Can Lambda Functions Be Used with `constexpr` in C ?

Lambda Functions and Constexpr

Question: Is it possible to use constexpr with lambda functions in C ?

In the following code example, the compiler reports an error related to requiring a constexpr function:

<code class="cpp">struct Test
{
  static const int value = []() -> int { return 0; } ();
};</code>

Answer:

Update (C 17): As of C 17, lambda functions are allowed in constant expressions.

Pre-C 17: Lambdas are not currently permitted in constant expressions in C 14. However, a proposal (N4487) has been made to allow certain lambda expressions and closure objects to appear in constant expressions.

If this proposal is accepted, it would introduce the following changes:

• Closure types would be considered literal types if their data members are all literal types.
• If no constexpr specifier is provided in the lambda declarator, the generated function call operator would be constexpr if it meets the requirements for a constexpr function.

This change would align with the behavior of implicitly defined constructors and assignment operator functions, which can be inferred as constexpr when appropriate.

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