improve for decl error messaging

src/Convert/ForDecl.hs

@@ -19,8 +19,6 @@ convert =
     traverseStmts convertStmt
 
 convertStmt :: Stmt -> Stmt
-convertStmt (For (Left []) cc asgns stmt) =
-    convertStmt $ For (Right []) cc asgns stmt
 convertStmt (For (Right []) cc asgns stmt) =
     convertStmt $ For inits cc asgns stmt
     where inits = Left [dummyDecl $ RawNum 0]
@@ -47,12 +45,9 @@ convertStmt (For (Right origPairs) cc asgns stmt) =
 convertStmt other = other
 
 splitDecl :: Decl -> (Decl, (LHS, Expr))
-splitDecl (decl @ (Variable _ _ _ _ Nil)) =
-    error $ "invalid for loop decl: " ++ show decl
-splitDecl (Variable d t ident a e) =
-    (Variable d t ident a Nil, (LHSIdent ident, e))
 splitDecl decl =
-    error $ "invalid for loop decl: " ++ show decl
+    (Variable d t ident a Nil, (LHSIdent ident, e))
+    where Variable d t ident a e = decl
 
 isComment :: Decl -> Bool
 isComment CommentDecl{} = True

src/Language/SystemVerilog/Parser/ParseDecl.hs

@@ -287,7 +287,7 @@ parseDTsAsDeclsOrAsgns tokens =
     forbidNonEqAsgn tokens $
     if hasLeadingAsgn || tripLookahead tokens
         then Right $ parseDTsAsAsgns tokens
-        else Left  $ parseDTsAsDecls tokens
+        else Left $ map checkDecl $ parseDTsAsDecls tokens
     where
         hasLeadingAsgn =
             -- if there is an asgn token before the next comma
@@ -295,6 +295,11 @@ parseDTsAsDeclsOrAsgns tokens =
                 (Just a, Just b) -> a > b
                 (Nothing, Just _) -> True
                 _ -> False
+        checkDecl :: Decl -> Decl
+        checkDecl (decl @ (Variable _ _ _ _ Nil)) =
+            error $ "for loop declaration missing initialization: "
+                ++ init (show decl)
+        checkDecl decl = decl
 
 -- internal parser for basic assignment lists
 parseDTsAsAsgns :: [DeclToken] -> [(LHS, Expr)]

test/error/for_loop_decl_no_init.sv

@@ -0,0 +1,6 @@
+// pattern: for loop declaration missing initialization
+module top;
+    initial
+        for (integer x; x < 3; x = x + 1)
+            ;
+endmodule