Improve generic argument inference for builtins (#598)

Fixes #487

The basic problem here is that the user writes something like:

```hlsl
float invSqrt2 = 1 / sqrt(2);
```

In this case the user knows that `sqrt()` is only defined for floating-point types, so they expect this to compile something like:

```hlsl
float invSqrt2 = float(1) / sqrt(float(2));
```

The challenge this creates for the Slang compiler is that we use generics to streamline our declarations of all the builtins, so that the scalar `sqrt()` function is actually declared as:

```hlsl
T sqrt<T:__BuiltinFloatingPointType>(T value);
```

The `__BuiltinFloatingPointType` is an `interface` defined as part of the standard library, such that only built-in floating-point types conform to it (that is, `half`, `float`, and `double`).

When generic argument inference applies to a call like `sqrt(2)`, we see an argument of type `int`, and try to infer `T=int`, which leads to a failure because `int` does not conform to `__BuiltinFloatingPointType`.

The point where this currently fails in in the logic to "join" two types for inference, which is supposed to pick the best type that can represent both of two input types. E.g., a join between `float` and `int3` would be `float3`, since both of those types can convert to it, and it is the "minimal" type with that property.

So, the goal here is simple: we want a "join" between `int` and `__BuiltinFloatingPointType` to yield the `float` type. The way we handle that in this change is to special case the join of a basic scalar type and an interface, by enumerating all the basic scalar types, filtering them for ones that support the chosen interface and can be implicitly converted from the argument type, and then picking the "best" of them (the comments in the code explain what "best" means in this context).

The technique used here could be generalized in the future to deal with user-defined types or more cases, but that would risk slowing down overload resolution even more, which is already the most expensive part of our semantic checking pass.

A test case has been added for the specific case of `sqrt()` applied to an `int` argument.

0 files changed, 0 insertions, 0 deletions