Can Lambda Functions Be Used in `constexpr` Contexts?

constexpr Support for Lambda Functions

constexpr support for lambda functions has been a widely discussed topic in the C community. Although lambdas are currently not allowed in constant expressions according to [expr.const]/(2.6), N4487, which is included in the working draft N4582, proposes to remove this restriction.

Proposed lambda-related changes:

• Allow lambdas in constant expressions.
• Consider a closure type as a literal type if all of its data members are literal types.
• If the constexpr specifier is omitted within the lambda declarator, the generated function call operator is considered constexpr if it meets the requirements of a constexpr function.

Example:

The following example will be valid once N4487 is accepted:

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

Workaround:

As a workaround before constexpr support is officially added, you can use a function template instead of a lambda:

<code class="c++">struct Test
{
  template <typename>
  static const int value = 0;
};</code>

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