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diff --git a/src/html/template/error.go b/src/html/template/error.go
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+// Copyright 2011 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 template
+
+import (
+	"fmt"
+	"text/template/parse"
+)
+
+// Error describes a problem encountered during template Escaping.
+type Error struct {
+	// ErrorCode describes the kind of error.
+	ErrorCode ErrorCode
+	// Node is the node that caused the problem, if known.
+	// If not nil, it overrides Name and Line.
+	Node parse.Node
+	// Name is the name of the template in which the error was encountered.
+	Name string
+	// Line is the line number of the error in the template source or 0.
+	Line int
+	// Description is a human-readable description of the problem.
+	Description string
+}
+
+// ErrorCode is a code for a kind of error.
+type ErrorCode int
+
+// We define codes for each error that manifests while escaping templates, but
+// escaped templates may also fail at runtime.
+//
+// Output: "ZgotmplZ"
+// Example:
+//   <img src="{{.X}}">
+//   where {{.X}} evaluates to `javascript:...`
+// Discussion:
+//   "ZgotmplZ" is a special value that indicates that unsafe content reached a
+//   CSS or URL context at runtime. The output of the example will be
+//     <img src="#ZgotmplZ">
+//   If the data comes from a trusted source, use content types to exempt it
+//   from filtering: URL(`javascript:...`).
+const (
+	// OK indicates the lack of an error.
+	OK ErrorCode = iota
+
+	// ErrAmbigContext: "... appears in an ambiguous URL context"
+	// Example:
+	//   <a href="
+	//      {{if .C}}
+	//        /path/
+	//      {{else}}
+	//        /search?q=
+	//      {{end}}
+	//      {{.X}}
+	//   ">
+	// Discussion:
+	//   {{.X}} is in an ambiguous URL context since, depending on {{.C}},
+	//  it may be either a URL suffix or a query parameter.
+	//   Moving {{.X}} into the condition removes the ambiguity:
+	//   <a href="{{if .C}}/path/{{.X}}{{else}}/search?q={{.X}}">
+	ErrAmbigContext
+
+	// ErrBadHTML: "expected space, attr name, or end of tag, but got ...",
+	//   "... in unquoted attr", "... in attribute name"
+	// Example:
+	//   <a href = /search?q=foo>
+	//   <href=foo>
+	//   <form na<e=...>
+	//   <option selected<
+	// Discussion:
+	//   This is often due to a typo in an HTML element, but some runes
+	//   are banned in tag names, attribute names, and unquoted attribute
+	//   values because they can tickle parser ambiguities.
+	//   Quoting all attributes is the best policy.
+	ErrBadHTML
+
+	// ErrBranchEnd: "{{if}} branches end in different contexts"
+	// Example:
+	//   {{if .C}}<a href="{{end}}{{.X}}
+	// Discussion:
+	//   Package html/template statically examines each path through an
+	//   {{if}}, {{range}}, or {{with}} to escape any following pipelines.
+	//   The example is ambiguous since {{.X}} might be an HTML text node,
+	//   or a URL prefix in an HTML attribute. The context of {{.X}} is
+	//   used to figure out how to escape it, but that context depends on
+	//   the run-time value of {{.C}} which is not statically known.
+	//
+	//   The problem is usually something like missing quotes or angle
+	//   brackets, or can be avoided by refactoring to put the two contexts
+	//   into different branches of an if, range or with. If the problem
+	//   is in a {{range}} over a collection that should never be empty,
+	//   adding a dummy {{else}} can help.
+	ErrBranchEnd
+
+	// ErrEndContext: "... ends in a non-text context: ..."
+	// Examples:
+	//   <div
+	//   <div title="no close quote>
+	//   <script>f()
+	// Discussion:
+	//   Executed templates should produce a DocumentFragment of HTML.
+	//   Templates that end without closing tags will trigger this error.
+	//   Templates that should not be used in an HTML context or that
+	//   produce incomplete Fragments should not be executed directly.
+	//
+	//   {{define "main"}} <script>{{template "helper"}}</script> {{end}}
+	//   {{define "helper"}} document.write(' <div title=" ') {{end}}
+	//
+	//   "helper" does not produce a valid document fragment, so should
+	//   not be Executed directly.
