In Java 19, JEP 427 "Pattern Matching for switch (Third Preview)" expands the grammar of a switch label to allow patterns with _guards_:

SwitchLabel:
    CaseOrDefaultLabel {: CaseOrDefaultLabel }
CaseOrDefaultLabel:
     case CaseElement {, CaseElement }
     default
CaseElement:
    CaseConstant
    Pattern { Guard }
    null
    default
Guard:
    when Expression

Simultaneously, JEP 405 "Record Patterns (Preview)" expands the grammar for `Pattern`:

Pattern:
    TypePattern
    ParenthesizedPattern
    RecordPattern

RecordPattern:
    ReferenceType RecordStructurePattern [ Identifier ]

RecordStructurePattern:
    ( [ RecordComponentPatternList ] )

Unfortunately, taken together these two grammars create an ambiguity.

record R(){}
switch(e) {
    case R() when when (true) -> ...
    ...
}

The label could be parsed as either:

    case (R() when) when (true) ->

or

    case (R()) when (when(true)) ->

(where `when` is a static boolean method).

There are three possible fixes to this ambiguity:

1. Require a named record pattern to use a new syntax, e.g. `R() as x`, so `case R() as when when (true)` is unambiguously `case (R() as when) when (true)`.
2. Exclude the identifier `when` from the universe of identifier names. (This would be a breaking change.)
3. Exclude the identifier `when` from the universe of unqualified method names. (This is similar to the treatment of `yield`, but is a breaking change.)