diff --git a/src/cmd/compile/internal/types2/builtins.go b/src/cmd/compile/internal/types2/builtins.go
index 7ba26509e8d..83d1743ee2f 100644
--- a/src/cmd/compile/internal/types2/builtins.go
+++ b/src/cmd/compile/internal/types2/builtins.go
@@ -332,13 +332,15 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 			return
 		}
 		var src Type
-		switch t := optype(y.typ).(type) {
+		switch t := under(y.typ).(type) {
 		case *Basic:
 			if isString(y.typ) {
 				src = universeByte
 			}
 		case *Slice:
 			src = t.elem
+		case *TypeParam:
+			check.error(x, "copy on generic operands not yet implemented")
 		}
 
 		if dst == nil || src == nil {
@@ -455,12 +457,12 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 		var valid func(t Type) bool
 		valid = func(t Type) bool {
 			var m int
-			switch t := optype(t).(type) {
+			switch t := under(t).(type) {
 			case *Slice:
 				m = 2
 			case *Map, *Chan:
 				m = 1
-			case *Union:
+			case *TypeParam:
 				return t.underIs(valid)
 			default:
 				return false
diff --git a/src/cmd/compile/internal/types2/expr.go b/src/cmd/compile/internal/types2/expr.go
index 1cb0ad4752b..bd35417c645 100644
--- a/src/cmd/compile/internal/types2/expr.go
+++ b/src/cmd/compile/internal/types2/expr.go
@@ -691,7 +691,7 @@ func (check *Checker) implicitTypeAndValue(x *operand, target Type) (Type, const
 		return nil, nil, _InvalidUntypedConversion
 	}
 
-	switch t := optype(target).(type) {
+	switch t := under(target).(type) {
 	case *Basic:
 		if x.mode == constant_ {
 			v, code := check.representation(x, t)
@@ -723,7 +723,7 @@ func (check *Checker) implicitTypeAndValue(x *operand, target Type) (Type, const
 		default:
 			return nil, nil, _InvalidUntypedConversion
 		}
-	case *Union:
+	case *TypeParam:
 		ok := t.underIs(func(t Type) bool {
 			target, _, _ := check.implicitTypeAndValue(x, t)
 			return target != nil
@@ -1197,7 +1197,7 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 			goto Error
 		}
 
-		switch utyp := optype(base).(type) {
+		switch utyp := under(base).(type) {
 		case *Struct:
 			if len(e.ElemList) == 0 {
 				break
diff --git a/src/cmd/compile/internal/types2/index.go b/src/cmd/compile/internal/types2/index.go
index 5a4dcb47417..d3e0c71f05d 100644
--- a/src/cmd/compile/internal/types2/index.go
+++ b/src/cmd/compile/internal/types2/index.go
@@ -199,7 +199,7 @@ func (check *Checker) sliceExpr(x *operand, e *syntax.SliceExpr) {
 
 	valid := false
 	length := int64(-1) // valid if >= 0
-	switch typ := optype(x.typ).(type) {
+	switch typ := under(x.typ).(type) {
 	case *Basic:
 		if isString(typ) {
 			if e.Full {
@@ -239,7 +239,7 @@ func (check *Checker) sliceExpr(x *operand, e *syntax.SliceExpr) {
 		valid = true
 		// x.typ doesn't change
 
-	case *Union, *TypeParam:
+	case *TypeParam:
 		check.error(x, "generic slice expressions not yet implemented")
 		x.mode = invalid
 		return
diff --git a/src/cmd/compile/internal/types2/predicates.go b/src/cmd/compile/internal/types2/predicates.go
index 5ff7840d6f9..2f108985857 100644
--- a/src/cmd/compile/internal/types2/predicates.go
+++ b/src/cmd/compile/internal/types2/predicates.go
@@ -25,10 +25,10 @@ func isGeneric(typ Type) bool {
 }
 
