TemplateType should accept a TemplateType derived from itself

src/Type/Generic/TemplateTypeArgumentStrategy.php

@@ -25,7 +25,7 @@ public function accepts(TemplateType $left, Type $right, bool $strictTypes): Tri
 			return TrinaryLogic::createNo();
 		}
 
-		return TrinaryLogic::createFromBoolean($left->equals($right))
+		return $left->isSuperTypeOf($right)
 			->or(TrinaryLogic::createFromBoolean($right->equals(new MixedType())));
 	}
 

src/Type/Generic/TemplateTypeTrait.php

@@ -3,6 +3,7 @@
 namespace PHPStan\Type\Generic;
 
 use PHPStan\TrinaryLogic;
+use PHPStan\Type\Generic\TemplateType;
 use PHPStan\Type\IntersectionType;
 use PHPStan\Type\MixedType;
 use PHPStan\Type\Traits\NonRemoveableTypeTrait;
@@ -124,7 +125,7 @@ public function accepts(Type $type, bool $strictTypes): TrinaryLogic
 
 	public function isSuperTypeOf(Type $type): TrinaryLogic
 	{
-		if ($type instanceof self || $type instanceof IntersectionType) {
+		if ($type instanceof TemplateType || $type instanceof IntersectionType) {
 			return $type->isSubTypeOf($this);
 		}
 
@@ -149,16 +150,12 @@ public function isSubTypeOf(Type $type): TrinaryLogic
 			return $type->isSuperTypeOf($this->getBound());
 		}
 
-		if ($this->equals($type)) {
-			return TrinaryLogic::createYes();
+		if ($this->getScope()->equals($type->getScope()) && $this->getName() === $type->getName()) {
+			return $type->getBound()->isSuperTypeOf($this->getBound());
 		}
 
-		if ($type->getBound()->isSuperTypeOf($this->getBound())->no() &&
-			$this->getBound()->isSuperTypeOf($type->getBound())->no()) {
-			return TrinaryLogic::createNo();
-		}
-
-		return TrinaryLogic::createMaybe();
+		return $type->getBound()->isSuperTypeOf($this->getBound())
+			->and(TrinaryLogic::createMaybe());
 	}
 
 	public function inferTemplateTypes(Type $receivedType): TemplateTypeMap

tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php

@@ -106,4 +106,14 @@ public function testBug2723(): void
 		]);
 	}
 
+	public function testBug5706(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-5706.php'], []);
+	}
+
+	public function testBug5844(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-5844.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Functions/data/bug-5706.php

@@ -0,0 +1,18 @@
+<?php declare(strict_types = 1);
+
+namespace Bug5706;
+
+/**
+ * @template T of string|int
+ * @param T $key
+ * @return T
+ */
+function bar($key)
+{
+    if (is_int($key)) {
+        return $key;
+    }
+
+    // String
+    return $key;
+}

tests/PHPStan/Rules/Functions/data/bug-5844.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace Bug5844;
+
+/**
+ * @template T of string|int
+ * @param T $value
+ * @return T
+ */
+function value($value)
+{
+	if (is_string($value)) {
+		return $value;
+	}
+
+	return $value;
+}

tests/PHPStan/Type/StringTypeTest.php

@@ -149,7 +149,7 @@ public function dataAccepts(): iterable
 				TemplateTypeVariance::createInvariant(),
 			)->toArgument(),
 			new StringType(),
-			TrinaryLogic::createNo(),
+			TrinaryLogic::createMaybe(),
 		];
 
 		yield [

tests/PHPStan/Type/TemplateTypeTest.php

@@ -12,6 +12,7 @@
 use PHPStan\Type\Generic\TemplateTypeHelper;
 use PHPStan\Type\Generic\TemplateTypeScope;
 use PHPStan\Type\Generic\TemplateTypeVariance;
+use PHPStan\Type\ObjectWithoutClassType;
 use stdClass;
 use Throwable;
 use function array_map;
@@ -31,37 +32,37 @@ public function dataAccepts(): array
 		);
 
