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A commonly requested feature is to be able to supply only some
of the arguments to a generic explicitly, while allowing the rest
to be inferred. A common example is a function that performs some
kind of conversion:
To convert<To, From>( From fromValue ) { .... }
A user would like to be able to call this operation like:
int i = convert<int>( 1.0f );
but the current Slang type checker requires all or none of the generic
arguments be supplied. Supplying all of the arguments is tedious:
int i = convert<int, float>( 1.0f );
In this case, the `float` type argument is redundant and could be
inferred from context. However, if the user tries to omit the generic
argument list:
int i = convert( 1.0f );
The current type-checker cannot infer the `int` type argument (even if
one might claim it *should* infer based on the desired result type).
This change adds support for the `convert<int>(...)` case, by allowing
a generic to be applied to a prefix of its explicit arguments, and then
inferring the remaining arguments from contextual information when that
"partially applied" generic is applied to value-level arguments.
Most of the changes are just plumbing: adding the notion of a partially
applied generic and then supporting them during overload resolution.
A single test case is included that covers the `convert`-style use case.
It is likely that more testing is needed to cover failure modes of this
feature.
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