Apply clang-tidy

test/base-test.cc

@@ -873,11 +873,11 @@ struct custom_container {
 
   using value_type = char;
 
-  size_t size() const { return 0; }
+  auto size() const -> size_t { return 0; }
   void resize(size_t) {}
 
   void push_back(char) {}
-  char& operator[](size_t) { return data; }
+  auto operator[](size_t) -> char& { return data; }
 };
 
 FMT_BEGIN_NAMESPACE