Don't use auto for function return types

lib/fizzy/stack.hpp

@@ -130,15 +130,15 @@ class OperandStack
 
     /// Returns the reference to the top item.
     /// Requires non-empty stack.
-    auto& top() noexcept
+    Value& top() noexcept
     {
         assert(size() != 0);
         return *m_top;
     }
 
     /// Returns the reference to the stack item on given position from the stack top.
     /// Requires index < size().
-    auto& operator[](size_t index) noexcept
+    Value& operator[](size_t index) noexcept
     {
         assert(index < size());
         return *(m_top - index);
@@ -150,7 +150,7 @@ class OperandStack
 
     /// Returns an item popped from the top of the stack.
     /// Requires non-empty stack.
-    auto pop() noexcept
+    Value pop() noexcept
     {
         assert(size() != 0);
         return *m_top--;