var in implicit lambdas shouldn't be accepted for source < 11

ADDITIONAL SYSTEM INFORMATION :
Linux x86_64

A DESCRIPTION OF THE PROBLEM :
"java -source 10" accepts Java syntax that was only introduced in Java 11 and that is rejected by "javac" from JDK 10.

This program
======================== foo.java ======================
public class foo { Readable r = (var b) -> 0; }
======================================================

gives a syntax error with javac from JDK 10:

$ javac foo.java
foo.java:1: error: 'var' is not allowed here
public class foo { Readable r = (var b) -> 0; }
                                 ^
1 error

But "javac -source 10" from JDK 11 fails to diagnose it:

$ javac -source 10 foo.java
warning: [options] bootstrap class path not set in conjunction with -source 10
1 warning


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
echo 'public class foo { Readable r = (var b) -> 0; }' > foo.java
javac -source 10 foo.java


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
An error diagnostic.
ACTUAL -
No error.

---------- BEGIN SOURCE ----------
public class foo { Readable r = (var b) -> 0; }

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
javac -source 9 foo.java

FREQUENCY : always