template parameter can't be a locally-declared type?

[Phil Endecott]

Dear Experts,

It seems that I'm not allowed to use a locally-declared type as an
implicit template parameter. Example:

struct Foo {
template <typename T>
Foo(const T& t) {}
};

void f() {
struct {
int a;
} s;
Foo x(s); // doesn't work; "no matching function"
}

void g() {
struct S {
int a;
};
S s;
Foo x(s); // also doesn't work, so it's the local scope not
// being anonymous that's the problem
}

struct stru {
int a;
};
void h() {
stru s;
Foo x(s); // does work.
}


This surprises me. Is this right? What's the rationale?


Cheers, Phil.

 

[James Kanze]

It seems that I'm not allowed to use a locally-declared type
as an implicit template parameter.

Nor as an explicit one. Instantiation types (and addresses)
must have external binding.

[...]

This surprises me. Is this right? What's the rationale?

Probably implementation problems. Unless the binding is
external, the definition isn't available in other translation
units. And orginaly, there was no requirement for the template
code to be available when you compiled---template instantiations
were always in another translation unit.