fix: extend colorOperand to cover all relative-color channel letters (a, w)

Co-authored-by: matthew-dean <414752+matthew-dean@users.noreply.github.com>
Agent-Logs-Url: https://github.com/less/less.js/sessions/4d1ac45c-4305-43e1-962f-991415b969e0

Author: Copilot

packages/less/lib/less/parser/parser.js

@@ -2282,11 +2282,13 @@ const Parser = function Parser(context, imports, fileInfo, currentIndex) {
             colorOperand: function () {
                 parserInput.save();
 
-                // hsl or rgb or lch operand
-                const match = parserInput.$re(/^[lchrgbs]\s+/);
+                // Single-letter channel keywords used in relative color syntax:
+                // r, g, b (rgb), h, s, l (hsl), h, w, b (hwb),
+                // l, a, b (lab/oklab), l, c, h (lch/oklch)
+                const match = parserInput.$re(/^([abcghlrsw])(?=\s|[,/*)]|$)/);
                 if (match) {
                     parserInput.forget();
-                    return new tree.Keyword(match[0]);
+                    return new tree.Keyword(match[1]);
                 }
 
                 parserInput.restore();

packages/test-data/tests-unit/color-functions/modern.css

@@ -34,3 +34,21 @@
 .color-rgb-div {
   background: rgb(from #0000FF calc(r / 2) g b);
 }
+.color-rgb-calc-sub-right {
+  background: rgb(from blue calc(100 - r) g b);
+}
+.color-rgb-calc-add-left {
+  background: rgb(from blue calc(r + 100) g b);
+}
+.color-lab-calc-a-add {
+  background: lab(from blue l calc(a + 5) b);
+}
+.color-lab-calc-a-sub {
+  background: lab(from blue l calc(5 - a) b);
+}
+.color-hwb-calc-w-add {
+  background: hwb(from blue h calc(w + 10) b);
+}
+.color-hwb-calc-w-sub {
+  background: hwb(from blue h calc(10 - w) b);
+}

packages/test-data/tests-unit/color-functions/modern.less

@@ -46,3 +46,30 @@
 .color-rgb-div {
   background: rgb(from #0000FF calc(r / 2) g b);
 }
+
+// Regression tests for issue #4316: calc() with channel as right-hand operand
+.color-rgb-calc-sub-right {
+  background: rgb(from blue calc(100 - r) g b);
+}
+
+.color-rgb-calc-add-left {
+  background: rgb(from blue calc(r + 100) g b);
+}
+
+// lab/oklab 'a' channel (was missing from character class)
+.color-lab-calc-a-add {
+  background: lab(from blue l calc(a + 5) b);
+}
+
+.color-lab-calc-a-sub {
+  background: lab(from blue l calc(5 - a) b);
+}
+
+// hwb 'w' channel (was missing from character class)
+.color-hwb-calc-w-add {
+  background: hwb(from blue h calc(w + 10) b);
+}
+
+.color-hwb-calc-w-sub {
+  background: hwb(from blue h calc(10 - w) b);
+}