2 files changed, 47 insertions, 0 deletions
diff --git a/tests/diagnostics/param-mutation.slang b/tests/diagnostics/param-mutation.slang
new file mode 100644
index 000000000..835e645c8
--- /dev/null
+++ b/tests/diagnostics/param-mutation.slang
@@ -0,0 +1,39 @@
+// param-mutation.slang
+
+// When a parameter is passed to `in`, it is mutable but generates a warning as it probably (?)
+// isn't what the programmer intended.
+
+//DIAGNOSTIC_TEST:SIMPLE:
+
+struct MutatingStruct
+{
+    [mutating] void setValue(int value) { m_value = value; }
+    int m_value;
+};
+
+int doThing(MutatingStruct s, int v)
+{
+    // Should generate a warning. 
+    s.setValue(v + 1);
+    return s.m_value;
+}
+
+// For non-copyable types (such as HitObject or NonCopyableStruct declared below), if passed as as `in`
+// should produce an error.
+// NOTE! This *doesn't* produce an error (or warning) because NonCopyable types are *implicitly* 
+// made *ref* when parsed as arguments. 
+
+[__NonCopyableType]
+struct NonCopyableStruct
+{
+    [mutating] void setValue(int value) { m_value = value; }
+    int m_value;
+};
+
+int doThing2(NonCopyableStruct s, int v)
+{
+    // Currently doesn't produce an error/warning because NonCopyableStruct is passed as *ref* implicitly.
+    s.setValue(v + 1);
+    return s.m_value;
+}
+
diff --git a/tests/diagnostics/param-mutation.slang.expected b/tests/diagnostics/param-mutation.slang.expected
new file mode 100644
index 000000000..a1fb34fb7
--- /dev/null
+++ b/tests/diagnostics/param-mutation.slang.expected
@@ -0,0 +1,8 @@
+result code = 0
+standard error = {
+tests/diagnostics/param-mutation.slang(17): warning 30068: mutating method 'setValue' called on `in` parameter 's'; changes will not be visible to caller. copy the parameter into a local variable if this behavior is intended
+    s.setValue(v + 1);
+              ^
+}
+standard output = {
+}