Deprecate explicitly passing null as an alpha value (#2049)

This allows us to reserve null to indicate a missing alpha channel
instead in future versions.

Author: nex3

CHANGELOG.md

@@ -2,8 +2,27 @@
 
 ### Dart API
 
+* Deprecate explicitly passing `null` as the alpha channel for
+  `SassColor.rgb()`, `SassColor.hsl()`, and `SassColor.hwb()`. Omitting the
+  `alpha` channel is still allowed. In future releases, `null` will be used to
+  indicate a [missing component]. This deprecation is named `null-alpha`.
+
+  [missing component]: https://developer.mozilla.org/en-US/docs/Web/CSS/color_value#missing_color_components
+
 * Include protocol buffer definitions when uploading the `sass` package to pub.
 
+### JS API
+
+* Deprecate explicitly passing `null` as the alpha channel for `new
+  SassColor()`. Omitting the `alpha` channel or passing `undefined` for it is
+  still allowed. In future releases, `null` will be used to indicate a [missing
+  component]. This deprecation is named `null-alpha`.
+
+  [missing component]: https://developer.mozilla.org/en-US/docs/Web/CSS/color_value#missing_color_components
+
+  (Note that this was already prohibited by the TypeScript types, but in
+  practice prior to this `null` was treated as `1`.)
+
 ## 1.64.2
 
 * No user-visible changes.

lib/src/deprecation.dart

@@ -5,6 +5,7 @@
 import 'package:collection/collection.dart';
 import 'package:pub_semver/pub_semver.dart';
 
+import 'io.dart';
 import 'util/nullable.dart';
 
 /// A deprecated feature in the language.
@@ -59,6 +60,10 @@ enum Deprecation {
       description:
           'Using !default or !global multiple times for one variable.'),
 
+  nullAlpha('null-alpha',
+      deprecatedIn: '1.62.3',
+      description: 'Passing null as alpha in the ${isJS ? 'JS' : 'Dart'} API.'),
+
   /// Deprecation for `@import` rules.
   import.future('import', description: '@import rules.'),
 

lib/src/functions/color.dart

@@ -600,7 +600,8 @@ Value _rgb(String name, List<Value> arguments) {
       fuzzyRound(_percentageOrUnitless(green, 255, "green")),
       fuzzyRound(_percentageOrUnitless(blue, 255, "blue")),
       alpha.andThen((alpha) =>
-          _percentageOrUnitless(alpha.assertNumber("alpha"), 1, "alpha")),
+              _percentageOrUnitless(alpha.assertNumber("alpha"), 1, "alpha")) ??
+          1,
       ColorFormat.rgbFunction);
 }
 
@@ -644,7 +645,8 @@ Value _hsl(String name, List<Value> arguments) {
       saturation.value.clamp(0, 100),
       lightness.value.clamp(0, 100),
       alpha.andThen((alpha) =>
-          _percentageOrUnitless(alpha.assertNumber("alpha"), 1, "alpha")),
+              _percentageOrUnitless(alpha.assertNumber("alpha"), 1, "alpha")) ??
+          1,
       ColorFormat.hslFunction);
 }
 
@@ -693,7 +695,8 @@ Value _hwb(List<Value> arguments) {
       whiteness.valueInRange(0, 100, "whiteness"),
       blackness.valueInRange(0, 100, "blackness"),
       alpha.andThen((alpha) =>
-          _percentageOrUnitless(alpha.assertNumber("alpha"), 1, "alpha")));
+              _percentageOrUnitless(alpha.assertNumber("alpha"), 1, "alpha")) ??
+          1);
 }
 
 Object /* SassString | List<Value> */ _parseChannels(

lib/src/io/interface.dart

@@ -20,7 +20,7 @@ bool get isMacOS => throw '';
 bool get hasTerminal => throw '';
 
 /// Whether we're running as JS (browser or Node.js).
-bool get isJS => throw '';
+const bool isJS = false;
 
 /// Whether we're running as Node.js (not browser or Dart VM).
 bool get isNode => throw '';

lib/src/io/js.dart

@@ -227,7 +227,7 @@ bool get isWindows => process?.platform == 'win32';
 
 bool get isMacOS => process?.platform == 'darwin';
 
