Added ConditionalType

src/Argument.php

@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Typhoon\Type;
+
+/**
+ * @api
+ * @psalm-immutable
+ */
+final class Argument
+{
+    /**
+     * @var non-empty-string
+     */
+    public readonly string $name;
+
+    /**
+     * @internal
+     * @psalm-internal Typhoon\Type
+     * @param non-empty-string $name
+     */
+    public function __construct(
+        string $name,
+    ) {
+        $this->name = $name;
+    }
+}

src/ConditionalType.php

@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Typhoon\Type;
+
+/**
+ * @api
+ * @psalm-immutable
+ * @implements Type<mixed>
+ */
+final class ConditionalType implements Type
+{
+    public readonly Argument|TemplateType $subject;
+
+    public readonly Type $is;
+
+    public readonly Type $if;
+
+    public readonly Type $else;
+
+    /**
+     * @internal
+     * @psalm-internal Typhoon\Type
+     */
+    public function __construct(
+        Argument|TemplateType $subject,
+        Type $is,
+        Type $if,
+        Type $else,
+    ) {
+        $this->subject = $subject;
+        $this->is = $is;
+        $this->if = $if;
+        $this->else = $else;
+    }
+
+    public function accept(TypeVisitor $visitor): mixed
+    {
+        return $visitor->visitConditional($this);
+    }
+}

src/TypeVisitor.php

@@ -155,6 +155,9 @@ public function visitValueOf(ValueOfType $type): mixed;
     /** @return TReturn */
     public function visitTemplate(TemplateType $type): mixed;
 
+    /** @return TReturn */
+    public function visitConditional(ConditionalType $type): mixed;
+
     /** @return TReturn */
     public function visitIntersection(IntersectionType $type): mixed;
 

src/types.php

@@ -457,6 +457,14 @@ public static function template(string $name): TemplateType
         return new TemplateType($name);
     }
 
+    /**
+     * @psalm-pure
+     */
+    public static function conditional(Argument|TemplateType $subject, Type $is, Type $if, Type $else): ConditionalType
+    {
+        return new ConditionalType($subject, $is, $if, $else);
+    }
+
     /**
      * @psalm-pure
      * @no-named-arguments
@@ -491,6 +499,15 @@ public static function union(Type $type1, Type $type2, Type ...$moreTypes): Unio
     {
         return new UnionType([$type1, $type2, ...$moreTypes]);
     }
+
+    /**
+     * @psalm-pure
+     * @param non-empty-string $name
+     */
+    public function arg(string $name): Argument
+    {
+        return new Argument($name);
+    }
 }
 
 /**