Fall back to the full branch type for a conditional guard when the partial set difference still overlaps the other branch

src/Analyser/ScopeOps.php

@@ -505,6 +505,15 @@
 			if ($remainder instanceof NeverType) {
 				continue;
 			}
+			if (
+				$remainder !== $holder->getType()
+				&& !$theirHolder->getType()->isSuperTypeOf($remainder)->no()
+			) {
+				// only part of their type was subtracted (e.g. null from
+				// string|null, but not non-empty-string from string), so the
+				// remainder still overlaps their branch and cannot prove ours
+				$remainder = $holder->getType();
+			}
 			if ($mergedExpressionTypes[$exprString]->getType()->equals($remainder)) {
 				// matching this guard later would not discriminate the branches -
 				// the merged scope already guarantees it

tests/PHPStan/Analyser/nsrt/bug-15483.php

@@ -0,0 +1,77 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug15483;
+
+use function PHPStan\Testing\assertType;
+
+interface Input
+{
+	public function isInteractive(): bool;
+}
+
+function prompt(): ?string
+{
+	return null;
+}
+
+function f(Input $input, mixed $option): void
+{
+	$title = \is_string($option) && $option !== '' ? $option : null;
+	if ($title === null && $input->isInteractive()) {
+		$title = prompt();
+	}
+	if ($title === null) {
+		return;
+	}
+	assertType('bool', $input->isInteractive());
+}
+
+function g(Input $input, mixed $option): void
+{
+	$title = \is_string($option) ? $option : null;
+	if ($title === null && $input->isInteractive()) {
+		$title = prompt();
+	}
+	if ($title === null) {
+		return;
+	}
+	assertType('bool', $input->isInteractive());
+}
+
+/** @param non-empty-string|null $title */
+function h(Input $input, ?string $title): void
+{
+	if ($title === null && $input->isInteractive()) {
+		$title = prompt();
+	}
+	if ($title === null) {
+		return;
+	}
+	assertType('bool', $input->isInteractive());
+}
+
+/** @param int<1, max>|null $count */
+function i(Input $input, ?int $count, ?int $other): void
+{
+	if ($count === null && $input->isInteractive()) {
+		$count = $other;
+	}
+	if ($count === null) {
+		return;
+	}
+	assertType('bool', $input->isInteractive());
+}
+
+/** @param non-empty-array<int>|null $items */
+function j(Input $input, ?array $items, ?array $other): void
+{
+	if ($items === null && $input->isInteractive()) {
+		$items = $other;
+	}
+	if ($items === null) {
+		return;
+	}
+	assertType('bool', $input->isInteractive());
+}

turbo-ext/src/ScopeOps.cpp

@@ -603,6 +603,15 @@ class ScopeOps
 			zv::Val remainder = typeCombinatorRemove(holderType(holder), holderType(theirHolder));
 			if (UNEXPECTED(remainder.isUndef())) return zv::Val();
 			if (remainder.ref().instanceOf(neverTypeCe)) continue;
+			if (Z_OBJ_P(remainder.raw()) != Z_OBJ_P(holderType(holder))) {
+				/* a partial subtraction that still overlaps their branch cannot
+				 * prove ours - fall back to our full type, like the twin */
+				zend_long theirIsSuperTypeOfRemainder;
+				if (UNEXPECTED(!isSuperTypeOfValue(holderType(theirHolder), remainder.raw(), &theirIsSuperTypeOfRemainder))) return zv::Val();
+				if (theirIsSuperTypeOfRemainder != PT_TRI_NO) {
+					remainder = zv::Val::copyOf(zv::Ref(holderType(holder)));
+				}
+			}
 			{
 				zv::Ref mergedHolder = zv::Ref(mergedSlot).deref();
 				if (UNEXPECTED(!pt_check_holder(mergedHolder.raw()))) return zv::Val();