Allow esac as the first pattern of a case branch

yash-cli/CHANGELOG-bin.md

@@ -12,6 +12,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- The shell's syntax now allows `esac` as the first pattern of a case branch
+  as in `case esac in (esac|case) echo ok; esac`. Previously, it was a syntax
+  error, but POSIX.1-2024 allows it.
 - The `bg` built-in now updates the `!` special parameter to the process ID of
   the background job, as required by POSIX.1-2024.
 - The `exec` built-in no longer exits the shell when the specified command is

yash-cli/tests/scripted_test/case-p.sh

@@ -333,7 +333,13 @@ test_reserved_word_as_pattern "$LINENO" then
 test_reserved_word_as_pattern "$LINENO" until
 test_reserved_word_as_pattern "$LINENO" while
 
-test_oE 'reserved word esac as (non-first) pattern'
+test_oE 'esac as first pattern'
+case esac in (esac) echo matched;; esac
+__IN__
+matched
+__OUT__
+
+test_oE 'esac as non-first pattern'
 case esac in -|esac) echo matched;; esac
 __IN__
 matched

yash-syntax/CHANGELOG.md

@@ -9,9 +9,19 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- In the `parser::Parser::case_item` method, the parser now accepts an `esac`
+  token as a pattern after an opening parenthesis, as required by POSIX.1-2024.
+  The previous version of POSIX did not allow `esac` as the first pattern, so
+  the method was returning `SyntaxError::EsacAsPattern` in that case.
 - Internal dependency versions:
     - futures-util 0.3.28 → 0.3.31
 
+### Deprecated
+
+- `parser::SyntaxError::EsacAsPattern`
+    - This variant is deprecated because this error condition is no longer
+      possible as described above.
+
 ## [0.12.0] - 2024-09-29
 
 ### Added

yash-syntax/src/parser/case.rs

@@ -36,7 +36,6 @@ impl Parser<'_, '_> {
     pub async fn case_item(&mut self) -> Result<Option<CaseItem>> {
         fn pattern_error_cause(token_id: TokenId) -> SyntaxError {
             match token_id {
-                Token(Some(Esac)) => SyntaxError::EsacAsPattern,
                 Token(_) => unreachable!(),
                 Operator(CloseParen) | Operator(Bar) | Operator(Newline) | EndOfInput => {
                     SyntaxError::MissingPattern
@@ -63,8 +62,7 @@ impl Parser<'_, '_> {
             Operator(OpenParen) => {
                 let next_token = self.take_token_auto(&[Esac]).await?;
                 match next_token.id {
-                    // TODO Allow `esac` if not in POSIXly-correct mode
-                    Token(keyword) if keyword != Some(Esac) => next_token.word,
+                    Token(_) => next_token.word,
                     _ => {
                         let cause = pattern_error_cause(next_token.id).into();
                         let location = next_token.word.location;
@@ -318,12 +316,10 @@ mod tests {
         let mut lexer = Lexer::from_memory("(esac)", Source::Unknown);
         let mut parser = Parser::new(&mut lexer, &EmptyGlossary);
 
-        let e = parser.case_item().now_or_never().unwrap().unwrap_err();
-        assert_eq!(e.cause, ErrorCause::Syntax(SyntaxError::EsacAsPattern));
-        assert_eq!(*e.location.code.value.borrow(), "(esac)");
-        assert_eq!(e.location.code.start_line_number.get(), 1);
-        assert_eq!(*e.location.code.source, Source::Unknown);
-        assert_eq!(e.location.range, 1..5);
+        let item = parser.case_item().now_or_never().unwrap().unwrap().unwrap();
+        assert_eq!(item.patterns.len(), 1);
+        assert_eq!(item.patterns[0].to_string(), "esac");
+        assert_eq!(item.body.0, []);
     }
 
     #[test]

yash-syntax/src/parser/error.rs

@@ -135,6 +135,7 @@ pub enum SyntaxError {
     /// The pattern is not a valid word token.
     InvalidPattern,
     /// The first pattern of a case item is `esac`.
+    #[deprecated = "this error no longer occurs"]
     EsacAsPattern,
     /// An `esac` or `;;` appears outside a case command.
     UnopenedCase,
@@ -219,6 +220,7 @@ impl SyntaxError {
             UnclosedPatternList => "The pattern list is not properly closed by a `)`",
             MissingPattern => "A pattern is missing in the `case` command",
             InvalidPattern => "The pattern is not a valid word token",
+            #[allow(deprecated)]
             EsacAsPattern => "`esac` cannot be the first of a pattern list",
             UnclosedCase { .. } => "The `case` command is missing its closing `esac`",
             UnmatchedParenthesis => "`)` is missing after `(`",
@@ -290,6 +292,7 @@ impl SyntaxError {
             UnopenedIf => "not in an `if` command",
             UnclosedIf { .. } => "expected `fi`",
             MissingIn { .. } => "expected `in`",
+            #[allow(deprecated)]
             EsacAsPattern => "needs quoting",
             UnopenedCase => "not in a `case` command",
             UnclosedCase { .. } => "expected `esac`",