diff --git a/src/Reflection/InitializerExprTypeResolver.php b/src/Reflection/InitializerExprTypeResolver.php
index abf5384f2a4..097176485d0 100644
--- a/src/Reflection/InitializerExprTypeResolver.php
+++ b/src/Reflection/InitializerExprTypeResolver.php
@@ -2335,8 +2335,13 @@ private function resolveCommonMath(Expr\BinaryOp $expr, Type $leftType, Type $ri
 		$leftNumberType = $leftType->toNumber();
 		$rightNumberType = $rightType->toNumber();
 
+		// a template type of mixed does not absorb the other operand in the union
+		$typesMixed = $types instanceof MixedType
+			|| $leftType instanceof MixedType
+			|| $rightType instanceof MixedType;
+
 		if (
-			!$types instanceof MixedType
+			!$typesMixed
 			&& (
 				$rightNumberType instanceof IntegerRangeType
 				|| $rightNumberType instanceof ConstantIntegerType
@@ -2397,14 +2402,14 @@ private function resolveCommonMath(Expr\BinaryOp $expr, Type $leftType, Type $ri
 
 		$resultType = TypeCombinator::union($leftNumberType, $rightNumberType);
 		if ($expr instanceof Expr\BinaryOp\Div) {
-			if ($types instanceof MixedType || $resultType->isInteger()->yes()) {
+			if ($typesMixed || $resultType->isInteger()->yes()) {
 				return new BenevolentUnionType([new IntegerType(), new FloatType()]);
 			}
 
 			return new UnionType([new IntegerType(), new FloatType()]);
 		}
 
-		if ($types instanceof MixedType
+		if ($typesMixed
 			|| $leftType instanceof BenevolentUnionType
 			|| $rightType instanceof BenevolentUnionType
 		) {
diff --git a/tests/PHPStan/Analyser/nsrt/bug-15388.php b/tests/PHPStan/Analyser/nsrt/bug-15388.php
new file mode 100644
index 00000000000..3187e30115a
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-15388.php
@@ -0,0 +1,53 @@
+<?php declare(strict_types = 1);
+
+namespace Bug15388;
+
+use function PHPStan\Testing\assertType;
+
+/** @param list<int> $vals */
+function foo(array $vals): void {
+	assertType('(float|int)', array_reduce($vals, function($carry, $val) {
+		return $carry + map($val);
+	}, 0));
+	assertType('(float|int)', array_reduce($vals, function($carry, $val) {
+		return $carry + $val;
+	}, 0));
+	assertType('(float|int)', array_reduce($vals, function($carry, int $val) {
+		return $carry - $val;
+	}, 0));
+	assertType('(float|int)', array_reduce($vals, function($carry, int $val) {
+		return $carry * $val;
+	}, 1));
+}
+
+function map(int $val): int {
+	return $val;
+}
+
+/**
+ * @template T
+ * @param T $t
+ * @param mixed $m
+ */
+function templateOfMixed($t, $m, int $i): void
+{
+	assertType('(float|int)', $m + $i);
+	assertType('(float|int)', $t + $i);
+	assertType('(float|int)', $i + $t);
+	assertType('(float|int)', $t + 1);
+	assertType('(float|int)', $t - $i);
+	assertType('(float|int)', $t - 1);
+	assertType('(float|int)', $t * $i);
+	assertType('(float|int)', $t / $i);
+	assertType('(float|int)', $t ** $i);
+	assertType('(float|int)', $t << $i);
+	assertType('int', $t % $i);
+
+	$a = $t;
+	$a += $i;
+	assertType('(float|int)', $a);
+
+	$a = $t;
+	$a++;
+	assertType('(float|int)', $a);
+}
diff --git a/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php b/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php
index daccbe3882c..bdd01c80cb2 100644
--- a/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php
+++ b/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php
@@ -542,6 +542,13 @@ public function testBug11507(): void
 		$this->analyse([__DIR__ . '/data/bug-11507.php'], []);
 	}
 
+	public function testBug15388(): void
+	{
+		$this->checkNullables = true;
+		$this->checkExplicitMixed = false;
+		$this->analyse([__DIR__ . '/data/bug-15388.php'], []);
+	}
+
 	public function testBug15245(): void
 	{
 		$this->checkNullables = true;
diff --git a/tests/PHPStan/Rules/Functions/data/bug-15388.php b/tests/PHPStan/Rules/Functions/data/bug-15388.php
new file mode 100644
index 00000000000..ff9841e83bc
--- /dev/null
+++ b/tests/PHPStan/Rules/Functions/data/bug-15388.php
@@ -0,0 +1,21 @@
+<?php declare(strict_types = 1);
+
+namespace Bug15388ReturnType;
+
+/** @param list<int> $vals */
+function foo(array $vals): int {
+	return array_reduce($vals, function($carry, $val) {
+		return $carry + map($val);
+	}, 0);
+}
+
+/** @param list<int> $vals */
+function bar(array $vals): int {
+	return array_reduce($vals, function($carry, $val) {
+		return $carry + $val;
+	}, 0);
+}
+
+function map(int $val): int {
+	return $val;
+}
diff --git a/turbo-ext/src/InitializerExprTypeResolver.cpp b/turbo-ext/src/InitializerExprTypeResolver.cpp
index a8dccb753ac..cb1e968af0e 100644
--- a/turbo-ext/src/InitializerExprTypeResolver.cpp
+++ b/turbo-ext/src/InitializerExprTypeResolver.cpp
@@ -2678,7 +2678,10 @@ class InitializerExprTypeResolver
 		IETR_VAL(leftNumberType, callOn(leftType, mToNumber));
 		IETR_VAL(rightNumberType, callOn(rightType, mToNumber));
 
-		bool typesMixed = isA(types.raw(), pt_ce_mixed_type);
+		// a template type of mixed does not absorb the other operand in the union
+		bool typesMixed = isA(types.raw(), pt_ce_mixed_type)
+			|| isA(leftType, pt_ce_mixed_type)
+			|| isA(rightType, pt_ce_mixed_type);
 		if (
 			!typesMixed
 			&& (