Bad error message when template function with bad parameter type & constraint is instantiated

[ichordev]

Let's say you tried to make a templated function, but one of the parameters' types isn't semantically correct. If you try to instantiate that function then you will only get an error at the point of instantiation telling you that you didn't 'match the template declaration', when you should really be getting an error from the malformed template. Here's an example:

auto templateFn(alias a)(T b)
if(true){}
alias broken = templateFn!("hello!");

Outputs:

Error: template instance `templateFn!"hello!"` does not match
template declaration `templateFn(alias a)(T b)`'

Note that if the constraint if(true) is removed, then the correct error messages are displayed. In this case:

Error: undefined identifier `T`
Error: template instance `templateFn!"hello!"` error instantiating

You can replace T in this example with any semantically incorrect type (i.e. where is(T) == false) and you get the result. It seems like when the parameter type is _error_ it gets silently allowed, and then it causes an error in the template constraint.