Eliminate non-generic types before inferring from unions

src/Type/UnionType.php

@@ -554,8 +554,28 @@ public function toArray(): Type
 	public function inferTemplateTypes(Type $receivedType): TemplateTypeMap
 	{
 		$types = TemplateTypeMap::createEmpty();
+		if ($receivedType instanceof UnionType) {
+			$myTypes = [];
+			$remainingReceivedTypes = [];
+			foreach ($receivedType->getTypes() as $receivedInnerType) {
+				foreach ($this->types as $type) {
+					if ($type->isSuperTypeOf($receivedInnerType)->yes()) {
+						$types = $types->union($type->inferTemplateTypes($receivedInnerType));
+						continue 2;
+					}
+					$myTypes[] = $type;
+				}
+				$remainingReceivedTypes[] = $receivedInnerType;
+			}
+			if (count($remainingReceivedTypes) === 0) {
+				return $types;
+			}
+			$receivedType = TypeCombinator::union(...$remainingReceivedTypes);
+		} else {
+			$myTypes = $this->types;
+		}
 
-		foreach ($this->types as $type) {
+		foreach ($myTypes as $type) {
 			$types = $types->union($type->inferTemplateTypes($receivedType));
 		}
 

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -5646,6 +5646,11 @@ public function dataBug4423(): array
 		return $this->gatherAssertTypes(__DIR__ . '/data/bug-4423.php');
 	}
 
+	public function dataGenericUnions(): array
+	{
+		return $this->gatherAssertTypes(__DIR__ . '/data/generic-unions.php');
+	}
+
 	/**
 	 * @dataProvider dataArrayFunctions
 	 * @param string $description
@@ -11258,6 +11263,7 @@ private function gatherAssertTypes(string $file): array
 	 * @dataProvider dataPseudoTypeOverrides
 	 * @dataProvider dataGenericTraits
 	 * @dataProvider dataBug4423
+	 * @dataProvider dataGenericUnions
 	 * @param string $assertType
 	 * @param string $file
 	 * @param mixed ...$args

tests/PHPStan/Analyser/data/generic-unions.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace GenericUnions;
+
+use function PHPStan\Analyser\assertType;
+
+class Foo
+{
+
+	/**
+	 * @template T
+	 * @param T|null $p
+	 * @return T
+	 */
+	public function doFoo($p)
+	{
+		if ($p === null) {
+			throw new \Exception();
+		}
+
+		return $p;
+	}
+
+	/**
+	 * @template T
+	 * @param T $p
+	 * @return T
+	 */
+	public function doBar($p)
+	{
+		return $p;
+	}
+
+	/**
+	 * @template T
+	 * @param T|int|float $p
+	 * @return T
+	 */
+	public function doBaz($p)
+	{
+		return $p;
+	}
+
+	/**
+	 * @param int|string $stringOrInt
+	 */
+	public function foo(
+		?string $nullableString,
+		$stringOrInt
+	): void
+	{
+		assertType('string', $this->doFoo($nullableString));
+		assertType('int|string', $this->doFoo($stringOrInt));
+
+		assertType('string|null', $this->doBar($nullableString));
+
+		assertType('int', $this->doBaz(1));
+		assertType('string', $this->doBaz('foo'));
+		assertType('float', $this->doBaz(1.2));
+		assertType('string', $this->doBaz($stringOrInt));
+	}
+
+}