8023679: Improve error message for '_' used as a lambda parameter name

Reviewed-by: jjg, dlsmith

src/share/classes/com/sun/tools/javac/parser/JavacParser.java

@@ -3002,8 +3002,9 @@ public class JavacParser implements Parser {
         int pos = token.pos;
         Name name;
         if (lambdaParameter && token.kind == UNDERSCORE) {
-            syntaxError(pos, "expected", IDENTIFIER);
+            log.error(pos, "underscore.as.identifier.in.lambda");
             name = token.name();
+            nextToken();
         } else {
             if (allowThisIdent) {
                 JCExpression pn = qualident(false);

src/share/classes/com/sun/tools/javac/resources/compiler.properties

@@ -2224,6 +2224,10 @@ compiler.warn.underscore.as.identifier=\
     ''_'' used as an identifier\n\
     (use of ''_'' as an identifier might not be supported in releases after Java SE 8)
 
+compiler.err.underscore.as.identifier.in.lambda=\
+    ''_'' used as an identifier\n\
+    (use of ''_'' as an identifier is forbidden for lambda parameters)
+
 compiler.err.enum.as.identifier=\
     as of release 5, ''enum'' is a keyword, and may not be used as an identifier\n\
     (use -source 1.4 or lower to use ''enum'' as an identifier)

test/tools/javac/diags/examples/UnderscoreInLambdaExpression.java

+/*
+ * Copyright (c) 2013, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+// key: compiler.err.underscore.as.identifier.in.lambda
+public class UnderscoreInLambdaExpression {
+    java.util.function.Function<String,String> f = _ -> "x";
+}