Weird template error in Phobos (after editing) that I can't quite get. Compiler bug?

[Atila Neves via Digitalmars-d-learn]

isInputRange looks like this:

template isInputRange(R)
{
     enum bool isInputRange = is(typeof(
     (inout int = 0)
     {
         R r = R.init;     // can define a range object
         if (r.empty) {}   // can test for empty
         r.popFront;       // can invoke popFront()
         auto h = r.front; // can get the front of the range
     }));
}


If I change the `enum bool` line to `enum bool isInputRange = 
true && is(typeof(`, all is fine.
If instead I:

enum foo = true;
enum bool isInputRange = foo && is(typeof(

Then:

std/range/primitives.d(352): Error: static assert  "Cannot put a 
char[] into a Appender!string."
std/format.d(1877):        instantiated from here: 
put!(Appender!string, char[])
std/format.d(1784):        instantiated from here: 
formatUnsigned!(Appender!string, ulong, char)
std/format.d(1755):        instantiated from here: 
formatIntegral!(Appender!string, ulong, char)
std/format.d(3778):        ... (3 instantiations, -v to show) ...
std/typecons.d(421):        instantiated from here: format!(char, 
ulong, ulong)
std/encoding.d(3468):        instantiated from here: Tuple!(BOM, 
"schema", ubyte[], "sequence")


Surely they should be identical? Obviously I was trying to do 
something else with an enum, but this is the reduced sample.

Atila