The following code fails:
template Foo()
{
   mixin("alias char[] TheType;");
   static if (is(TheType : char[]))
       static assert(true);
   else
       static assert(false);
}

alias Foo!() instance1;

mixins1.d(7): static assert  is false

The following similar programs all compile:
// 1. Replace the mixin with the statement itself
template Foo()
{
   alias char[] TheType;
   static if (is(TheType : char[]))
       static assert(true);
   else
       static assert(false);
}

alias Foo!() instance1;

// 2. Add a dummy declaration at the beginning of the template
template Foo()
{
   char[] dummy;
   mixin("alias char[] TheType;");
   static if (is(TheType : char[]))
       static assert(true);
   else
       static assert(false);
}

alias Foo!() instance1;

// 3. Replace static if with static assert
template Foo()
{
   mixin("alias char[] TheType;");
   static assert(is(TheType : char[]));
}

alias Foo!() instance1;