Support generic interface methods (#251)

* improve diagnostic messages and prevent fatal errors from crashing the compiler.

* fix top level exception catching.

* spelling fix

* change wording of invalidSwizzleExpr diagnostic

* add speculative GenericsApp expr parsing

* add new test case of cascading generics call.

* Fixing bugs in compiling cascaded generic function calls.

Add implementation of DeclaredSubTypeWitness::SubstituteImpl()

This is not needed by the type checker, but needed by IR specialization. When input source contains cascading generic function call, the arguments to `specialize` instruction is currently represented as a substitution. The arg values of this subsittution can be a `DeclaredSubTypeWitness` when a generic function uses one of its generic parameter to specialize another generic function. When the top level generics function is being specialized, this substitution argument, which is a `DeclaredSubTypeWitness`, needs to be substituted with the witness that used to specialize the top level function in the specialized specialize instruction as well.

* add a test case for cascading generic function call.

* parser bug fix

* fixes #255

* add test case for issue #255

* Generate missing `specialize` instruction when calling a generic method from an interface constraint.

When calling a generic method via an interface, we should be generating the following ir:
...
f = lookup_interface_method(...)
f_s = specailize(f, declRef)
...

This commit fixes this  `emitFuncRef` function to emit the needed `specialize` instruction.

* fixes #260

This fix follows the second apporach in the disucssion. It generated mangled name for specialized functions by appending new substitution type names to the original mangled name.

* Disabling removing and re-inserting specailized functions in getSpecalizeFunc()

I am not sure why it is needed, it seems HLSL and GLSL backends are generating forward declarations anyways, so the order of functions in IRModule shouldn't matter.

* cleanup and complete test cases.

* fix warnings

1 files changed, 38 insertions, 0 deletions

diff --git a/tests/bugs/nested-generics-method-call.slang b/tests/bugs/nested-generics-method-call.slang
new file mode 100644
index 000000000..d1e80da57
--- /dev/null
+++ b/tests/bugs/nested-generics-method-call.slang
@@ -0,0 +1,38 @@
+//TEST(compute):COMPARE_COMPUTE:-xslang -use-ir
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):dxbinding(0),glbinding(0),out
+
+RWStructuredBuffer<float> outputBuffer;
+
+interface IBase
+{
+    float get();
+}
+
+struct BaseImpl : IBase
+{
+    float get() { return 1.0; }
+};
+
+struct S
+{
+    __generic<T:IBase>
+    float eval(T obj)
+    {
+        return obj.get();
+    }
+};
+
+__generic<T : IBase>
+float test(T obj)
+{
+    S s;
+    return s.eval<T>(obj);
+}
+
+[numthreads(4, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    BaseImpl base;
+	float outVal = test<BaseImpl>(base);
+	outputBuffer[dispatchThreadID.x] = outVal;
+}
\ No newline at end of file