A \grammarterm{requires-expression} is a prvalue of type \tcode{bool}

whose value is described below.

\begin{example}

A \grammarterm{simple-requirement} asserts

the validity of an \grammarterm{expression}.

\begin{note}

The enclosing \grammarterm{requires-expression} will evaluate to \keyword{false}

if substitution of template arguments into the \grammarterm{expression} fails.

A \grammarterm{compound-requirement} asserts properties

of template arguments (if any) and verification of

semantic properties proceed in the following order:

\end{example}

\begin{example}

\begin{codeblock}

template<typename T> concept C = requires (T a) {