Fix bugs related to mutating implementations of interface methods (#1461)

There are two main bug fixes here:

* We were failing to diagnose when code calls a `[mutating]` method on a value that doesn't support mutation (that is an r-value instead of an l-value).

* We had a bug in the synthesis logic for interface requirements where we used the *result* type of the requirement in place of each of the *parameter* types.

The second bug made synthesis often produce incorrect signatures with `void` parameters.

The first bug meant that even though a `[mutating]` method should not be able to satisfy a non-`[mutating]` method (and we had code to enforce this for the "exact match" case), when we go on to try and synthesize a non-`[mutating]` method that satisfies the requirement by delegating to the user-written one, it would end up succeeding, because nothing was stopping a non-`[mutating]` method from calling a `[mutating]` one.

In each case this code adds a fix and a test case to confirm it.

2 files changed, 48 insertions, 0 deletions

diff --git a/tests/diagnostics/interfaces/mutating-impl-of-non-mutating-req.slang b/tests/diagnostics/interfaces/mutating-impl-of-non-mutating-req.slang
new file mode 100644
index 000000000..2dbe45ccb
--- /dev/null
+++ b/tests/diagnostics/interfaces/mutating-impl-of-non-mutating-req.slang
@@ -0,0 +1,42 @@
+// mutating-impl-of-non-mutating-req.slang
+
+//DIAGNOSTIC_TEST:SIMPLE:-target hlsl -entry main
+
+interface IThing
+{
+    int processValue(int inValue);
+}
+
+struct Counter : IThing
+{
+    int state;
+
+    [mutating] int processValue(int inValue)
+    {
+        int result = state;
+        state += inValue;
+        return state;
+    }
+}
+
+int helper<T : IThing>(T thing, int value)
+{
+    return thing.processValue(value);
+}
+
+int test(int value)
+{
+    Counter counter = { value };
+    return helper(counter, value);
+}
+
+cbuffer C
+{
+    int gValue;
+}
+
+[shader("fragment")]
+int main() : SV_Target
+{
+    return test(gValue);
+}
\ No newline at end of file
diff --git a/tests/diagnostics/interfaces/mutating-impl-of-non-mutating-req.slang.expected b/tests/diagnostics/interfaces/mutating-impl-of-non-mutating-req.slang.expected
new file mode 100644
index 000000000..922a6c826
--- /dev/null
+++ b/tests/diagnostics/interfaces/mutating-impl-of-non-mutating-req.slang.expected
@@ -0,0 +1,6 @@
+result code = -1
+standard error = {
+tests/diagnostics/interfaces/mutating-impl-of-non-mutating-req.slang(10): error 38100: type 'Counter' does not provide required interface member 'processValue'
+}
+standard output = {
+}