Issue an error if a varying lvalue is passed to a reference function parameter.

(Previously, we crashed.)

expr.cpp

@@ -2599,6 +2599,17 @@ FunctionCallExpr::GetValue(FunctionEmitContext *ctx) const {
 
         const Type *paramType = ft->GetParameterType(i); 
 
+        const Type *argLValueType = argExpr->GetLValueType();
+        if (argLValueType != NULL &&
+            dynamic_cast<const PointerType *>(argLValueType) != NULL &&
+            argLValueType->IsVaryingType() &&
+            dynamic_cast<const ReferenceType *>(paramType) != NULL) {
+            Error(argExpr->pos, "Illegal to pass a \"varying\" lvalue to a "
+                  "reference parameter of type \"%s\".",
+                  paramType->GetString().c_str());
+            return NULL;
+        }
+
         // Do whatever type conversion is needed
         argExpr = TypeConvertExpr(argExpr, paramType,
                                   "function call argument");

tests_errors/varying-lvalue-to-ref.ispc

@@ -1,4 +1,4 @@
-// ffofoof
+// Illegal to pass a "varying" lvalue to a reference parameter
 
 void inc(float &x) { ++x; }