Document #: | P3310R0 |

Date: | 2024-05-21 |

Project: | Programming Language C++ |

Audience: |
EWG, CWG |

Reply-to: |
Matheus Izvekov <mizvekov@gmail.com> |

This paper aims to address some lingering issues introduced with the adoption of P0522R0 into C++17, later made a defect report, which relaxed the rules on how templates are matched to template template parameters, but didn’t concern itself with how partial ordering would be affected.

As a result, it invalidated several perfectly legitimate prior uses, creating compatibility issues with old code, forcing implementors to amend the rules, and overall slowing adoption.

We will point out two separate issues and their proposed solutions, which can be picked and voted independently, with the intention that they be adopted as resolutions to [CWG2398], which is the core issue tracking this problem.

The intent is to keep old code working, not to change best practices. With that said, these proposed rules should keep things more consistent and explainable, as candidates having template template parameters with more non-pack parameters will be more specialized than those with fewer, and those taking pack parameters will be less specialized than those than don’t.

If any of the solutions is adopted, it is suggested to bump the version for the feature testing macro.

Consider the following example:

```
template<class T1> struct A;
template<template<class T2> class TT1, class T3>
struct A<TT1<T3>>; // #1
template<template<class T4, class T5> class TT2, class T6, class T7>
struct A<TT2<T6, T7>> {}; // #2
template<class T8, class T9 = float> struct B;
template struct A<B<int>>;
```

Prior to [P0522R0], with the more strict rules,
only the partial specialization candidate
`#2`

would be
considered, since `B`

has two template
parameters, `T8`

and
`T9`

, and the template template
parameter in
`#1`

has only
`T2`

as a template parameter, so
`#1`

doesn’t
match.

After P0522R0,
`#1`

starts
being accepted as a candidate, since
`B`

can be used just fine in place of
`TT1`

in the specialization `A<TT1<T3>>`

:
The default argument
`float`

for
`T9`

allows
`B`

to be specialized with just a
single argument.

However, the rules for partial ordering operate considering only the
candidates themselves, without access to
`B`

, which has the default template
argument. When looking at only those candidates, it’s not obvious the
specialization of `TT1`

would work, if
that were to be replaced with a template with two parameters.

The only clue is the fact that these two candidates are being considered together: this must mean that default arguments are somehow involved.

But nonetheless, as things stand, these candidates cannot be ordered, resulting in ambiguity.

Maintaining the pre-P0522 semantics of picking
`#2`

would be
ideal: Besides that it would be a compatible change, it is logical that
this is the most specialized candidate.

When performing template argument deduction such that
`P`

and
`A`

are specializations of either
class templates or template template parameters:

```
P = TT1<X1, ..., Xn> A = TT2<Y1, ..., Yn>
```

Under the current rules, `TT1`

will
be deduced as `TT2`

.

This paper proposes that in this case,
`TT1`

will be deduced as a new
template, synthesized from and almost identical to
`TT2`

, except that the specialization
arguments
(`Y1, ..., Yn`

)
will be used as default arguments in this new invented TT2, replacing
any existing ones, effectively as if it had been written as:

`template<class T1 = Y1, ..., class Tn = Yn> class TT2`

Conversely, a template template parameter can also be deduced as a class template:

```
template <class T1, class T2 = float> struct A;
template <class T3> struct B;
template <template <class T4> class TT1, class T5> struct B<TT1<T5>>; // #1
template <class T6, class T7> struct B<A<T6, T7>>; // #2
template struct B<A<int>>;
```

When partially ordering
`#1`

versus
`#2`

, TT1 will
be deduced as a synthesized template re-declaration of A with `<T6, T7>`

as default arguments.

When these proposed rules are applied to class template deduction, this example, which is unrelated to partial ordering, also becomes valid:

```
template<class T, class U> struct A {};
<int, float> v;
Atemplate<template<class> class TT> void f(TT<int>);
void g() { f(v); }
```

Here, ‘TT’ picks a synthesized re-declaration of ‘A’ which has ‘float’ as the default argument for the second parameter.

