diff --git a/src/Type/TypeCombinator.php b/src/Type/TypeCombinator.php
index 03ddffd988..cd776efa5f 100644
--- a/src/Type/TypeCombinator.php
+++ b/src/Type/TypeCombinator.php
@@ -3,8 +3,10 @@
 namespace PHPStan\Type;
 
 use PHPStan\Type\Accessory\AccessoryArrayListType;
+use PHPStan\Type\Accessory\AccessoryLowercaseStringType;
 use PHPStan\Type\Accessory\AccessoryNonEmptyStringType;
 use PHPStan\Type\Accessory\AccessoryType;
+use PHPStan\Type\Accessory\AccessoryUppercaseStringType;
 use PHPStan\Type\Accessory\HasOffsetType;
 use PHPStan\Type\Accessory\HasOffsetValueType;
 use PHPStan\Type\Accessory\HasPropertyType;
@@ -452,7 +454,10 @@ private static function compareTypesInUnion(Type $a, Type $b): ?array
 			&& ($b->describe(VerbosityLevel::value()) === 'non-empty-string'
 			|| $b->describe(VerbosityLevel::value()) === 'non-falsy-string')
 		) {
-			return [null, new StringType()];
+			return [null, self::intersect(
+				new StringType(),
+				...self::getAccessoryCaseStringTypes($b),
+			)];
 		}
 
 		if (
@@ -461,7 +466,10 @@ private static function compareTypesInUnion(Type $a, Type $b): ?array
 			&& ($a->describe(VerbosityLevel::value()) === 'non-empty-string'
 				|| $a->describe(VerbosityLevel::value()) === 'non-falsy-string')
 		) {
-			return [new StringType(), null];
+			return [self::intersect(
+				new StringType(),
+				...self::getAccessoryCaseStringTypes($a),
+			), null];
 		}
 
 		if (
@@ -469,10 +477,11 @@ private static function compareTypesInUnion(Type $a, Type $b): ?array
 			&& $a->getValue() === '0'
 			&& $b->describe(VerbosityLevel::value()) === 'non-falsy-string'
 		) {
-			return [null, new IntersectionType([
+			return [null, self::intersect(
 				new StringType(),
 				new AccessoryNonEmptyStringType(),
-			])];
+				...self::getAccessoryCaseStringTypes($b),
+			)];
 		}
 
 		if (
@@ -480,15 +489,32 @@ private static function compareTypesInUnion(Type $a, Type $b): ?array
 			&& $b->getValue() === '0'
 			&& $a->describe(VerbosityLevel::value()) === 'non-falsy-string'
 		) {
-			return [new IntersectionType([
+			return [self::intersect(
 				new StringType(),
 				new AccessoryNonEmptyStringType(),
-			]), null];
+				...self::getAccessoryCaseStringTypes($a),
+			), null];
 		}
 
 		return null;
 	}
 
+	/**
+	 * @return array<Type>
+	 */
+	private static function getAccessoryCaseStringTypes(Type $type): array
+	{
+		$accessory = [];
+		if ($type->isLowercaseString()->yes()) {
+			$accessory[] = new AccessoryLowercaseStringType();
+		}
+		if ($type->isUppercaseString()->yes()) {
+			$accessory[] = new AccessoryUppercaseStringType();
+		}
+
+		return $accessory;
+	}
+
 	private static function unionWithSubtractedType(
 		Type $type,
 		?Type $subtractedType,
diff --git a/tests/PHPStan/Analyser/nsrt/bug-12312.php b/tests/PHPStan/Analyser/nsrt/bug-12312.php
new file mode 100644
index 0000000000..f67d1aa3cb
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-12312.php
@@ -0,0 +1,140 @@
+<?php declare(strict_types = 1);
+
+namespace Bug12312;
+
+use function PHPStan\Testing\assertType;
+
+class HelloWorld
+{
+	/**
+	 * @param lowercase-string $s
+	 */
+	public function sayLowercase(string $s): void
+	{
+		if ($s != '') {
+			assertType('lowercase-string&non-empty-string', $s);
+		}
+		assertType('lowercase-string', $s);
+	}
+
+	/**
+	 * @param lowercase-string $s
+	 */
+	public function sayLowercase2(string $s): void
+	{
+		if ('' != $s) {
+			assertType('lowercase-string&non-empty-string', $s);
+		}
+		assertType('lowercase-string', $s);
+	}
+
+	/**
+	 * @param lowercase-string&non-empty-string $s
+	 */
+	public function sayLowercase3(string $s): void
+	{
+		if ($s != '0') {
+			assertType('lowercase-string&non-falsy-string', $s);
+		}
+		assertType('lowercase-string&non-empty-string', $s);
+	}
+
+	/**
+	 * @param lowercase-string&non-empty-string $s
+	 */
+	public function sayLowercase4(string $s): void
+	{
+		if ('0' != $s) {
+			assertType('lowercase-string&non-falsy-string', $s);
+		}
+		assertType('lowercase-string&non-empty-string', $s);
+	}
+
+	/**
+	 * @param uppercase-string $s
+	 */
+	public function sayUppercase(string $s): void
+	{
+		if ($s != '') {
+			assertType('non-empty-string&uppercase-string', $s);
+		}
+		assertType('uppercase-string', $s);
+	}
+
+	/**
+	 * @param uppercase-string $s
+	 */
+	public function sayUppercase2(string $s): void
+	{
+		if ('' != $s) {
+			assertType('non-empty-string&uppercase-string', $s);
+		}
+		assertType('uppercase-string', $s);
+	}
+
+	/**
+	 * @param uppercase-string&non-empty-string $s
+	 */
+	public function sayUppercase3(string $s): void
+	{
+		if ($s != '0') {
+			assertType('non-falsy-string&uppercase-string', $s);
+		}
+		assertType('non-empty-string&uppercase-string', $s);
+	}
+
+	/**
+	 * @param uppercase-string&non-empty-string $s
+	 */
+	public function sayUppercase4(string $s): void
+	{
+		if ('0' != $s) {
+			assertType('non-falsy-string&uppercase-string', $s);
+		}
+		assertType('non-empty-string&uppercase-string', $s);
+	}
+
+	/**
+	 * @param lowercase-string&uppercase-string $s
+	 */
+	public function sayBoth(string $s): void
+	{
+		if ($s != '') {
+			assertType('lowercase-string&non-empty-string&uppercase-string', $s);
+		}
+		assertType('lowercase-string&uppercase-string', $s);
+	}
+
+	/**
+	 * @param lowercase-string&uppercase-string $s
+	 */
+	public function sayBoth2(string $s): void
+	{
+		if ('' != $s) {
+			assertType('lowercase-string&non-empty-string&uppercase-string', $s);
+		}
+		assertType('lowercase-string&uppercase-string', $s);
+	}
+
+	/**
+	 * @param lowercase-string&uppercase-string&non-empty-string $s
+	 */
+	public function sayBoth3(string $s): void
+	{
+		if ($s != '0') {
+			assertType('lowercase-string&non-falsy-string&uppercase-string', $s);
+		}
+		assertType('lowercase-string&non-empty-string&uppercase-string', $s);
+	}
+
+	/**
+	 * @param lowercase-string&uppercase-string&non-empty-string $s
+	 */
+	public function sayBoth4(string $s): void
+	{
+		if ('0' != $s) {
+			assertType('lowercase-string&non-falsy-string&uppercase-string', $s);
+		}
+		assertType('lowercase-string&non-empty-string&uppercase-string', $s);
+	}
+}