Fixes #/issues/989

Tests/Applications/C844/Prg/C844.prg

@@ -1,5 +1,6 @@
-// 844. Problem with iif() and NULL - /vo11 disabled
-// error XS0029: Cannot implicitly convert type 'object' to 'string'
+// 844. Problem with iif() and NULL - /vo10 disabled
+// error XS0029: Cannot implicitly convert type 'object' to 'string'     
+#pragma options("vo10", off)
 FUNCTION Start() AS VOID
 LOCAL c AS STRING
 c := iif(FALSE, "", NULL)  // error XS0029

Tests/Applications/C845/Prg/C845.prg

@@ -1,5 +1,6 @@
-// 844. Problem with iif() and NULL - /vo11 enabled
+// 844. Problem with iif() and NULL - /vo10 enabled
 // error XS0029: Cannot implicitly convert type 'object' to 'string'
+#pragma options("vo10", on)
 FUNCTION Start() AS VOID
 LOCAL c AS STRING
 c := iif(FALSE, "", NULL)  // error XS0029

Tests/xSharp Tests.viproj

@@ -106884,7 +106884,7 @@ VO6=0
 VO7=0
 VO8=0
 VO9=0
-VO10=1
+VO10=0
 VO11=0
 VO12=0
 VO13=0

XSharp/src/Compiler/XSharpCodeAnalysis/Binder/Binder_Operators.cs

@@ -1010,19 +1010,34 @@ private BoundExpression AdjustConstantType(BoundExpression expr, TypeSymbol type
         public void VODetermineIIFTypes(ConditionalExpressionSyntax node, DiagnosticBag diagnostics,
             ref BoundExpression trueExpr, ref BoundExpression falseExpr)
         {
-            if (trueExpr is null || trueExpr.Type is null)
+            // a combination of null and a value type is not allowed
+            // in that case we replace the null with a default value.
+            var trueNull = trueExpr?.Type is null;
+            var falseNull = falseExpr?.Type is null;
+            if (trueNull && falseNull)
+            {
+                return;
+            }
+            else if (trueNull && !falseExpr.Type.IsReferenceType)
             {
                 if (Compilation.Options.HasRuntime)
+                {
                     trueExpr = new BoundDefaultExpression(trueExpr.Syntax, Compilation.UsualType());
-                else
-                    trueExpr = new BoundDefaultExpression(trueExpr.Syntax, Compilation.ObjectType);
+                    trueNull = false;
+                }
             }
-            if (falseExpr is null || falseExpr.Type is null)
+            else if (falseNull && !trueExpr.Type.IsReferenceType)
             {
                 if (Compilation.Options.HasRuntime)
+                {
                     falseExpr = new BoundDefaultExpression(falseExpr.Syntax, Compilation.UsualType());
-                else
-                    falseExpr = new BoundDefaultExpression(falseExpr.Syntax, Compilation.ObjectType);
+                    falseNull = false;
+                }
+            }
+            if (trueNull || falseNull)
+            {
+                // this happens with the combination of a null and a reference type.
+                return;
             }
             // Determine underlying types. For literal numbers this may be Byte, Short, Int or Long
             TypeSymbol trueType = VOGetType(trueExpr);