+	ErrEndContext
+
+	// ErrNoSuchTemplate: "no such template ..."
+	// Examples:
+	//   {{define "main"}}<div {{template "attrs"}}>{{end}}
+	//   {{define "attrs"}}href="{{.URL}}"{{end}}
+	// Discussion:
+	//   Package html/template looks through template calls to compute the
+	//   context.
+	//   Here the {{.URL}} in "attrs" must be treated as a URL when called
+	//   from "main", but you will get this error if "attrs" is not defined
+	//   when "main" is parsed.
+	ErrNoSuchTemplate
+
+	// ErrOutputContext: "cannot compute output context for template ..."
+	// Examples:
+	//   {{define "t"}}{{if .T}}{{template "t" .T}}{{end}}{{.H}}",{{end}}
+	// Discussion:
+	//   A recursive template does not end in the same context in which it
+	//   starts, and a reliable output context cannot be computed.
+	//   Look for typos in the named template.
+	//   If the template should not be called in the named start context,
+	//   look for calls to that template in unexpected contexts.
+	//   Maybe refactor recursive templates to not be recursive.
+	ErrOutputContext
+
+	// ErrPartialCharset: "unfinished JS regexp charset in ..."
+	// Example:
+	//     <script>var pattern = /foo[{{.Chars}}]/</script>
+	// Discussion:
+	//   Package html/template does not support interpolation into regular
+	//   expression literal character sets.
+	ErrPartialCharset
+
+	// ErrPartialEscape: "unfinished escape sequence in ..."
+	// Example:
+	//   <script>alert("\{{.X}}")</script>
+	// Discussion:
+	//   Package html/template does not support actions following a
+	//   backslash.
+	//   This is usually an error and there are better solutions; for
+	//   example
+	//     <script>alert("{{.X}}")</script>
+	//   should work, and if {{.X}} is a partial escape sequence such as
+	//   "xA0", mark the whole sequence as safe content: JSStr(`\xA0`)
+	ErrPartialEscape
+
+	// ErrRangeLoopReentry: "on range loop re-entry: ..."
+	// Example:
+	//   <script>var x = [{{range .}}'{{.}},{{end}}]</script>
+	// Discussion:
+	//   If an iteration through a range would cause it to end in a
+	//   different context than an earlier pass, there is no single context.
+	//   In the example, there is missing a quote, so it is not clear
+	//   whether {{.}} is meant to be inside a JS string or in a JS value
+	//   context.  The second iteration would produce something like
+	//
+	//     <script>var x = ['firstValue,'secondValue]</script>
+	ErrRangeLoopReentry
+
+	// ErrSlashAmbig: '/' could start a division or regexp.
+	// Example:
+	//   <script>
+	//     {{if .C}}var x = 1{{end}}
+	//     /-{{.N}}/i.test(x) ? doThis : doThat();
+	//   </script>
+	// Discussion:
+	//   The example above could produce `var x = 1/-2/i.test(s)...`
+	//   in which the first '/' is a mathematical division operator or it
+	//   could produce `/-2/i.test(s)` in which the first '/' starts a
+	//   regexp literal.
+	//   Look for missing semicolons inside branches, and maybe add
+	//   parentheses to make it clear which interpretation you intend.
+	ErrSlashAmbig
+)
+
+func (e *Error) Error() string {
+	switch {
+	case e.Node != nil:
+		loc, _ := (*parse.Tree)(nil).ErrorContext(e.Node)
+		return fmt.Sprintf("html/template:%s: %s", loc, e.Description)
+	case e.Line != 0:
+		return fmt.Sprintf("html/template:%s:%d: %s", e.Name, e.Line, e.Description)
+	case e.Name != "":
+		return fmt.Sprintf("html/template:%s: %s", e.Name, e.Description)
+	}
+	return "html/template: " + e.Description
+}
+
+// errorf creates an error given a format string f and args.
+// The template Name still needs to be supplied.
+func errorf(k ErrorCode, node parse.Node, line int, f string, args ...interface{}) *Error {
+	return &Error{k, node, "", line, fmt.Sprintf(f, args...)}
+}