Diagnose use of an implicit cast as an argument for an `out` parameter (#516)

Work on #499

Two big fixes here:

* The logic for checking constraints on `out` arguments wasn't actually triggering because it relied on function parameters being given an `OutType` if they are marked `out`, but the code wasn't actually doing that. Fixing the computation of types for functions resolved that issue.

* Next, I added a specific diagnostic to follow up the "expected an l-value" error to let the user know that their argument was implicitly converted, and that is why it doesn't count as an l-value in Slang's rules.

I've added a test case to ensure that we retain this diagnostic until we can do a true fix for the issue.

The right long-term fix is to have an AST representation of all the implicit casts involved (e.g., in both directions for an `inout` parameter), and then have the IR generate explicit code for the conversions in each direction (the `LoweredVal` representation can handle this sort of thing).

1 files changed, 15 insertions, 2 deletions

diff --git a/source/slang/syntax.cpp b/source/slang/syntax.cpp
index 9d29e7d21..5e855de29 100644
--- a/source/slang/syntax.cpp
+++ b/source/slang/syntax.cpp
@@ -2081,9 +2081,22 @@ void Type::accept(IValVisitor* visitor, void* extra)
         funcType->setSession(session);
 
         funcType->resultType = GetResultType(declRef);
-        for (auto pp : GetParameters(declRef))
+        for (auto paramDeclRef : GetParameters(declRef))
         {
-            funcType->paramTypes.Add(GetType(pp));
+            auto paramDecl = paramDeclRef.getDecl();
+            auto paramType = GetType(paramDeclRef);
+            if( paramDecl->FindModifier<OutModifier>() )
+            {
+                if(paramDecl->FindModifier<InOutModifier>() || paramDecl->FindModifier<InModifier>())
+                {
+                    paramType = session->getInOutType(paramType);
+                }
+                else
+                {
+                    paramType = session->getOutType(paramType);
+                }
+            }
+            funcType->paramTypes.Add(paramType);
         }
 
         return funcType;