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.

1 files changed, 8 insertions, 0 deletions

diff --git a/tests/diagnostics/methods/mutating-method-on-rvalue.slang.expected b/tests/diagnostics/methods/mutating-method-on-rvalue.slang.expected
new file mode 100644
index 000000000..878882bd3
--- /dev/null
+++ b/tests/diagnostics/methods/mutating-method-on-rvalue.slang.expected
@@ -0,0 +1,8 @@
+result code = -1
+standard error = {
+tests/diagnostics/methods/mutating-method-on-rvalue.slang(13): error 30050: mutating method 'increment' cannot be called on an immutable value
+tests/diagnostics/methods/mutating-method-on-rvalue.slang(13): note 30049: a 'this' parameter is an immutable parameter by default in Slang; apply the `[mutating]` attribute to the function declaration to opt in to a mutable `this`
+tests/diagnostics/methods/mutating-method-on-rvalue.slang(25): error 30050: mutating method 'increment' cannot be called on an immutable value
+}
+standard output = {
+}