-bool get isJS => true;
+const bool isJS = true;
 
 /// The fs module object, used to check whether this has been loaded as Node.
 ///

lib/src/io/vm.dart

@@ -22,7 +22,7 @@ bool get isMacOS => io.Platform.isMacOS;
 
 bool get hasTerminal => io.stdout.hasTerminal;
 
-bool get isJS => false;
+const bool isJS = false;
 
 bool get isNode => false;
 

lib/src/js/value/color.dart

@@ -8,19 +8,20 @@ import '../../util/nullable.dart';
 import '../../util/number.dart';
 import '../../value.dart';
 import '../reflection.dart';
+import '../utils.dart';
 
 /// The JavaScript `SassColor` class.
 final JSClass colorClass = () {
   var jsClass = createJSClass('sass.SassColor', (Object self, _Channels color) {
     if (color.red != null) {
       return SassColor.rgb(fuzzyRound(color.red!), fuzzyRound(color.green!),
-          fuzzyRound(color.blue!), color.alpha);
+          fuzzyRound(color.blue!), _handleNullAlpha(color.alpha));
     } else if (color.saturation != null) {
-      return SassColor.hsl(
-          color.hue!, color.saturation!, color.lightness!, color.alpha);
+      return SassColor.hsl(color.hue!, color.saturation!, color.lightness!,
+          _handleNullAlpha(color.alpha));
     } else {
-      return SassColor.hwb(
-          color.hue!, color.whiteness!, color.blackness!, color.alpha);
+      return SassColor.hwb(color.hue!, color.whiteness!, color.blackness!,
+          _handleNullAlpha(color.alpha));
     }
   });
 
@@ -68,6 +69,12 @@ final JSClass colorClass = () {
   return jsClass;
 }();
 
+/// Converts an undefined [alpha] to 1.
+///
+/// This ensures that an explicitly null alpha will produce a deprecation
+/// warning when passed to the Dart API.
+num? _handleNullAlpha(num? alpha) => isUndefined(alpha) ? 1 : alpha;
+
 @JS()
 @anonymous
 class _Channels {

lib/src/parse/stylesheet.dart

@@ -2232,7 +2232,7 @@ abstract class StylesheetParser extends Parser {
         red,
         green,
         blue,
-        alpha,
+        alpha ?? 1,
         // Don't emit four- or eight-digit hex colors as hex, since that's not
         // yet well-supported in browsers.
         alpha == null ? SpanColorFormat(scanner.spanFrom(start)) : null);

lib/src/value/color.dart

@@ -7,7 +7,10 @@ import 'dart:math' as math;
 import 'package:meta/meta.dart';
 import 'package:source_span/source_span.dart';
 
+import '../deprecation.dart';
+import '../evaluation_context.dart';
 import '../exception.dart';
+import '../io.dart';
 import '../util/number.dart';
 import '../value.dart';
 import '../visitor/interface/value.dart';
@@ -98,52 +101,63 @@ class SassColor extends Value {
 
   /// Creates an RGB color.
   ///
+  /// Passing `null` to [alpha] is deprecated, and will change behavior in
+  /// future versions of Dart Sass to represent a [missing component] instead of
+  /// being equivalent to `1`. Callers who want to create opaque colors should
+  /// explicitly pass `1` or not pass [alpha] at all.
+  ///
+  /// [missing component]: https://developer.mozilla.org/en-US/docs/Web/CSS/color_value#missing_color_components
+  ///
   /// Throws a [RangeError] if [red], [green], and [blue] aren't between `0` and
   /// `255`, or if [alpha] isn't between `0` and `1`.
-  SassColor.rgb(int red, int green, int blue, [num? alpha])
-      : this.rgbInternal(red, green, blue, alpha);
+  SassColor.rgb(int red, int green, int blue, [num? alpha = 1])
+      : this.rgbInternal(red, green, blue, _handleNullAlpha(alpha));
 
