tests: Add more tests for function definitions.

Approved-by: Francisco Casas (@fcasas)
Approved-by: Giovanni Mascellani (@giomasce)
Approved-by: Henri Verbeet (@hverbeet)
Approved-by: Alexandre Julliard (@julliard)
Merge-Request: https://gitlab.winehq.org/wine/vkd3d/-/merge_requests/77

tests/hlsl-function.shader_test

@@ -106,6 +106,9 @@ float4 main() : sv_target
     return 0;
 }
 
+% The function must have been at least declared before calling it. It may have
+% been declared with a different but compatible type, though.
+
 [pixel shader fail]
 
 float4 main() : sv_target
@@ -120,6 +123,64 @@ void func()
 
 [pixel shader todo]
 
+void func();
+
+float4 main() : sv_target
+{
+    func();
+    return 0;
+}
+
+void func()
+{
+}
+
+[pixel shader fail]
+
+void func(float arg);
+
+float4 main() : sv_target
+{
+    func();
+    return 0;
+}
+
+void func()
+{
+}
+
+[pixel shader todo]
+
+/* This is something of an internal test: we need to make sure that we use the
+ * correct variables for a function's arguments and returns regardless of
+ * whether it's been defined yet.
+ *
+ * Also, make sure that we can handle the case where the argument names differ.
+ */
+
+float2 concat(float x, float y);
+
+float2 func(void)
+{
+    return concat(0.1, 0.2);
+}
+
+float2 concat(float a, float b)
+{
+    return float2(a, b);
+}
+
+float4 main() : sv_target
+{
+    return float4(func(), concat(0.3, 0.4));
+}
+
+[test]
+todo draw quad
+todo probe all rgba (0.1, 0.2, 0.3, 0.4)
+
+[pixel shader todo]
+
 float func(in float a, out float b, inout float c)
 {
     c -= 0.2;