Disambiguate subscript decls by preferring the one with more accessors. (#5046)

2 files changed, 89 insertions, 0 deletions

diff --git a/source/slang/slang-check-overload.cpp b/source/slang/slang-check-overload.cpp
index 9b3d6baff..3b36b270b 100644
--- a/source/slang/slang-check-overload.cpp
+++ b/source/slang/slang-check-overload.cpp
@@ -1199,6 +1199,8 @@ namespace Slang
         return parent;
     }
 
+    // Returns -1 if left is preferred, 1 if right is preferred, and 0 if they are equal.
+    //
     int SemanticsVisitor::CompareLookupResultItems(
         LookupResultItem const& left,
         LookupResultItem const& right)
@@ -1283,6 +1285,45 @@ namespace Slang
                     return -1;
         }
 
+        // If both are subscript decls, prefer the one that provides more
+        // accessors.
+        if (auto leftSubscriptDecl = left.declRef.as<SubscriptDecl>())
+        {
+            if (auto rightSubscriptDecl = right.declRef.as<SubscriptDecl>())
+            {
+                auto leftAccessorCount = leftSubscriptDecl.getDecl()->getMembersOfType<AccessorDecl>().getCount();
+                auto rightAccessorCount = rightSubscriptDecl.getDecl()->getMembersOfType<AccessorDecl>().getCount();
+                auto decl1IsSubsetOfDecl2 = [=](SubscriptDecl* decl1, SubscriptDecl* decl2)
+                    {
+                        for (auto accessorDecl1 : decl1->getMembersOfType<AccessorDecl>())
+                        {
+                            bool found = false;
+                            for (auto accessorDecl2 : decl2->getMembersOfType<AccessorDecl>())
+                            {
+                                if (accessorDecl1->astNodeType == accessorDecl2->astNodeType)
+                                {
+                                    found = true;
+                                    break;
+                                }
+                            }
+                            if (!found)
+                                return false;
+                        }
+                        return true;
+                    };
+                if (leftAccessorCount > rightAccessorCount
+                    && decl1IsSubsetOfDecl2(rightSubscriptDecl.getDecl(), leftSubscriptDecl.getDecl()))
+                {
+                    return -1;
+                }
+                else if (rightAccessorCount > leftAccessorCount
+                    && decl1IsSubsetOfDecl2(leftSubscriptDecl.getDecl(), rightSubscriptDecl.getDecl()))
+                {
+                    return 1;
+                }
+            }
+        }
+
         // TODO: We should generalize above rules such that in a tie a declaration
         // A::m is better than B::m when all other factors are equal and
         // A inherits from B.
diff --git a/tests/language-feature/overloaded-subscript.slang b/tests/language-feature/overloaded-subscript.slang
new file mode 100644
index 000000000..f396f4f66
--- /dev/null
+++ b/tests/language-feature/overloaded-subscript.slang
@@ -0,0 +1,48 @@
+//TEST(compute):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK):-slang -compute -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK):-vk -compute -shaderobj -output-using-type
+
+// Test that we can disambiguiate subscript decls by prefering the candidate that contains a super set of
+// accessors than the other candidates.
+interface IBuf<T>
+{
+    T read(int x);
+}
+interface IRWBuf<T> : IBuf<T>
+{
+    [mutating]
+    void write(int x, T v);
+}
+
+extension<T, U : IBuf<T>> U
+{
+    __subscript(int x) -> T { get { return read(x); } }
+}
+
+extension<T, U : IRWBuf<T>> U
+{
+    __subscript(int x)->T { get { return read(x); } set { write(x, newValue); } }
+}
+
+struct MyArray<T> : IRWBuf<T>
+{
+    T data[4];
+    T read(int x) { return data[x]; }
+    [mutating]
+    void write(int x, T v) { data[x] = v; }
+}
+
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<int> outputBuffer;
+
+[numthreads(1,1,1)]
+void computeMain()
+{
+    MyArray<int> arr = {};
+    arr[0] = 1;
+    arr[1] = 2;
+    // CHECK: 1
+    // CHECK: 2
+    outputBuffer[0] = arr[0];
+    outputBuffer[1] = arr[1];
+}