Add a more specific diagnostic message when passing concrete value to interface-typed output parameter (#6788)

* More specific diagnostic for invalid concrete-to-interface arg coercion

* Add test for the new error message

* Fix typo in expected test result

2 files changed, 57 insertions, 0 deletions

diff --git a/tests/diagnostics/concrete-argument-to-output-interface.slang b/tests/diagnostics/concrete-argument-to-output-interface.slang
new file mode 100644
index 000000000..cfe447d94
--- /dev/null
+++ b/tests/diagnostics/concrete-argument-to-output-interface.slang
@@ -0,0 +1,49 @@
+//DIAGNOSTIC_TEST:SIMPLE:
+
+interface IThing
+{
+    [mutating]
+    void f();
+
+    void g();
+}
+
+struct MyThing: IThing
+{
+    int a = 0;
+
+    [mutating]
+    void f()
+    {
+        a = 10;
+    }
+
+    void g() {}
+}
+
+void g(IThing x)
+{
+    x.g();
+}
+
+void f(inout IThing x)
+{
+    x.f();
+}
+
+void h<T:IThing>(inout T x)
+{
+    x.f();
+}
+
+void entry()
+{
+    MyThing concrete;
+    IThing interfaceTyped = MyThing();
+
+    g(interfaceTyped); // No error
+    g(concrete); // No error
+    f(interfaceTyped); // No error
+    h(concrete); // No error
+    f(concrete); // ERROR!
+}
diff --git a/tests/diagnostics/concrete-argument-to-output-interface.slang.expected b/tests/diagnostics/concrete-argument-to-output-interface.slang.expected
new file mode 100644
index 000000000..b979d01a3
--- /dev/null
+++ b/tests/diagnostics/concrete-argument-to-output-interface.slang.expected
@@ -0,0 +1,8 @@
+result code = -1
+standard error = {
+tests/diagnostics/concrete-argument-to-output-interface.slang(48): error 30077: argument passed to parameter 'x' is of concrete type 'MyThing', but interface-typed output parameters require interface-typed arguments. To allow passing a concrete type to this function, you can replace 'IThing x' with a generic 'T x' and a 'where T : IThing' constraint.
+    f(concrete); // ERROR!
+     ^
+}
+standard output = {
+}