Bug 2849 – Length of a local immutable(char[]) cannot be a template value argument

struct bar(int n){} void main(){ immutable(char[]) x = "bug"; bar!(x.length) z; } ----- fog.d(3): Error: expression (x = "bug").length is not a valid template value arg ument fog.d(4): Error: template instance fog.bar!((x = "bug").length) error instantiat ing ========== Yet here are two very similar cases which work: struct bar(int n){} immutable(char[]) x1 = "bug"; void main(){ immutable(char[]) x2 = "bug"; enum q= x2.length; bar!(x1.length) z; // ok, it's a global bar!(q) z2; //ok, it's been copied to an enum } Probably related to issue #2841 and issue #2414.

It's because it doesn't try to const fold it. In this case, the init value is constant, so it could possibly work. In other cases, it could be initialized from something non-constant (eg, a function parameter). Regardless, it should say that it's not constant, rather than just that it's not a valid template parameter.

(In reply to comment #1) > It's because it doesn't try to const fold it. In this case, > the init value is constant, so it could possibly work. In other cases, > it could be initialized from something non-constant (eg, a function > parameter). Regardless, it should say that it's not constant, rather > than just that it's not a valid template parameter. That first sample now works, but w.r.t. errors I can't reproduce the bad error message. E.g.: struct bar (int n){} void main() { char[] x = "bug".dup; bar!(x.length) z; } test.d(5): Error: variable x cannot be read at compile time test.d(5): Error: expression x.length is not a valid template value argument test.d(5): Error: template instance test.bar!(x.length) error instantiating Fixed?