Fix [mutating] generic methods (#1618)

Slang generates code that turns the implicit `this` parameter of a method into an explicit parameter. The logic that decides whether that parameter should be `inout` is a bit involved, and there was a bug where a generic method would lead to the use of an `in` modifier (the default) and override the `inout` modifier that was requested by the method itself.

This change fixes the logic to treat generic declarations in the parent chain of a leaf method as having no bearing on whether an implicit `this` parameter should be `inout` or not.

A test case is included that breaks with the old behavior, and demonstrates that a generic `[mutating]` method can now work correctly.

3 files changed, 62 insertions, 0 deletions

diff --git a/source/slang/slang-lower-to-ir.cpp b/source/slang/slang-lower-to-ir.cpp
index fefec805c..b2a7529c5 100644
--- a/source/slang/slang-lower-to-ir.cpp
+++ b/source/slang/slang-lower-to-ir.cpp
@@ -2231,6 +2231,14 @@ ParameterDirection getThisParamDirection(Decl* parentDecl, ParameterDirection de
         return defaultDirection;
     }
 
+    // A parent generic declaration should not change the
+    // mutating-ness of the inner declaration.
+    //
+    if( as<GenericDecl>(parentDecl) )
+    {
+        return defaultDirection;
+    }
+
     // For now we make any `this` parameter default to `in`.
     //
     return kParameterDirection_In;
diff --git a/tests/bugs/mutating/mutating-generic-method.slang b/tests/bugs/mutating/mutating-generic-method.slang
new file mode 100644
index 000000000..18f4d315b
--- /dev/null
+++ b/tests/bugs/mutating/mutating-generic-method.slang
@@ -0,0 +1,50 @@
+//TEST(compute):COMPARE_COMPUTE_EX:-slang -compute
+//TEST(compute,vulkan):COMPARE_COMPUTE_EX:-vk -slang -compute
+
+// Confirm that a generic method marked `[mutating]`
+// produces an `inout` parameter for `this`.
+
+interface IGenerator
+{
+    [mutating] int generate();
+}
+
+struct SimpleGenerator : IGenerator
+{
+    int state = 0;
+
+    [mutating] int generate() { return state++; }
+}
+
+
+struct Reducer
+{
+    int state = 0;
+    [mutating] void update<S : IGenerator>(inout S g)
+    {
+        state = (state << 8) | state;
+        state = state + g.generate();
+    }
+}
+
+int test(int val)
+{
+    SimpleGenerator g = { 0 };
+    Reducer r = { val+1 };
+
+    r.update(g);
+    r.update(g);
+
+    return r.state;
+}
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out
+RWStructuredBuffer<int> outputBuffer;
+
+[numthreads(4, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    int tid = dispatchThreadID.x;
+    int val = test(tid);
+    outputBuffer[tid] = val;
+}
\ No newline at end of file
diff --git a/tests/bugs/mutating/mutating-generic-method.slang.expected.txt b/tests/bugs/mutating/mutating-generic-method.slang.expected.txt
new file mode 100644
index 000000000..5ac358274
--- /dev/null
+++ b/tests/bugs/mutating/mutating-generic-method.slang.expected.txt
@@ -0,0 +1,4 @@
+10102
+20203
+30304
+40405