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, 7 insertions, 2 deletions

diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index 2879d7fa2..3cc0fd0f5 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -1576,7 +1576,7 @@ namespace Slang
             //
             auto synParamDecl = m_astBuilder->create<ParamDecl>();
             synParamDecl->nameAndLoc = paramDeclRef.getDecl()->nameAndLoc;
-            synParamDecl->type.type = resultType;
+            synParamDecl->type.type = paramType;
 
             // We need to add the parameter as a child declaration of
             // the method we are building.
@@ -1599,7 +1599,12 @@ namespace Slang
         // if any.
         //
         ThisExpr* synThis = nullptr;
-        if( !requiredMemberDeclRef.getDecl()->hasModifier<HLSLStaticModifier>() )
+        if( requiredMemberDeclRef.getDecl()->hasModifier<HLSLStaticModifier>() )
+        {
+            auto synStaticModifier = m_astBuilder->create<HLSLStaticModifier>();
+            synFuncDecl->modifiers.first = synStaticModifier;
+        }
+        else
         {
             // For a non-`static` requirement, we need a `this` parameter.
             //