Fix PHP Warning: Undefined property $value in extract()

== What ==

When a Less input file passes a list node to extract(), we access
the internal array value and use that to access the given index.

When a Less input file passes something else to extract(), the value
should be treated as an array containing that 1 node (which means index
1 is the given node, and any other index returns null). This last
part was not working correctly in our implementation.

== Before ==

```
$ bin/lessc test/Fixtures/less.php/less/T391735-extract-non-array.less
PHP Warning: Undefined property: Less_Tree_Call::$value in lib/Less/Functions.php
PHP Warning: Undefined property: Less_Tree_DetachedRuleset::$value in lib/Less/Functions.php
```

A bare slash can also cause the error (as reported at T391735), although
only with `--math=parens-division`, because otherwise the slash operation
is already compiled away before the function call is compiled. Note that
"test/Fixtures/less.php" is registered in test/fixtures.php with math=always.

```
@x: 20px;

.foo-one {
    .foo(@x / 1px);
}

.foo(@value: false) when not (@value = false)
  and not (extract(@value, 1) = false)
  and not (extract(@value, 2) = false)
  and not (extract(@value, 3) = false)
  and not (extract(@value, 4) = false)
  and not (extract(@value, 5) = false) {
    foo: @value;
}
```
```
$ bin/less --math=parens-division tmp.less
PHP Warning: Undefined property: Less_Tree_Operation::$value in lib/Less/Functions.php
```

== How ==

We already did the right thing in Functions::length(), where we list
the two Less_Tree classes that represent list nodes and have internal
array values.

Update our implementation to match Less.js 3.13, which moved this
logic to getItemsFromNode() and re-uses it in Functions::extract().

Bug: T391735
Change-Id: Id64ff9f13039571caf475d2e4085d2ef06654508

Author: Krinkle

lib/Less/Functions.php

@@ -941,33 +941,41 @@ public function shade( $color, $amount = null ) {
 	}
 
 	/**
-	 * @see less-3.13.1.js#functions _SELF
+	 * @see less-3.13.1.js#getItemsFromNode
+	 */
+	private function getItemsFromNode( Less_Tree $node ) {
+		// handle non-array values as an array of length 1
+		// return 'undefined' if index is invalid
+		//
+		// NOTE: Less.js uses duck-typing `isArray(node.value)`, which would cause warnings in PHP,
+		// and potentially bugs for Less_Tree classes with a $value that is only sometimes an array.
+		// Instead, check for Less_Tree classes that always implement an array $value.
+		return ( $node instanceof Less_Tree_Expression || $node instanceof Less_Tree_Value )
+			? $node->value
+			: [ $node ];
+	}
+
+	/**
+	 * @see less-3.13.1.js#_SELF
 	 */
 	public function _self( $args ) {
 		return $args;
 	}
 
+	/**
+	 * @see less-3.13.1.js#extract
+	 */
 	public function extract( $values, $index ) {
-		$index = (int)$index->value - 1; // (1-based index)
-		// handle non-array values as an array of length 1
-		// return 'undefined' if index is invalid
-		if ( !( $values instanceof Less_Tree_Color ) && is_array( $values->value ) ) {
-			if ( isset( $values->value[$index] ) ) {
-				return $values->value[$index];
-			}
-			return null;
-
-		} elseif ( (int)$index === 0 ) {
-			return $values;
-		}
-
-		return null;
+		// (1-based index)
+		$index = (int)$index->value - 1;
+		return $this->getItemsFromNode( $values )[ $index ] ?? null;
 	}
 
+	/**
+	 * @see less-3.13.1.js#length
+	 */
 	public function length( $values ) {
-		$n = ( $values instanceof Less_Tree_Expression || $values instanceof Less_Tree_Value ) ?
-			count( $values->value ) : 1;
-		return new Less_Tree_Dimension( $n );
+		return new Less_Tree_Dimension( count( $this->getItemsFromNode( $values ) ) );
 	}
 
 	/**

test/Fixtures/less.php/css/T391735-extract-non-array.css

@@ -0,0 +1,12 @@
+.foo-one-div {
+  foo: 1;
+}
+.foo-one-detached {
+  foo: 1;
+}
+.foo-two {
+  foo: 2;
+}
+.foo-includes-false {
+  color: purple;
+}

test/Fixtures/less.php/less/T391735-extract-non-array.less

@@ -0,0 +1,24 @@
+@x: 20px;
+
+.foo-one-div {
+    .foo(calc(@x / 1px));
+}
+.foo-one-detached {
+    .foo( { color: blue } );
+}
+.foo-two {
+    .foo(calc(@x / 1px) 2px);
+}
+.foo-includes-false {
+    color: purple;
+    .foo(@x 2px false 4px);
+}
+
+.foo(@value: false) when not (@value = false)
+  and not (extract(@value, 1) = false)
+  and not (extract(@value, 2) = false)
+  and not (extract(@value, 3) = false)
+  and not (extract(@value, 4) = false)
+  and not (extract(@value, 5) = false) {
+    foo: length(@value);
+}