3

$ scala -Xsource:3
Welcome to Scala 2.13.13 -Xsource:3.0.0 (OpenJDK 64-Bit Server VM, Java 1.8.0_392).
Type in expressions for evaluation. Or try :help.

scala> def f = {
     |   val x: Boolean = ???
     | 
     |   x
     |   && x
     | }
     | 
         && x
         ^
On line 5: error: not found: value &&
         && x
            ^
On line 5: error: postfix operator x needs to be enabled
       by making the implicit value scala.language.postfixOps visible.
       This can be achieved by adding the import clause 'import scala.language.postfixOps'
       or by setting the compiler option -language:postfixOps.
       See the Scaladoc for value scala.language.postfixOps for a discussion
       why the feature needs to be explicitly enabled.
       Line starts with an operator that in future
       will be taken as an infix expression continued from the previous line.
       To force the previous interpretation as a separate statement,
       add an explicit `;`, add an empty line, or remove spaces after the operator.

3-cross

$ scala -Xsource:3-cross
Welcome to Scala 2.13.13 -Xsource:3-cross (OpenJDK 64-Bit Server VM, Java 1.8.0_392).
Type in expressions for evaluation. Or try :help.

scala> def f = {
     |   val x: Boolean = ???
     | 
     |   x
     |   && x
     | }
     | 
def f: Boolean

no-option

$ scala
Welcome to Scala 2.13.13 (OpenJDK 64-Bit Server VM, Java 1.8.0_392).
Type in expressions for evaluation. Or try :help.

scala> def f = {
     |   val x: Boolean = ???
     | 
     |   x
     |   && x
     | }
     | 
         && x
         ^
On line 5: error: not found: value &&
         && x
            ^
On line 5: error: postfix operator x needs to be enabled
       by making the implicit value scala.language.postfixOps visible.
       This can be achieved by adding the import clause 'import scala.language.postfixOps'
       or by setting the compiler option -language:postfixOps.
       See the Scaladoc for value scala.language.postfixOps for a discussion
       why the feature needs to be explicitly enabled.