 		return [
-			[
+			0 => [
 				$templateType('T', new ObjectType('DateTime')),
 				new ObjectType('DateTime'),
 				TrinaryLogic::createYes(),
-				TrinaryLogic::createNo(),
+				TrinaryLogic::createMaybe(),
 			],
-			[
+			1 => [
 				$templateType('T', new ObjectType('DateTimeInterface')),
 				new ObjectType('DateTime'),
 				TrinaryLogic::createYes(),
-				TrinaryLogic::createNo(),
+				TrinaryLogic::createMaybe(),
 			],
-			[
+			2 => [
 				$templateType('T', new ObjectType('DateTime')),
 				$templateType('T', new ObjectType('DateTime')),
 				TrinaryLogic::createYes(),
 				TrinaryLogic::createYes(),
 			],
-			[
+			3 => [
 				$templateType('T', new ObjectType('DateTime'), 'a'),
 				$templateType('T', new ObjectType('DateTime'), 'b'),
 				TrinaryLogic::createMaybe(),
-				TrinaryLogic::createNo(),
+				TrinaryLogic::createMaybe(),
 			],
-			[
+			4 => [
 				$templateType('T', null),
 				new MixedType(),
 				TrinaryLogic::createYes(),
 				TrinaryLogic::createYes(),
 			],
-			[
+			5 => [
 				$templateType('T', null),
 				new IntersectionType([
 					new ObjectWithoutClassType(),
@@ -70,6 +71,21 @@ public function dataAccepts(): array
 				TrinaryLogic::createYes(),
 				TrinaryLogic::createYes(),
 			],
+			'accepts itself with a sub-type union bound' => [
+				$templateType('T', new UnionType([
+					new IntegerType(),
+					new StringType(),
+				])),
+				$templateType('T', new IntegerType()),
+				TrinaryLogic::createYes(),
+				TrinaryLogic::createYes(),
+			],
+			'accepts itself with a sub-type object bound' => [
+				$templateType('T', new ObjectWithoutClassType()),
+				$templateType('T', new ObjectType('stdClass')),
+				TrinaryLogic::createYes(),
+				TrinaryLogic::createYes(),
+			],
 		];
 	}
 
@@ -146,7 +162,7 @@ public function dataIsSuperTypeOf(): array
 				$templateType('T', new ObjectType('DateTime')),
 				$templateType('T', new ObjectType('DateTimeInterface')),
 				TrinaryLogic::createMaybe(), // (T of DateTime) isSuperTypeTo (T of DateTimeInterface)
-				TrinaryLogic::createMaybe(), // (T of DateTimeInterface) isSuperTypeTo (T of DateTime)
+				TrinaryLogic::createYes(), // (T of DateTimeInterface) isSuperTypeTo (T of DateTime)
 			],
 			6 => [
 				$templateType('T', new ObjectType('DateTime')),
@@ -211,49 +227,49 @@ public function dataIsSuperTypeOf(): array
 				TrinaryLogic::createMaybe(),
 				TrinaryLogic::createMaybe(),
 			],
-			[
+			15 => [
 				$templateType('T', new MixedType(true)),
 				$templateType('U', new UnionType([new IntegerType(), new StringType()])),
 				TrinaryLogic::createMaybe(),
 				TrinaryLogic::createMaybe(),
 			],
-			[
+			16 => [
 				$templateType('T', new MixedType(true)),
 				$templateType('U', new BenevolentUnionType([new IntegerType(), new StringType()])),
 				TrinaryLogic::createMaybe(),
 				TrinaryLogic::createMaybe(),
 			],
-			[
+			17 => [
 				$templateType('T', new ObjectType(stdClass::class)),
 				$templateType('U', new BenevolentUnionType([new IntegerType(), new StringType()])),
 				TrinaryLogic::createNo(),
 				TrinaryLogic::createNo(),
 			],
-			[
+			18 => [
 				$templateType('T', new BenevolentUnionType([new IntegerType(), new StringType()])),
 				$templateType('T', new BenevolentUnionType([new IntegerType(), new StringType()])),
 				TrinaryLogic::createYes(),
 				TrinaryLogic::createYes(),
 			],
-			[
+			19 => [
 				$templateType('T', new UnionType([new IntegerType(), new StringType()])),
 				$templateType('T', new UnionType([new IntegerType(), new StringType()])),
 				TrinaryLogic::createYes(),
 				TrinaryLogic::createYes(),
 			],
-			[
+			20 => [
 				$templateType('T', new UnionType([new IntegerType(), new StringType()])),
 				$templateType('T', new BenevolentUnionType([new IntegerType(), new StringType()])),
-				TrinaryLogic::createMaybe(),
-				TrinaryLogic::createMaybe(),
+				TrinaryLogic::createYes(),
+				TrinaryLogic::createYes(),
 			],
-			[
+			21 => [
 				$templateType('T', new UnionType([new IntegerType(), new StringType()])),
 				$templateType('T', new IntegerType()),
-				TrinaryLogic::createMaybe(),
+				TrinaryLogic::createYes(),
 				TrinaryLogic::createMaybe(),
 			],
-			[
+			22 => [
 				$templateType('T', new BenevolentUnionType([new IntegerType(), new StringType()])),
 				new UnionType([new BooleanType(), new FloatType(), new IntegerType(), new StringType(), new NullType()]),
 				TrinaryLogic::createMaybe(),