1 files changed, 72 insertions, 0 deletions
diff --git a/test/typeparam/double.go b/test/typeparam/double.go
new file mode 100644
index 0000000..fbbe602
--- /dev/null
+++ b/test/typeparam/double.go
@@ -0,0 +1,72 @@
+// run
+
+// Copyright 2021 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
+
+import (
+	"fmt"
+	"reflect"
+)
+
+type Number interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64 | ~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr | ~float32 | ~float64
+}
+
+type MySlice []int
+type MyFloatSlice []float64
+
+type _SliceOf[E any] interface {
+	~[]E
+}
+
+func _DoubleElems[S _SliceOf[E], E Number](s S) S {
+	r := make(S, len(s))
+	for i, v := range s {
+		r[i] = v + v
+	}
+	return r
+}
+
+// Test use of untyped constant in an expression with a generically-typed parameter
+func _DoubleElems2[S _SliceOf[E], E Number](s S) S {
+	r := make(S, len(s))
+	for i, v := range s {
+		r[i] = v * 2
+	}
+	return r
+}
+
+func main() {
+	arg := MySlice{1, 2, 3}
+	want := MySlice{2, 4, 6}
+	got := _DoubleElems[MySlice, int](arg)
+	if !reflect.DeepEqual(got, want) {
+		panic(fmt.Sprintf("got %s, want %s", got, want))
+	}
+
+	// constraint type inference
+	got = _DoubleElems[MySlice](arg)
+	if !reflect.DeepEqual(got, want) {
+		panic(fmt.Sprintf("got %s, want %s", got, want))
+	}
+
+	got = _DoubleElems(arg)
+	if !reflect.DeepEqual(got, want) {
+		panic(fmt.Sprintf("got %s, want %s", got, want))
+	}
+
+	farg := MyFloatSlice{1.2, 2.0, 3.5}
+	fwant := MyFloatSlice{2.4, 4.0, 7.0}
+	fgot := _DoubleElems(farg)
+	if !reflect.DeepEqual(fgot, fwant) {
+		panic(fmt.Sprintf("got %s, want %s", fgot, fwant))
+	}
+
+	fgot = _DoubleElems2(farg)
+	if !reflect.DeepEqual(fgot, fwant) {
+		panic(fmt.Sprintf("got %s, want %s", fgot, fwant))
+	}
+}