Support partial inference of generic arguments (#2404)

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.

2 files changed, 54 insertions, 0 deletions

diff --git a/tests/language-feature/generics/partial-generic-argument-inference.slang b/tests/language-feature/generics/partial-generic-argument-inference.slang
new file mode 100644
index 000000000..4ee50b88d
--- /dev/null
+++ b/tests/language-feature/generics/partial-generic-argument-inference.slang
@@ -0,0 +1,49 @@
+//TEST(compute):COMPARE_COMPUTE: -shaderobj -output-using-type
+//TEST(compute):COMPARE_COMPUTE: -vk -shaderobj -output-using-type
+
+// This test confirms that we can provide a subset of the required generic
+// arguments to a generic function, and have the rest be inferred from the
+// types of the value arguments.
+
+interface IConvertible
+{
+    __init( int value );
+
+    property value : int { get; }
+}
+
+ToType convert< ToType : IConvertible, FromType : IConvertible >( FromType fromVal )
+{
+    return ToType( fromVal.value*0x10 );
+}
+
+struct A : IConvertible
+{
+    int value;
+    __init(int v) { this.value = v; }
+}
+
+struct B : IConvertible
+{
+    int value;
+    __init(int v) { this.value = v; }
+}
+
+
+int test( int value )
+{
+    A a = A(value);
+    B b = convert<B>(a);
+
+    return a.value + b.value;
+}
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<int> outputBuffer;
+
+[numthreads(4, 1, 1)]
+void computeMain(int3 dispatchThreadID : SV_DispatchThreadID)
+{
+    int tid = dispatchThreadID.x;
+    outputBuffer[tid] = test(tid);
+}
diff --git a/tests/language-feature/generics/partial-generic-argument-inference.slang.expected.txt b/tests/language-feature/generics/partial-generic-argument-inference.slang.expected.txt
new file mode 100644
index 000000000..d7be6af66
--- /dev/null
+++ b/tests/language-feature/generics/partial-generic-argument-inference.slang.expected.txt
@@ -0,0 +1,5 @@
+type: int32_t
+0
+17
+34
+51