diff --git a/src/Analyser/TypeSpecifier.php b/src/Analyser/TypeSpecifier.php
index bcb9dafae0..159a523599 100644
--- a/src/Analyser/TypeSpecifier.php
+++ b/src/Analyser/TypeSpecifier.php
@@ -1303,6 +1303,28 @@ private function specifyTypesForConstantStringBinaryExpression(
 			)->setRootExpr($rootExpr);
 		}
 
+		if (
+			$context->false()
+			&& $exprNode instanceof FuncCall
+			&& $exprNode->name instanceof Name
+			&& in_array(strtolower((string) $exprNode->name), [
+				'trim', 'ltrim', 'rtrim',
+				'mb_trim', 'mb_ltrim', 'mb_rtrim',
+			], true)
+			&& isset($exprNode->getArgs()[0])
+			&& $constantStringValue === ''
+		) {
+			return $this->create(
+				$exprNode->getArgs()[0]->value,
+				TypeCombinator::intersect(
+					new StringType(),
+					new AccessoryNonEmptyStringType(),
+				),
+				$context->negate(),
+				$scope,
+			)->setRootExpr($rootExpr);
+		}
+
 		return null;
 	}
 
diff --git a/tests/PHPStan/Analyser/nsrt/bug-12973-php84.php b/tests/PHPStan/Analyser/nsrt/bug-12973-php84.php
new file mode 100644
index 0000000000..cfbe20ce83
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-12973-php84.php
@@ -0,0 +1,29 @@
+<?php // lint >= 8.4
+
+namespace Bug12973Php84;
+
+use function PHPStan\Testing\assertType;
+
+function mbtrim($value): void
+{
+	if (mb_trim($value) === '') {
+		assertType('mixed', $value);
+	} else {
+		assertType('non-empty-string', $value);
+	}
+	assertType('mixed', $value);
+
+	if (mb_ltrim($value) === '') {
+		assertType('mixed', $value);
+	} else {
+		assertType('non-empty-string', $value);
+	}
+	assertType('mixed', $value);
+
+	if (mb_rtrim($value) === '') {
+		assertType('mixed', $value);
+	} else {
+		assertType('non-empty-string', $value);
+	}
+	assertType('mixed', $value);
+}
diff --git a/tests/PHPStan/Analyser/nsrt/bug-12973.php b/tests/PHPStan/Analyser/nsrt/bug-12973.php
new file mode 100644
index 0000000000..14116eef93
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-12973.php
@@ -0,0 +1,79 @@
+<?php
+
+namespace Bug12973;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @return non-empty-string|null
+ */
+function nullIfBlank(?string $value): ?string
+{
+	if ($value !== null) {
+		return \trim($value) === '' ? null : $value;
+	}
+
+	return null;
+}
+
+function trimMixed($value): void
+{
+	if (trim($value) === '') {
+		assertType('mixed', $value);
+	} else {
+		assertType('non-empty-string', $value);
+	}
+	assertType('mixed', $value);
+
+}
+
+function trimTypes(string $value): void
+{
+	if (trim($value) === '') {
+		assertType('string', $value);
+	} else {
+		assertType('non-empty-string', $value);
+	}
+	assertType('string', $value);
+
+	if (trim($value) !== '') {
+		assertType('non-empty-string', $value);
+	} else {
+		assertType('string', $value);
+	}
+	assertType('string', $value);
+}
+
+function ltrimTypes(string $value): void
+{
+	if (ltrim($value) === '') {
+		assertType('string', $value);
+	} else {
+		assertType('non-empty-string', $value);
+	}
+	assertType('string', $value);
+
+	if (ltrim($value) !== '') {
+		assertType('non-empty-string', $value);
+	} else {
+		assertType('string', $value);
+	}
+	assertType('string', $value);
+}
+
+function rtrimTypes(string $value): void
+{
+	if (rtrim($value) === '') {
+		assertType('string', $value);
+	} else {
+		assertType('non-empty-string', $value);
+	}
+	assertType('string', $value);
+
+	if (rtrim($value) !== '') {
+		assertType('non-empty-string', $value);
+	} else {
+		assertType('string', $value);
+	}
+	assertType('string', $value);
+}
diff --git a/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php b/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php
index 0daa90073a..c1fac8e311 100644
--- a/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php
+++ b/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php
@@ -378,4 +378,11 @@ public function testBug9401(): void
 		]);
 	}
 
+	public function testBug12973(): void
+	{
+		$this->checkNullables = true;
+		$this->checkExplicitMixed = true;
+		$this->analyse([__DIR__ . '/../../Analyser/nsrt/bug-12973.php'], []);
+	}
+
 }