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The catch with these operations is that they return a vector based on the scalar of the element type of the texture.
That is, given `Texture2D<float> t` the operation `t.GatherRed(...)` should return a `float4`.
The ideal way to solve this would use associated types, but we aren't there yet, so I am using extension declarations.
An extension can "capture" the identity of the element type, like so:
__generic<T, let N : int> __extension Texture2D<vector<T,N> > { ... }
That extension will match `Texture2D<float3>` and correctly capture `T == float`, so that we can use it in other operations.
Getting this working required a bunch of changes:
- Actually emit the relevant extension declarations in the stdlib
- Fix the parser to be able to parse `Texture2D<vector<T,N> >` (that is, a nested generic app).
- I actually went ahead and significantly overhauled the expression parser while I was there, because I just couldn't deal with the existing code any longer.
- Added support for general-case lookup to look through `__extension` declarations. I had logic in place to special-case this for looking up "constructors" but hadn't done anything for general member lookup yet.
- This required some annoying holes to be punched through the layers, because lookup might need to invoke semantic analysis to ensure that an extension has been checked.
- There is some first-pass code trying to support looking up a `typedef` nested inside the `vector` type. This is a nice idea in principle, but the problem is that the `Texture2D<T>` definition would be looking up `T.Element` and not `float4.Element`, and that means we'd need machinery for doing lookup *through* interface conformances for a type parameter like `T`
The big gotcha here is that none of this logic applies to `Texture2D<float>` (the original case I mentioned) because I am matching vector types and not scalars.
Matching scalars *should* be as easy as:
__generic<T : __BuitlinScalarType> __extension Texture2D<T> { ... }
But I'd need to confirm that interface constraints like that actually work, or else that extension would *also* apply to `Texture2D<float4>` and break everything.
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For context: a `DeclRef` is supposed to capture both a pointer to a particualr declaration, and also any information needed to specialize that declaration for a context (e.g., generic parameter substitutions).
The existing approach had a hiearchy of specialized decl-ref types that mirrored the AST hierarchy, but that led to a lot of boilerplate where you had to recapitulate the exact same hierarchy.
The new appraoch basically treats `DeclRef<T>` as a sort of "smart pointer" in that it wraps a pointer to a `T` (the declaration), plus a side field for the specialization info, and then allows it to be cast as needed to other types (where the pointer cast would be allowed), while carrying along the side info.
To enable this, all the things that used to be member functions of declaration-reference types are now free functions that take a `DeclRef<T>` for some specific `T` as a parameter.
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