diff --git a/src/Convert/SizeCast.hs b/src/Convert/SizeCast.hs
index 49a7f02..7943e8f 100644
--- a/src/Convert/SizeCast.hs
+++ b/src/Convert/SizeCast.hs
@@ -84,8 +84,6 @@ traverseExprM =
                 sizesMatch = numberToInteger s == Just (numberBitLength n)
                 fallback = convertCastM (Number s) (Number n)
                 num = return . Number
-        convertExprM (orig @ (Cast (Right DimsFn{}) _)) =
-            return orig
         convertExprM (Cast (Right (Ident x)) e) = do
             typeMap <- get
             -- can't convert this cast yet because x could be a typename
@@ -93,7 +91,9 @@ traverseExprM =
                 then return $ Cast (Right $ Ident x) e
                 else convertCastM (Ident x) e
         convertExprM (Cast (Right s) e) =
-            convertCastM s e
+            if isSimpleExpr s
+                then convertCastM s e
+                else return $ Cast (Right s) e
         convertExprM (Cast (Left (IntegerVector _ Signed rs)) e) =
             convertCastWithSigningM (dimensionsSize rs) e Signed
         convertExprM (Cast (Left (IntegerVector _ _ rs)) e) =
@@ -121,6 +121,17 @@ traverseExprM =
             let args = Args [e] []
             return $ Call (Ident f) args
 
+isSimpleExpr :: Expr -> Bool
+isSimpleExpr =
+    null . execWriter . collectNestedExprsM collectUnresolvedExprM
+    where
+        collectUnresolvedExprM :: Expr -> Writer [Expr] ()
+        collectUnresolvedExprM (expr @ PSIdent{}) = tell [expr]
+        collectUnresolvedExprM (expr @ CSIdent{}) = tell [expr]
+        collectUnresolvedExprM (expr @ DimsFn{}) = tell [expr]
+        collectUnresolvedExprM (expr @ DimFn {}) = tell [expr]
+        collectUnresolvedExprM _ = return ()
+
 castFn :: Expr -> Signing -> Description
 castFn e sg =
     PackageItem $
diff --git a/test/basic/package_function_cast.sv b/test/basic/package_function_cast.sv
new file mode 100644
index 0000000..b79ecea
--- /dev/null
+++ b/test/basic/package_function_cast.sv
@@ -0,0 +1,20 @@
+package P;
+    function automatic logic [7:0] f(input logic [2:0] p);
+        logic [7:0] r;
+        r = '0;
+        r[p+:2] = '1;
+        return r;
+    endfunction
+endpackage
+
+module top;
+    logic [2:0] p;
+    logic [7:0] q;
+    assign q = P::f(p);
+    initial begin
+        $monitor("%0d, p=%b q=%b", $time, p, q);
+        #1 p = 0;
+        while (p != 7)
+            #1 p = p + 1;
+    end
+endmodule
diff --git a/test/basic/package_function_cast.v b/test/basic/package_function_cast.v
new file mode 100644
index 0000000..f9d26b8
--- /dev/null
+++ b/test/basic/package_function_cast.v
@@ -0,0 +1,18 @@
+module top;
+    function automatic [7:0] f;
+        input [2:0] p;
+        begin
+            f = 7'b0;
+            f[p+:2] = 2'b11;
+        end
+    endfunction
+    reg [2:0] p;
+    wire [7:0] q;
+    assign q = f(p);
+    initial begin
+        $monitor("%0d, p=%b q=%b", $time, p, q);
+        #1 p = 0;
+        while (p != 7)
+            #1 p = p + 1;
+    end
+endmodule