Constant expression required

This error message is usually seen in programming languages like C++ or Java when a constant expression is expected but a non-constant expression is provided. A constant expression is an expression that can be evaluated at compile-time and does not change during the program's execution.

One common example of a constant expression is a literal value like 42 or "hello". On the other hand, a variable or a function call cannot be used as a constant expression if it changes its value during the program's execution.

To fix this error, you need to ensure that only constant expressions are used in contexts where they are expected. This may involve declaring variables as constants using the `const` keyword, or using preprocessor macros or template metaprogramming techniques to evaluate expressions at compile-time.