Asking for the opinion of DMD developers

[Elie Morisse]

On Sunday, 13 October 2019 at 10:51:52 UTC, Stefanos Baziotis 
wrote:
> [1] https://github.com/dlang/DIPs/pull/176
> [2] 
> https://forum.dlang.org/post/dnyqxmgdazczwmmvayjx@forum.dlang.org

I read your DIP and the review thread.

> if (arg is templateInstance && arg.TD is 
> aliasTemplateDeclaration) {

Checking that a TemplateDeclaration is an "alias template 
declaration" is easy : TemplateDeclaration.onemember && 
TemplateDeclaration.onemember.isAliasDeclaration()

However since they may be more than one TemplateDeclaration 
overload, your templateInstance cannot exist yet and you don't 
know which TemplateDeclaration overload should be probed..

A possible solution like C++'s, but taking into account the 
multiplicity of TemplateDeclaration overloads would be:

  - If the first round of arg deduction fails, try transforming 
the TypeFunction by substituting each function parameter that 
failed matching with a dummy instance from each 
TemplateDeclaration overload that contains an alias (one 
substitution attempt per overload) and redo the arg deduction. 
For each function parameter, no more than one TemplateDeclaration 
overload must match or else it's ambiguous.
  - If every combination of "one substitution per param" fails, 
keep substituting the remaining non-matching already substituted 
function parameters until there's no deeper alias template (i.e 
no more alias Temp(T) = SomeOtherAliasTemp(T)).

Downside : there may be A LOT of possible combinations.