Member predicate ConceptIdExpr::getTemplateArgument

Gets template argument at index index passed to the concept, if any.

For example, if:

template<typename T, T X> concept C = ...;
...
requires { C<int, 1>; };

then getTemplateArgument(0) yields int, and getTemplateArgument(1) yields 1.

If the concept id is a type constraint, then getTemplateArgument(0) will not yield a result.