[cfe-dev] Template error without instantiation?

[Handa, Ramneek]

Thanks John. 

Making small immaterial adjustments to code:

x--start--x

class A {};

class B {};

void THIS_IS_AN_ERROR(A& myexp) {
}

template <typename T>
void mytest(const B &value)
{
   THIS_IS_AN_ERROR(A(value));
}

template <typename T>
void mytest(const T &value) 
{
   //Do something useful, ignore what we had here earlier
}

x--end--x

It's a case of template specialization where I would like the compiler
to throw an error if B is being passed to mytest.
If not I would like it to do the *useful thing*.

Could you please point me to where in the standard document does it say
that compiler can reject a template if it has no valid instantiation,
without actually being asked to instantiate it?

Regards.

-----Original Message-----
From: cfe-dev-bounces at cs.uiuc.edu [mailto:cfe-dev-bounces at cs.uiuc.edu]
On Behalf Of John McCall
Sent: Thursday, April 25, 2013 2:17 PM
To: Ramneek Handa
Cc: clang-dev Developers
Subject: Re: [cfe-dev] Template error without instantiation?

On Apr 24, 2013, at 8:02 AM, Ramneek Handa <ramneekhanda at gmail.com>
wrote:
>>> Hey folks,
>>> Can you help check which compiler behaves correctly here? To my
>>> untrained mind it seems to be a clang issue but seems too basic to
have
>>> been missed unintentionally?
>>> 
>>> Regards,
>>> Ramneek
>>> 
>>> x- start -x
>>> class A {};
>>> 
>>> class B {};
>>> 
>>> void myfun(A& myexp) {
>>> }
>>> 
>>> template <typename T>
>>> void mytest(const B &value)
>>> {
>>> myfun(A(value));
>>> }

The behavior of both compilers is consistent with the standard.

This template has no valid instantiation;  there is no type T which will
allow
an A to be constructed from a const l-value of type B.  A template with
no valid
instantiation is ill-formed with no diagnostic required (unless
instantiated, of
course).

Clang is somewhat more aggressive than GCC about diagnosing errors
within
templates at definition time rather than at instantiation time.

>>> template <typename T>
>>> void mytest(const T &value)
>>> {
>>> myfun(A(value));
>>> }

This template does have a valid instantiation:  for example, T=A.
Therefore it
is not ill-formed.

John.
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