A template definition in block scope is not allowed, but it would at least allow for something like:
template<class M, class N>
{
M m;
N n;
} adhocstruct= { expression for m, expression for n};
adhocstruct.m ...
...only when this proposal comes through:
In contrast, generic lambda's which are just like templates are allowed in block scope.
And the capture list of a lambda is in fact exactly this: a ad hoc structure with auto typed members that have a name, unfortunately only inside the lambda scope.
So why not have something like a capture list of a lambda with outside access of its named auto typed members?
A kind of structured binding with initializing expressions for each member?
auto f()
{
auto adhocstruct=[m=expression_m, n=expression_n];
g(adhocstruct.m+adhocstruct.n);
h(adhocstruct);
return adhocstruct;
}
auto sometransform()
{
...
return [result=expression, tolerance=expression, moved=std::move(movedfrom)];
}
decltype(sometransform()) sometransformresultvalue;
Using tuples for that has a major drawback of not being able to give descriptive names for members.
Using predefined structs has three major drawbacks: more typing/code (DRY), less locality and no auto member type deduction.