• d-bugmail puremagic.com (52/52) Oct 17 2022 https://issues.dlang.org/show_bug.cgi?id=23424

d-bugmail puremagic.com writes:

https://issues.dlang.org/show_bug.cgi?id=23424

          Issue ID: 23424
           Summary: template with overloads not clear why it doesn't match
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: diagnostic
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: schveiguy gmail.com

If I create a template that accepts a type, and try to instantiate it with
something other than a type, it fails with a clear message:

```d
template T(X) { int v; }

void main() {
    T!(1).v = 5;
}
```

Error: template instance `T!1` does not match template declaration `T(X)`

However, if I create just one overload, it no longer helps me with the clear
message:

```d
template T(X) { int v; }
template T() { int v; }

void main() {
    T!(1).v = 5;
}
```

Error: template `onlineapp.T` does not match any template declaration

It tells me the template FQN, but the "does not match any template declaration"
is puzzling -- yes, it doesn't match any overloaded declaration, but it seems
to suggest there is no declaration.

Comparing to a failure to match a function template:

```d
void T(X)() { int v; }
void T()() { int v; }

void main() {
    T!(1)();
}
```

Error: none of the overloads of template `onlineapp.T` are callable using
argument types `!(1)()`
       Candidates are: `T(X)()`
                       `T()()`

This is very clear and informative. It also identifies possible things that it
tries, just like the non-overloaded template example.

A similar approach should be done for non-function templates.

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