I've been trying to correctly implement the interpreter
patten/expression templates in D (for reference this is a summary of the
C++ interpreter patten can be found here
http://www.drdobbs.com/184401627). I've run into a problem and I'm not
sure if it's a compiler bug or not. The testcase is:
struct BinaryOp(L,string op,R) {
pragma(msg,"Instansiating " ~ typeof(this).stringof);
BinaryOp!(typeof(this),s,R1) opBinary(string s,R1)(R1 r) {
pragma(msg,"Instansiating BinaryOp.opBinary ~L.stringof ~ op ~
R1.stringof);
return typeof(return)();
}
}
struct Leaf {
BinaryOp!(typeof(this),s,R) opBinary(string s,R)(R r) {
pragma(msg,"Instansiating leaf.opBinary(" ~ R.stringof ~ ")");
return typeof(return)();
}
};
void main() {
Leaf v1,v2,v3;
pragma(msg,"");
pragma(msg,"======= This Compiles ======");
v1*(v2*v3);
pragma(msg,"");
pragma(msg,"======= This Doesn't ======");
(v1*v2)*v3;
}
Output:
======= This Compiles ======
Instansiating BinaryOp!(Leaf,s,Leaf)
Instansiating leaf.opBinary(Leaf)
Instansiating BinaryOp!(Leaf,s,BinaryOp!(Leaf,s,Leaf))
Instansiating leaf.opBinary(BinaryOp!(Leaf,s,Leaf))
======= This Doesn't ======
Error: recursive template expansion for template argument
BinaryOp!(Leaf,s,Leaf)
I've tracked the problem down to the return type of BinaryOp.opBinary.
Clearly putting BinaryOp!(typeof(this),...) would be a Bad Thing in the
main template body but opBinary is a template that may or may not be
instantiated so it shouldn't automatically lead to runaway
instantiation. It seems the compiler is a little bit overzealous in
making sure that such runaway instantiations do not happen.
Is this a bug? Should I file it? Here's what I think a minimal test case
might look like:
struct A(T1) {
void templateFunc(T2)(T2 a) {
alias A!(typeof(this)) error;
}
}
void main() {
A!int a;
a.templateFunc!int(0);
}