V1080. Call of 'std::is_constant_evaluated' function always returns the same value.

The diagnostic rule warns that the 'std::is_constant_evaluated' function's return value has no ability to ever change. This may lead to unreachable code.

This rule covers two cases:

• The function is called in the context, that is always the compile-time context, and returns 'true'.
• The function is called in the context, that is always the runtime context, and returns 'false'.

Example: we need to write a function that would contain two versions of one algorithm - one version for compile-time, the other version for runtime. We'll use the 'std::is_constant_evaluated' function to differentiate between these two implementations.

#include <type_traits>

constexpr void foo()
{
  constexpr auto context = std::is_constant_evaluated();
  //....
  if (context)
  {
    // compile-time logic
  }
  else
  {
    // runtime logic
  }
}

Here the 'std::is_constant_evaluated' function always returns 'true'. The 'else' branch contains unreachable code.

The opposite situation may happen if we remove the 'costexpr' specifier from the 'context' variable and from the 'foo' function.

#include <type_traits>

void foo()
{
  auto context = std::is_constant_evaluated();
  //....
  if (context)
  {
    // compile-time logic
  }
  else
  {
    // runtime logic
  }
}

Here the 'context' variable is always 'false'. The code becomes unreachable as early as the 'then' branch.

The 'std::is_constant_evaluated' always returns 'true' if called in one of the following locations:

• inside 'static_assert';
• inside the 'consteval' function;
• inside the 'if constexpr' condition.

The 'std::is_constant_evaluated' always returns 'true' if called here:

• inside a function that is not marked with the 'constexpr' / 'consteval' specifiers.

This diagnostic is classified as:

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