Fix Issue 11455 - Overriding template methods should raise a compile …

[ntrel]

…error

@RazvanN7 This implements Improving Error Messages (view)

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	11455	normal	Overriding template methods should raise a compile error

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#15350"

[RazvanN7]

This seems to break Vectorflow:

src/vectorflow/layers.d(476,19): Error: a function template cannot be `override`
--
  | src/vectorflow/neurallayer.d(243,19): Error: a function template cannot be `override`
  | src/vectorflow/neurallayer.d(246,19): Error: a function template cannot be `override`

[RazvanN7]

Maybe a bit more info would come in handy. Something along the lines of: "Function templates are never virtual, therefore they cannot override anything." and then add an error supplemental stating: "override used to be accepted and ignored on function templates. Please delete the override keyword."

[RazvanN7]

Also, maybe we should use a deprecation first. Vectorflow looks unmaintained, I'll raise a PR to fix the failure, but chances are it's gonna get ignored.

[RazvanN7]

PR: Netflix/vectorflow#35