test/gen: add some FromStrings test cases

These are from the latest version of the design draft.

There are still a couple of FromStrings test cases that don't yet work.

Change-Id: I057875820d8250012c06faeabda4637f0585b6f8
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go2-dev/+/763941
Reviewed-by: Ian Lance Taylor <iant@google.com>

test/gen/g008.go2

@@ -0,0 +1,41 @@
+// 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 Setter interface {
+	Set(string)
+}
+
+func FromStrings(type T Setter)(s []string) []T {
+	result := make([]T, len(s))
+	for i, v := range s {
+		result[i].Set(v)
+	}
+	return result
+}
+
+type Settable int
+
+func (p *Settable) Set(s string) {
+	*p = 0
+}
+
+func F() {
+	defer func() {
+		if recover() == nil {
+			panic("did not panic as expected")
+		}
+	}()
+	// This should type check but should panic at run time,
+	// because it will make a slice of *Settable and then call
+	// Set on a nil value.
+	FromStrings(*Settable)([]string{"1"})
+}
+
+func main() {
+	F()
+}

test/gen/g009.go2

@@ -0,0 +1,43 @@
+// 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
+
+import (
+	"log"
+	"strconv"
+)
+
+type Setter2(type B) interface {
+	Set(string)
+	type *B
+}
+
+func FromStrings2(type T interface{}, PT Setter2(T))(s []string) []T {
+	result := make([]T, len(s))
+	for i, v := range s {
+		p := PT(&result[i])
+		p.Set(v)
+	}
+	return result
+}
+
+type Settable int
+
+func (p *Settable) Set(s string) {
+	i, err := strconv.Atoi(s)
+	if err != nil {
+		log.Fatal(err)
+	}
+	*p = Settable(i)
+}
+
+func main() {
+	s := FromStrings2(Settable, *Settable)([]string{"1"})
+	if len(s) != 1 || s[0] != 1 {
+		log.Fatalf("got %v, want %v", s, []int{1})
+	}
+}

test/gen/g010.go2

@@ -0,0 +1,38 @@
+// 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
+
+import (
+	"log"
+	"strconv"
+)
+
+func FromStrings3(type T)(s []string, set func(*T, string)) []T {
+	results := make([]T, len(s))
+	for i, v := range s {
+		set(&results[i], v)
+	}
+	return results
+}
+
+type Settable int
+
+func (p *Settable) Set(s string) {
+	i, err := strconv.Atoi(s)
+	if err != nil {
+		log.Fatal(err)
+	}
+	*p = Settable(i)
+}
+
+func main() {
+	s := FromStrings3(Settable)([]string{"1"},
+		func(p *Settable, s string) { p.Set(s) })
+	if len(s) != 1 || s[0] != 1 {
+		log.Fatalf("got %v, want %v", s, []int{1})
+	}
+}