expand: don't panic on ${var@U}, ${var@u}, ${var@L}, ${var@K}, ${var@k}

The parser accepts these @-expansion operators, but the expander only
handled Q, E, a, A, and P, panicking on the rest. Implement U, u, and L
(uppercase-all, uppercase-first, lowercase-all), and treat K and k as
unimplemented no-ops so they don't crash either.

Author: mvdan

expand/param.go

@@ -337,6 +337,18 @@ func (cfg *Config) paramExp(pe *syntax.ParamExp) (string, error) {
 				}
 			case "P":
 				// TODO: implement prompt expansion (\u, \h, \w, etc.).
+			case "U":
+				str = strings.ToUpper(str)
+			case "u":
+				rs := []rune(str)
+				if len(rs) > 0 {
+					rs[0] = unicode.ToUpper(rs[0])
+					str = string(rs)
+				}
+			case "L":
+				str = strings.ToLower(str)
+			case "K", "k":
+				// TODO: implement, like @A but listing keys for assoc arrays.
 			default:
 				panic(fmt.Sprintf("unexpected @%s param expansion", arg))
 			}

interp/interp_test.go

@@ -735,6 +735,22 @@ var runTests = []runTest{
 		`export e=1; echo "${e@A}"`,
 		"declare -x e=1\n #IGNORE bash always single-quotes",
 	},
+	{
+		`a=Hello; echo "${a@U}"`,
+		"HELLO\n",
+	},
+	{
+		`a=hello; echo "${a@u}"`,
+		"Hello\n",
+	},
+	{
+		`a=HELLO; echo "${a@L}"`,
+		"hello\n",
+	},
+	{
+		`a=foo; echo "<${a@K}><${a@k}>"`,
+		"<foo><foo>\n #IGNORE not implemented; must not panic",
+	},
 	{
 		"declare a; a+=(b); echo ${a[@]} ${#a[@]}",
 		"b 1\n",