New switch errors.

src/parse_error.rs

@@ -117,6 +117,8 @@ pub enum ParseErrorType {
     DuplicatedSwitchCase,
     /// The default case of a `switch` statement is not the last.
     WrongSwitchDefaultCase,
+    /// The case condition of a `switch` statement is not appropriate.
+    WrongSwitchCaseCondition,
     /// Missing a property name for custom types and maps.
     ///
     /// Never appears under the `no_object` feature.
@@ -198,6 +200,7 @@ impl ParseErrorType {
             Self::DuplicatedProperty(_) => "Duplicated property in object map literal",
             Self::DuplicatedSwitchCase => "Duplicated switch case",
             Self::WrongSwitchDefaultCase => "Default switch case is not the last",
+            Self::WrongSwitchCaseCondition => "Default switch case cannot have condition",
             Self::PropertyExpected => "Expecting name of a property",
             Self::VariableExpected => "Expecting name of a variable",
             Self::Reserved(_) => "Invalid use of reserved keyword",

src/parser.rs

@@ -829,6 +829,13 @@ fn parse_switch(
             (Token::Underscore, pos) if def_stmt.is_none() => {
                 def_pos = *pos;
                 eat_token(input, Token::Underscore);
+
+                let (if_clause, if_pos) = match_token(input, Token::If);
+
+                if if_clause {
+                    return Err(PERR::WrongSwitchCaseCondition.into_err(if_pos));
+                }
+
                 (None, None)
             }
             (Token::Underscore, pos) => return Err(PERR::DuplicatedSwitchCase.into_err(*pos)),