When a parameter is deduced against multiple arguments, consistency must be enforced.

Given this example:

```
template<class T> struct N { using type = T; };
template<class T1, class T2, class T3> struct A;
template<template<class, class> class TT1,
class T1, class T2, class T3, class T4>
struct A<TT1<T1, T2>, TT1<T3, T4>,
typename N<TT1<T1, T2>>::type> {};
template<template<class> class UU1,
template<class> class UU2,
class U1, class U2>
struct A<UU1<U1>, UU2<U2>, typename N<UU1<U1>>::type>;
template<class T8, class T9 = float> struct B;
template struct A<B<int>, B<int>, B<int>>;
```

This was accepted prior to P0522R0, but picking `<T1, T2>`

for the default arguments of `TT1`

will result in this example being accepted, but result in rejection of
the symmetric example where `N<TT1<T1, T2>>`

is replaced with `N<TT1<T1, T4>>`

,
which is also accepted pre-P0522.

There is no known mechanically simple and sound way to solve this. It would not be simple, but sound, if the solution were to include a mechanism to deduce that T2 and T4 must be the same type. Unless there is sufficient motivation to try to salvage this situation, it’s proposed that both forms be rejected, for consistency.

To achieve this consistency, multiple deductions must be combined by stripping out differing default arguments.

Modify §13.10.3.6 13.10.3.6 [temp.deduct.type]/8:

8 A template type argument T, a template template argument TT, or a template non-type argument i can be deduced if P and A have one of the following forms:

cv_{opt} T

T*

T&

T&&

T_{opt} [i_{opt}]

T_{opt} (T_{opt}) noexcept(i_{opt})

T_{opt} T_{opt}::*

TT_{opt}<T>

TT_{opt}<i>

TT_{opt}<TT_{opt}>

TT_{opt}<>

T*

T&

T&&

T

T

T

TT

TT

TT

TT

TT_{opt}

(8.1) -
T_{opt} represents a type or parameter-type-list that either
satisfies these rules recursively, is a non-deduced context in P or A,
or is the same non-dependent type in P and A,

(8.2) -
TT_{opt} represents either a class template or a template
template parameter,

[Example:

```
template<class T, class U> struct A {};
template<template<class> class TT> void f(TT<int>);
void g() {
A<int, float> v;
// OK, TT = A, 'float' is used as default argument the second parameter
f(v); }
```

- end example]

(8.3) -
When matching TT_{opt} specializations, TT_{opt} will be
deduced as if it’s template argument list in A were used as the default
arguments for it’s template parameters, up to the last parameter which
has a corresponding argument. When multiple such arguments are deduced,
only default arguments which are common to all deductions are kept.

Append examples to §13.4.4 13.4.4 [temp.arg.template]/4:

[Example:

```
template <class, class = float> struct A;
template <class> struct B;
template <template <class> class TT, class T> struct B<TT<T>>; // #1
template <class T, class U> struct B<A<T, U>>; // #2
template struct B<A<int>>; // selects #2
template <class> struct C;
template <template <class> class TT, class T> struct C<TT<T>>; // #3
template <class T, class U> struct C<A<T, U>>; // #4 template struct C<A<int>>; // selects #4
```

- end example]

[Example:

```
template<class T> struct N { using type = T; };
template<class, class = float> struct A;
template<class, class, class> struct B;
template<template<class, class> class TT,
class T1, class T2, class T3, class T4>
struct B<TT<T1, T2>, TT<T3, T4>,
typename N<TT<T1, T2>>::type>;
template<template<class> class UU1,
template<class> class UU2,
class U1, class U2>
struct B<UU1<U1>, UU2<U2>, typename N<UU1<U1>>::type>;
template struct B<A<int>, A<int>, A<int>>; // ambiguous
template<class, class, class> struct C;
template<template<class, class> class TT,
class T1, class T2, class T3, class T4>
struct C<TT<T1, T2>, TT<T3, T4>,
typename N<TT<T1, T4>>::type>;
template<template<class> class UU1,
template<class> class UU2,
class U1, class U2>
struct C<UU1<U1>, UU2<U2>, typename N<UU1<U1>>::type>;
template struct C<A<int>, A<int>, A<int>>; // ambiguous
```