 func is(typ Type, what BasicInfo) bool {
-	switch t := optype(typ).(type) {
+	switch t := under(typ).(type) {
 	case *Basic:
 		return t.info&what != 0
-	case *Union:
+	case *TypeParam:
 		return t.underIs(func(t Type) bool { return is(t, what) })
 	}
 	return false
@@ -56,7 +56,7 @@ func isNumericOrString(typ Type) bool { return is(typ, IsNumeric|IsString) }
 // are not fully set up.
 func isTyped(typ Type) bool {
 	// isTyped is called with types that are not fully
-	// set up. Must not call Basic()!
+	// set up. Must not call asBasic()!
 	// A *Named or *instance type is always typed, so
 	// we only need to check if we have a true *Basic
 	// type.
@@ -97,18 +97,19 @@ func comparable(T Type, seen map[Type]bool) bool {
 	seen[T] = true
 
 	// If T is a type parameter not constrained by any type
-	// list (i.e., it's operational type is the top type),
+	// (i.e., it's operational type is the top type),
 	// T is comparable if it has the == method. Otherwise,
 	// the operational type "wins". For instance
 	//
 	//     interface{ comparable; type []byte }
 	//
 	// is not comparable because []byte is not comparable.
+	// TODO(gri) this code is not 100% correct (see comment for TypeSet.IsComparable)
 	if t := asTypeParam(T); t != nil && optype(t) == theTop {
 		return t.Bound().IsComparable()
 	}
 
-	switch t := optype(T).(type) {
+	switch t := under(T).(type) {
 	case *Basic:
 		// assume invalid types to be comparable
 		// to avoid follow-up errors
@@ -124,24 +125,22 @@ func comparable(T Type, seen map[Type]bool) bool {
 		return true
 	case *Array:
 		return comparable(t.elem, seen)
-	case *Union:
+	case *TypeParam:
 		return t.underIs(func(t Type) bool {
 			return comparable(t, seen)
 		})
-	case *TypeParam:
-		return t.Bound().IsComparable()
 	}
 	return false
 }
 
 // hasNil reports whether a type includes the nil value.
 func hasNil(typ Type) bool {
-	switch t := optype(typ).(type) {
+	switch t := under(typ).(type) {
 	case *Basic:
 		return t.kind == UnsafePointer
 	case *Slice, *Pointer, *Signature, *Interface, *Map, *Chan:
 		return true
-	case *Union:
+	case *TypeParam:
 		return t.underIs(hasNil)
 	}
 	return false
diff --git a/src/cmd/compile/internal/types2/testdata/check/typeparams.go2 b/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
index 123567682a3..8a7f6eb2c25 100644
--- a/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
@@ -119,7 +119,7 @@ func _[T interface{ [10]int | *[20]int | []int }](x T, i int) { _ = x[i]; _ = x[
 // slicing
 // TODO(gri) implement this
 
-func _[T interface{ ~string }] (x T, i, j, k int) { _ = x /* ERROR invalid operation */ [i:j:k] }
+func _[T interface{ ~string }] (x T, i, j, k int) { _ = x /* ERROR generic slice expressions not yet implemented */ [i:j:k] }
 
 // len/cap built-ins
 
diff --git a/src/cmd/compile/internal/types2/typeset.go b/src/cmd/compile/internal/types2/typeset.go
index 6ff85639748..4aee8e40972 100644
--- a/src/cmd/compile/internal/types2/typeset.go
+++ b/src/cmd/compile/internal/types2/typeset.go
@@ -28,6 +28,7 @@ func (s *TypeSet) IsTop() bool { return len(s.methods) == 0 && s.types == nil }
 func (s *TypeSet) IsMethodSet() bool { return s.types == nil && !s.IsComparable() }
 
 // IsComparable reports whether each type in the set is comparable.
+// TODO(gri) this is not correct - there may be s.types values containing non-comparable types
 func (s *TypeSet) IsComparable() bool {
 	_, m := s.LookupMethod(nil, "==")
 	return m != nil