1 files changed, 94 insertions, 0 deletions
diff --git a/test/typeparam/absdiff.go b/test/typeparam/absdiff.go
new file mode 100644
index 0000000..9c83eff
--- /dev/null
+++ b/test/typeparam/absdiff.go
@@ -0,0 +1,94 @@
+// run
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+type Numeric interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64 |
+		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
+		~float32 | ~float64 |
+		~complex64 | ~complex128
+}
+
+// numericAbs matches numeric types with an Abs method.
+type numericAbs[T any] interface {
+	Numeric
+	Abs() T
+}
+
+// AbsDifference computes the absolute value of the difference of
+// a and b, where the absolute value is determined by the Abs method.
+func absDifference[T numericAbs[T]](a, b T) T {
+	d := a - b
+	return d.Abs()
+}
+
+// orderedNumeric matches numeric types that support the < operator.
+type orderedNumeric interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64 |
+		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
+		~float32 | ~float64
+}
+
+// Complex matches the two complex types, which do not have a < operator.
+type Complex interface {
+	~complex64 | ~complex128
+}
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// // orderedAbs is a helper type that defines an Abs method for
+// // ordered numeric types.
+// type orderedAbs[T orderedNumeric] T
+//
+// func (a orderedAbs[T]) Abs() orderedAbs[T] {
+// 	if a < 0 {
+// 		return -a
+// 	}
+// 	return a
+// }
+//
+// // complexAbs is a helper type that defines an Abs method for
+// // complex types.
+// type complexAbs[T Complex] T
+//
+// func (a complexAbs[T]) Abs() complexAbs[T] {
+// 	r := float64(real(a))
+// 	i := float64(imag(a))
+// 	d := math.Sqrt(r*r + i*i)
+// 	return complexAbs[T](complex(d, 0))
+// }
+//
+// // OrderedAbsDifference returns the absolute value of the difference
+// // between a and b, where a and b are of an ordered type.
+// func orderedAbsDifference[T orderedNumeric](a, b T) T {
+// 	return T(absDifference(orderedAbs[T](a), orderedAbs[T](b)))
+// }
+//
+// // ComplexAbsDifference returns the absolute value of the difference
+// // between a and b, where a and b are of a complex type.
+// func complexAbsDifference[T Complex](a, b T) T {
+// 	return T(absDifference(complexAbs[T](a), complexAbs[T](b)))
+// }
+
+func main() {
+	// // For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+	// if got, want := orderedAbsDifference(1.0, -2.0), 3.0; got != want {
+	// 	panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	// }
+	// if got, want := orderedAbsDifference(-1.0, 2.0), 3.0; got != want {
+	// 	panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	// }
+	// if got, want := orderedAbsDifference(-20, 15), 35; got != want {
+	// 	panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	// }
+	//
+	// if got, want := complexAbsDifference(5.0+2.0i, 2.0-2.0i), 5+0i; got != want {
+	// 	panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	// }
+	// if got, want := complexAbsDifference(2.0-2.0i, 5.0+2.0i), 5+0i; got != want {
+	// 	panic(fmt.Sprintf("got = %v, want = %v", got, want))
+	// }
+}