syntax: add support for the Bash 5.2 @k expansion operator

Updates #921.

syntax/filetests_test.go

@@ -2557,6 +2557,25 @@ var fileTests = []testCase{
 			}),
 		),
 	},
+	{
+		Strs: []string{`${a@K} ${b@k}`},
+		bash: call(
+			word(&ParamExp{
+				Param: lit("a"),
+				Exp: &Expansion{
+					Op:   OtherParamOps,
+					Word: litWord("K"),
+				},
+			}),
+			word(&ParamExp{
+				Param: lit("b"),
+				Exp: &Expansion{
+					Op:   OtherParamOps,
+					Word: litWord("a"),
+				},
+			}),
+		),
+	},
 	{
 		Strs: []string{`${a@Q} ${b@#}`},
 		mksh: call(

syntax/parser.go

@@ -1375,7 +1375,7 @@ func (p *Parser) paramExpExp() *Expansion {
 			p.curErr("@ expansion operator requires a literal")
 		}
 		switch p.val {
-		case "a", "u", "A", "E", "K", "L", "P", "U":
+		case "a", "k", "u", "A", "E", "K", "L", "P", "U":
 			if !p.lang.isBash() {
 				p.langErr(p.pos, "this expansion operator", LangBash)
 			}
@@ -1385,7 +1385,7 @@ func (p *Parser) paramExpExp() *Expansion {
 			}
 		case "Q":
 		default:
-			p.curErr("invalid @ expansion operator")
+			p.curErr("invalid @ expansion operator %q", p.val)
 		}
 	}
 	return &Expansion{Op: op, Word: p.getWord()}

syntax/parser_test.go

@@ -1798,7 +1798,7 @@ var shellTests = []errorCase{
 	},
 	{
 		in:   "echo ${foo@bar}",
-		bash: `1:12: invalid @ expansion operator #NOERR at runtime`,
+		bash: `1:12: invalid @ expansion operator "bar" #NOERR at runtime`,
 	},
 	{
 		in:   "echo ${foo@'Q'}",
@@ -1961,6 +1961,10 @@ var shellTests = []errorCase{
 		in:   "echo ${foo@K}",
 		mksh: `1:12: this expansion operator is a bash feature`,
 	},
+	{
+		in:   "echo ${foo@k}",
+		mksh: `1:12: this expansion operator is a bash feature`,
+	},
 	{
 		in:   "echo ${foo@L}",
 		mksh: `1:12: this expansion operator is a bash feature`,