[temp.constr.fold]

# 13 Templates [[temp]](./#temp)

## 13.5 Template constraints [[temp.constr]](temp.constr#fold)

### 13.5.2 Constraints [[temp.constr.constr]](temp.constr.constr#temp.constr.fold)

#### 13.5.2.5 Fold expanded constraint [temp.constr.fold]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L1920)

A [*fold expanded constraint*](#def:constraint,fold_expanded "13.5.2.5 Fold expanded constraint [temp.constr.fold]") is formed from a constraint C and
a [*fold-operator*](expr.prim.fold#nt:fold-operator "7.5.7 Fold expressions [expr.prim.fold]") which can either be && or ||[.](#1.sentence-1)

A fold expanded constraint is a pack expansion ([[temp.variadic]](temp.variadic "13.7.4 Variadic templates"))[.](#1.sentence-2)

Let N be the number of elements
in the pack expansion parameters ([[temp.variadic]](temp.variadic "13.7.4 Variadic templates"))[.](#1.sentence-3)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L1928)

A fold expanded constraint whose [*fold-operator*](expr.prim.fold#nt:fold-operator "7.5.7 Fold expressions [expr.prim.fold]") is && is satisfied if it is a valid pack expansion and
if N=0 or if for each i where 0≤i<N in increasing order,C is satisfied
when replacing each pack expansion parameter
with the corresponding ith element[.](#2.sentence-1)

No substitution takes place for any i greater than
the smallest i for which the constraint is not satisfied[.](#2.sentence-2)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L1938)

A fold expanded constraint whose [*fold-operator*](expr.prim.fold#nt:fold-operator "7.5.7 Fold expressions [expr.prim.fold]") is || is satisfied if it is a valid pack expansion,N>0, and if for i where 0≤i<N in increasing order,
there is a smallest i for which C is satisfied
when replacing each pack expansion parameter
with the corresponding ith element[.](#3.sentence-1)

No substitution takes place for any i greater than
the smallest i for which the constraint is satisfied[.](#3.sentence-2)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L1948)

[*Note [1](#note-1)*:

If the pack expansion expands packs of different size,
then it is invalid and the fold expanded constraint is not satisfied[.](#4.sentence-1)

— *end note*]

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L1954)

Two fold expanded constraints are [*compatible for subsumption*](#def:subsumption,compatible_for "13.5.2.5 Fold expanded constraint [temp.constr.fold]") if their respective constraints both contain
an equivalent unexpanded pack ([[temp.over.link]](temp.over.link "13.7.7.2 Function template overloading"))[.](#5.sentence-1)