encoding/xml: truncate generic type names

xml names can't have any of '[],' in them, which might appear in
generic type names. Truncate at the first '[' so the names are still valid.

Fixes #48318

Change-Id: I110ff4269f763089467e7cf84b0f0c5075fb44b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/349349
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/encoding/xml/marshal.go

@@ -494,6 +494,10 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 	}
 	if start.Name.Local == "" {
 		name := typ.Name()
+		if i := strings.IndexByte(name, '['); i >= 0 {
+			// Truncate generic instantiation name. See issue 48318.
+			name = name[:i]
+		}
 		if name == "" {
 			return &UnsupportedTypeError{typ}
 		}

test/typeparam/issue48318.go

@@ -0,0 +1,33 @@
+// run -gcflags=-G=3
+
+// 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 (
+	"encoding/xml"
+	"fmt"
+)
+
+type A[T, U any] struct {
+	Name T `xml:"name"`
+	Data U `xml:"data"`
+}
+
+func main() {
+	src := &A[string, int]{Name: "name", Data: 1}
+	data, err := xml.Marshal(src)
+	if err != nil {
+		panic(err)
+	}
+	dst := &A[string, int]{}
+	err = xml.Unmarshal(data, dst)
+	if err != nil {
+		panic(err)
+	}
+	if *src != *dst {
+		panic(fmt.Sprintf("wanted %#v got %#v", src, dst))
+	}
+}