Re: [G++] templates not instantiated early enough

[Gabriel Dos Reis <gdr at integrable-solutions dot net>]

Chris Lattner <sabre@nondot.org> writes:

| I've been tracking down a G++ bug when using my target, and I think that
| the problem needs to be fixed in the generic portion of the C++ front-end
| (which I know very little about).
| 
| In particular, consider this testcase:
| 
| template<typename Ty>
| struct some_template {};
| 
| void foo(some_template<int>);
| 
| void bar() {
|   foo;
| }
| 
| Now it appears that G++ is delaying instantiating the some_template type
| until it actually needs it.

When the declaration of foo is seen, the completeness of the class
type some_template<int> is not needed, only its declaration -- which
the compiler synthetizes.

|  In particular, the prototype for foo doesn't
| get emitted to the backend until foo is referenced in "bar".

I don't understand this.  When foo is declared, it gets assigned a
type, which is FUNCTION_TYPE (void, some_template<int>).

|  This happens
| when the parser sees the reference to foo, and decides that it really
| is referring to the foo function in finish_id_expression.
| 
| The problem is that, in this testcase (which is reduced from a
| non-pointless larger testcase), the some_template is never instantiated
| with an int type.

It does not need to.  Its completeness is not required.  It is only
referenced in a context of parameter-type-list, in a declaration
of foo and no call if made to that function.

|  This causes problems for my backend, which needs to
| look at the types for arguments for foo.

If your backend unconditionally assumes that any type referenced in a
function declaration ought to be a complete type, then it contains a bug.  

-- Gaby