• Andrej Mitrovic (18/18) Aug 21 2011 void foo(T)(T t) if(is(X == struct)) { }
• Timon Gehr (7/25) Aug 22 2011 No, because that would defeat the main purpose of 'is'. 'is' is to a

Andrej Mitrovic <andrej.mitrovich gmail.com> writes:

void foo(T)(T t) if(is(X == struct)) { }

void main()
{
    foo(4);
}

This prints out:

test.d(9): Error: template test.foo(T) if (is(X == struct)) does not
match any function template declaration
test.d(9): Error: template test.foo(T) if (is(X == struct)) cannot
deduce template function
from argument types !()(int)

which is your typical template instantiation error. The real issue is
that the symbol "X" doesn't actually exist.

I've had a template I was refactoring and I've accidentally mistyped
the name of the type, e.g.:

void foo(T)(T r) if (is(T == struct) || is(t == class))  // t should have been T

I was hoping for a better error message here. Can the compiler check
that `is` has an expression with valid symbols in it?

Timon Gehr <timon.gehr gmx.ch> writes:

On 08/22/2011 04:16 AM, Andrej Mitrovic wrote:
 void foo(T)(T t) if(is(X == struct)) { }

 void main()
 {
      foo(4);
 }

 This prints out:

 test.d(9): Error: template test.foo(T) if (is(X == struct)) does not
 match any function template declaration
 test.d(9): Error: template test.foo(T) if (is(X == struct)) cannot
 deduce template function
 from argument types !()(int)

 which is your typical template instantiation error. The real issue is
 that the symbol "X" doesn't actually exist.

 I've had a template I was refactoring and I've accidentally mistyped
 the name of the type, e.g.:

 void foo(T)(T r) if (is(T == struct) || is(t == class))  // t should have been
T

 I was hoping for a better error message here. Can the compiler check
 that `is` has an expression with valid symbols in it?

No, because that would defeat the main purpose of 'is'. 'is' is to a 
large extent there to allow you to check for conditions that are not 
necessarily semantically valid.

However, the compiler could catch that the result of some parts of the 
constraint does not actually depend on any template arguments to improve 
the error message. Maybe you could write an enhancement request for this.