Allow passing sized array to unsized array parameter. (#4744)

1 files changed, 41 insertions, 1 deletions

diff --git a/docs/user-guide/02-conventional-features.md b/docs/user-guide/02-conventional-features.md
index fe0fdb854..bd8f0435c 100644
--- a/docs/user-guide/02-conventional-features.md
+++ b/docs/user-guide/02-conventional-features.md
@@ -111,7 +111,7 @@ In some cases the element count is then inferred from the initial value of a var
 int a[] = { 1, 2, 3 };
 ```
 
-In other cases, the result is a _runtime-sized_ array, where the actual element count will be determined later:
+In other cases, the result is a _unsized_ array, where the actual element count will be determined later:
 
 ```hlsl
 // the type of `b` is `int[]`
@@ -119,6 +119,46 @@ void f( int b[] )
 { ... }
 ```
 
+It is allowed to pass a sized array as argument to an unsized array parameter when calling a function.
+
+Array types has a `getCount()` memeber function that returns the length of the array.
+
+```hlsl
+int f( int b[] )
+{
+    return b.getCount(); // Note: all arguments to `b` must be resolvable to sized arrays.
+}
+
+void test()
+{
+    int arr[3] = { 1, 2, 3 };
+    int x = f(arr); // OK, passing sized array to unsized array parameter, x will be 3.
+}
+```
+
+Please note that if a function calls `getCount()` method on an unsized array parameter, then all
+calls to that function must provide a sized array argument, otherwise the compiler will not be able
+to resolve the size and will report an error. The following code shows an example of valid and
+invalid cases.
+
+```hlsl
+int f( int b[] )
+{
+    return b.getCount();
+}
+int g( int b[] )
+{
+    return f(b); // transitive calls are allowed.
+}
+uniform int unsizedParam[];
+void test()
+{
+    g(unsizedParam); // Not OK, `unsizedParam` doesn't have a known size at compile time.
+    int arr[3];
+    g(arr); // OK.
+}
+```
+
 There are more limits on how runtime-sized arrays can be used than on arrays of statically-known element count.
 
 > #### Note ####