- end example]

Modify example in §13.7.8 13.7.8 [temp.alias]/2:

[Example 1:

```
-13,7 +13,8
template<template<class> class TT>
void f(TT<int>);
-f(v); // error: Vec not deduced
+// OK, TT = vector, Alloc<int> used as default argument for second parameter
+f(v);
template<template<class,class> class TT> void g(TT<int, Alloc<int>>);
```

— end example]

Consider the following example:

```
template<template<class ...T1s> class TT1> struct A {};
template<class T2> struct B;
template struct A<B>; // #1
template<template<class T3> class TT2> struct C {};
template<class ...T4s> struct D;
template struct C<D>; // #2
```

Before [P0522R0],
`#1`

was
valid, and
`#2`

was
invalid.

After P0522R0,
`#1`

stayed
valid, and
`#2`

became
valid.

Now consider this partial ordering example:

```
template<class T1> struct A;
template<template<class ...T2s> class TT1, class T3>
struct A<TT1<T3>>; // #1
template<template<class T4> class TT2, class T5>
struct A<TT2<T5>> {}; // #2
template<class T6> struct B;
template struct A<B<int>>;
```

Before P0522R0,
`#2`

was
picked.

However, since the same rules which determine a template can bind to a template template parameter, are also used to determine one template template parameter is at least as specialized as another, and since P0522R0 made no special provisions in these rules for the latter, this example now becomes ambiguous.

But this is undesirable, because it is a breaking change, and also
because logically
`#2`

is more
specialized.

The new paragraph inserted with P0522R0, which defines the ‘at least as specialized as’ rules in terms of function template rewrite, leads to this confusion:

The template argument list deduction rules accept packs in parameters
and no packs in arguments, while the reverse is rejected, but in order
to accept both
`#1`

and
`#2`

from the
first example, both directions need to be accepted. An exception had to
be carved out in 13.4.4
[temp.arg.template]/3
for this.

We propose a solution which will restore the semantics of the second example, by specifying that during partial ordering, packs must match non-packs in only one direction.

Now consider this last example:

```
template <template <class... > class TT1> struct A { static constexpr int V = 0; };
template <template <class > class TT2> struct A<TT2> { static constexpr int V = 1; };
template <template <class, class> class TT3> struct A<TT3> { static constexpr int V = 2; };
template <class ... > struct B;
template <class > struct C;
template <class, class > struct D;
template <class, class, class> struct E;
static_assert(A<B>::V == 0);
static_assert(A<C>::V == 1);
static_assert(A<D>::V == 2);
static_assert(A<E>::V == 0);
```

Before P0522R0, this is accepted.

After P0522R0, this became wholly invalid: The partial
specializations are not more specialized than the primary template.
Also, the `A<B>`

specialization becomes ambiguous.

With the proposed solution, the primary template becomes less specialized again.

But the other issue will remain: The `A<B>`

specialization will stay ambiguous. A solution to this last problem is
left for future work.

We propose changing the deduction rules such that, only during partial ordering, packs matching to non-packs isn’t accepted both ways, that it should only be accepted in the argument to parameter direction, which is the opposite direction it’s normally accepted in the deduction of template argument lists.

We also propose scratching the classic pack exception clause in [temp.arg.template]/3{.sref}, in favor of explicitly specifying that outside of partial ordering, packs matching to non-packs must be accepted both ways, parameter to argument as well as argument to parameter.

