struct {
 // or Signature type by calling SetTypeParams. Setting a type parameter on more
 // than one type will result in a panic.
 //
-// The bound argument can be nil, and set later via SetConstraint.
+// The constraint argument can be nil, and set later via SetConstraint.
 func NewTypeParam(obj *TypeName, constraint Type) *TypeParam {
 	return (*Checker)(nil).newTypeParam(obj, constraint)
 }