Fix the help example, read padding past the shape gate, catch a stray closing brace, and stop the fieldless hint naming a spelling that fails

cmd/fejkdata/main.go

@@ -30,8 +30,8 @@ const usage = `Usage: fejkdata [flags] <path|template>
 An argument containing a { token, or a JSON object, array or string, is a
 template; any other argument is a path (a path never contains a brace, a bracket
 or a quote). Templates reach the data by reference from the root —
-{/sv_SE.person.first} — whether the data is shipped or layered with --data-path. A
+{/sv_SE.person.last} — whether the data is shipped or layered with --data-path. An
-bracket that is not valid JSON names no template and no path.
+argument carrying one of those characters but no valid JSON names neither.
 
   -d, --data-path D      a data directory to layer over the shipped data (repeatable; last wins on a clash)
   -h, --help             print this help, then exit
@@ -208,7 +208,7 @@ func (in invocation) check() (argKind, error) {
 		return argPath, nil
 	}
 	if len(in.paths) != 1 {
-		return argPath, fmt.Errorf("expected one path, got %d", len(in.paths))
+		return argPath, fmt.Errorf("expected one path or template, got %d", len(in.paths))
 	}
 	return classify(in.paths[0])
 }
@@ -276,10 +276,10 @@ const (
 // quote, so no path collides with any of those spellings, and the JSON gate is
 // valid-JSON so a copied bracket names nothing rather than swallowing an argument.
 func classify(arg string) (argKind, error) {
-	if strings.ContainsRune(arg, '{') || (isJSONStart(arg) && json.Valid([]byte(arg))) {
+	if strings.ContainsRune(arg, '{') || (isJSONStart(strings.TrimSpace(arg)) && json.Valid([]byte(arg))) {
 		return argTemplate, nil
 	}
-	if i := strings.IndexAny(arg, `[]"`); i >= 0 {
+	if i := strings.IndexAny(arg, `[]}"`); i >= 0 {
 		return argPath, fmt.Errorf("%q holds a %q, which no path may, and it is not valid JSON, so it names no template either", arg, arg[i:i+1])
 	}
 	return argPath, nil

template.go

@@ -165,7 +165,11 @@ func checkArm(name string, fields map[string]node) error {
 			return fmt.Errorf("%q is an option and can never be a field", a.key)
 		}
 		if len(fields) == 0 {
-			return fmt.Errorf("no field %q; a token names a sibling field, and a bare string has none — write {/%s} to reference the data", a.key, name)
+			hint := ""
+			if hintableRef(name) {
+				hint = fmt.Sprintf(" — write {/%s} to reference the data", name)
+			}
+			return fmt.Errorf("no field %q; a token names a sibling field, and this template has none%s", a.key, hint)
 		}
 		return fmt.Errorf("no field %q", a.key)
 	}
@@ -175,6 +179,17 @@ func checkArm(name string, fields map[string]node) error {
 	return nil
 }
 
+// hintableRef reports whether {/name} is a reference the grammar accepts, so the
+// hint never names a spelling that fails too.
+func hintableRef(name string) bool {
+	for _, seg := range strings.Split(name, ".") {
+		if checkName(seg) != nil {
+			return false
+		}
+	}
+	return true
+}
+
 // tokenOperands lists the fields one {token} body reads as operands, empty for a
 // field token or a builtin that reads none.
 func tokenOperands(body string) []string {