Modify §13.10.3.6 13.10.3.6 [temp.deduct.type]/9:

9
If P has a form that contains <T>~~or~~, <i>, or <TT_{opt}>,
then each argument P_{i} of the respective template argument
list of P is compared with the corresponding argument A_{i} of
the corresponding template argument list of A. If the template argument
list of P contains a pack expansion that is not the last template
argument, the entire template argument list is a non-deduced context.
~~If P~~_{i} is a pack
expansion, then the pattern of P_{i} is compared with each
remaining argument in the template argument list of A. Each comparison
deduces template arguments for subsequent positions in the template
parameter packs expanded by P_{i}. During partial ordering, if
A_{i} was originally a pack expansion:

(9.1) - if
P does not contain a template argument corresponding to A_{i}
then A_{i} is ignored;

(9.2) -
otherwise, if P_{i} is not a pack expansion, template argument
deduction fails.

(9.1)
Except when deducing the
argument list of X as specified in 13.4.4
[temp.arg.template]/4,
while checking deduced template arguments, if P_{i}
is a pack expansion, then the pattern of P_{i} is compared with
each remaining argument in the template argument list of A. Each
comparison deduces template arguments for subsequent positions in the
template parameter packs expanded by P_{i}. During partial
ordering, if A_{i} was originally a pack expansion:

(9.1.1) -
if P does not contain a template argument corresponding to A_{i}
then A_{i} is ignored;

(9.1.2) -
otherwise, if P_{i} is not a pack expansion, template argument
deduction fails.

(9.2)
When deducing the argument
list of X as specified in 13.4.4
[temp.arg.template]/4,
if A_{i} is a pack expansion, then the pattern of A_{i}
is compared with each remaining argument in the template argument list
of P. Each comparison deduces template arguments for subsequent
positions in the template parameter packs expanded by A_{i}. If
P_{i} was originally a pack expansion:

(9.2.1) -
if A does not contain a
template argument corresponding to P_{i} then P_{i} is
ignored;

(9.2.2) -
otherwise, if
A_{i} is not a pack expansion, template argument deduction
fails.

[Note: (9.2) is the reverse of (9.1) - end note]

Modify §13.4.4 13.4.4 [temp.arg.template]/3:

3
A template-argument matches a template template-parameter P when P is at
least as specialized as the template-argument A. In this comparison, if
P is unconstrained, the constraints on A are not considered. ~~If P contains a template
parameter pack, then A also matches P if each of A’s template parameters
matches the corresponding template parameter in the template-head of
P~~. Two template parameters match if they are of the same
kind (type, non-type, template), for non-type template-parameters, their
types are equivalent (13.7.7.2
[temp.over.link]),
and for template template-parameters, each of their corresponding
template-parameters matches, recursively. When P’s template-head
contains a template parameter pack (13.7.4
[temp.variadic]),
the template parameter pack will match zero or more template parameters
or template parameter packs in the template-head of A with the same type
and form as the template parameter pack in P (ignoring whether those
template parameters are template parameter packs).

Append note and example to §13.4.4 13.4.4 [temp.arg.template]/4:

[Note: 13.10.3.6 [temp.deduct.type]/9 has a special case for this deduction - end note]

[Example:

```
template<class T1> struct A;
template<template<class ...> class TT, class T> struct A<TT<T>>; // #1
template<template<class > class TT, class T> struct A<TT<T>>; // #2
template<class> struct B;
template struct A<B<int>>; // selects #2
template <template <class...> class TT> struct C;
// This partial specialization is more specialized than the primary template.
template <template <class> class TT> struct C<TT>;
template <template <class> class TT> struct D;
// This partial specialization is NOT more specialized than the primary template. template <template <class...> class TT> struct D<TT>;
```

- end example]

[CWG2398] Jason Merrill. 2016-12-03. Template template parameter
matching and deduction.

https://wg21.link/cwg2398

[P0522R0] James Touton, Hubert Tong. 2016-11-11. DR: Matching of
template template-arguments excludes compatible templates.

https://wg21.link/p0522r0