Bug 11219 – isExpression should work on non-type template instantiations

Given a templated type: struct Foo(int x) { enum myValue = x; } I can check if something is an instantiation of this type: template isFoo(T) { enum isFoo = is(T == Foo!U, U...); } But when the template acts only as a namespace: template Bar(T) { enum myValue = x; } the isExpression recognizes Bar!T as not a type, and the comparisons return false. A useful idiom is to store compile-time values in such a template (typetuples that don't auto-flatten, for instance), and later being able to tell if the value being passed is an instantiation of template A or template B: template Rectangle(float w, float h) { enum values = TypeTuple!(w,h); } template Circle(float radius) { enum values = TypeTuple!(radius); } template doStuff(alias A) { // Behave differently for a rectangle than for a circle. }

This cannot work - in this code: template Rectangle(float w, float h) { enum values = TypeTuple!(w,h); } Rectangle!(1.0f, 1.0f) _is_ TypeTuple!(1.0f, 1.0f) - the wrapping template is gone once semantic is evaluated. You're asking for something impossible - determining if a literal _could_ be produced by a template. eg template x(int a) { enum x = a; } static assert(is(x!3 == x!N, N)); Compiler sees it as static assert(is(3 == x!N, N));