   /// Like [SassColor.rgb], but also takes a [format] parameter.
   ///
   /// @nodoc
   @internal
   SassColor.rgbInternal(this._red, this._green, this._blue,
-      [num? alpha, this.format])
-      : _alpha = alpha == null
-            ? 1
-            : fuzzyAssertRange(alpha.toDouble(), 0, 1, "alpha") {
+      [num alpha = 1, this.format])
+      : _alpha = fuzzyAssertRange(alpha.toDouble(), 0, 1, "alpha") {
     RangeError.checkValueInInterval(red, 0, 255, "red");
     RangeError.checkValueInInterval(green, 0, 255, "green");
     RangeError.checkValueInInterval(blue, 0, 255, "blue");
   }
 
   /// Creates an HSL color.
   ///
+  /// Passing `null` to [alpha] is deprecated, and will change behavior in
+  /// future versions of Dart Sass to represent a [missing component] instead of
+  /// being equivalent to `1`. Callers who want to create opaque colors should
+  /// explicitly pass `1` or not pass [alpha] at all.
+  ///
+  /// [missing component]: https://developer.mozilla.org/en-US/docs/Web/CSS/color_value#missing_color_components
+  ///
   /// Throws a [RangeError] if [saturation] or [lightness] aren't between `0`
   /// and `100`, or if [alpha] isn't between `0` and `1`.
-  SassColor.hsl(num hue, num saturation, num lightness, [num? alpha])
-      : this.hslInternal(hue, saturation, lightness, alpha);
+  SassColor.hsl(num hue, num saturation, num lightness, [num? alpha = 1])
+      : this.hslInternal(hue, saturation, lightness, _handleNullAlpha(alpha));
 
   /// Like [SassColor.hsl], but also takes a [format] parameter.
   ///
   /// @nodoc
   @internal
   SassColor.hslInternal(num hue, num saturation, num lightness,
-      [num? alpha, this.format])
+      [num alpha = 1, this.format])
       : _hue = hue % 360,
         _saturation =
             fuzzyAssertRange(saturation.toDouble(), 0, 100, "saturation"),
         _lightness =
             fuzzyAssertRange(lightness.toDouble(), 0, 100, "lightness"),
-        _alpha = alpha == null
-            ? 1
-            : fuzzyAssertRange(alpha.toDouble(), 0, 1, "alpha");
+        _alpha = fuzzyAssertRange(alpha.toDouble(), 0, 1, "alpha");
 
   /// Creates an HWB color.
   ///
   /// Throws a [RangeError] if [whiteness] or [blackness] aren't between `0` and
   /// `100`, or if [alpha] isn't between `0` and `1`.
-  factory SassColor.hwb(num hue, num whiteness, num blackness, [num? alpha]) {
+  factory SassColor.hwb(num hue, num whiteness, num blackness,
+      [num? alpha = 1]) {
     // From https://www.w3.org/TR/css-color-4/#hwb-to-rgb
     var scaledHue = hue % 360 / 360;
     var scaledWhiteness =
@@ -171,6 +185,21 @@ class SassColor extends Value {
         toRgb(scaledHue - 1 / 3), alpha);
   }
 
+  /// Prints a deprecation warning if [alpha] is explicitly `null`.
+  static num _handleNullAlpha(num? alpha) {
+    if (alpha != null) return alpha;
+
+    warnForDeprecation(
+        'Passing null for alpha in the ${isJS ? 'JS' : 'Dart'} API is '
+        'deprecated.\n'
+        'To preserve current behavior, pass 1${isJS ? ' or undefined' : ''} '
+        'instead.'
+        '\n'
+        'More info: https://sass-lang.com/d/null-alpha',
+        Deprecation.nullAlpha);
+    return 1;
+  }
+
   SassColor._(this._red, this._green, this._blue, this._hue, this._saturation,
       this._lightness, this._alpha)
       : format = null;

pkg/sass_api/CHANGELOG.md

@@ -7,6 +7,10 @@
 
 * All uses of classes from the `tuple` package have been replaced by record
   types.
+  
+## 7.2.2
+
+* No user-visible changes.
 
 ## 7.2.1