keep explicitly unconnected port bindings

These are now kept while still supporting optional trailing commas and
without introducing pass-through inconsistencies.

CHANGELOG.md

@@ -7,6 +7,7 @@
 * Fixed module-level localparams being needlessly inlined when forming longest
   static prefixes, which could cause deep recursion and run out of memory on
   some designs
+* Fixed overzealous removal of explicitly unconnected ports (e.g., `.a()`)
 * Fixed unneeded scoping of constant function calls used in type lookups
 * `/*/` is no longer interpreted as a self-closing block comment, e.g.,
   `$display("a"/*/,"b"/* */);` previously printed "ab", but now prints "a"

src/Convert/ResolveBindings.hs

@@ -62,9 +62,13 @@ mapInstance parts (Instance m paramBindings x rs portBindings) =
 
         paramBindings' = map checkParam $
             resolveBindings (msg "parameter overrides") paramNames paramBindings
-        portBindings' = filter ((/= Nil) . snd) $
-            resolveBindings (msg "port connections") portNames $
-            concatMap expandStar portBindings
+        portBindings' = resolveBindings (msg "port connections") portNames $
+            concatMap expandStar $
+            -- drop the trailing comma in positional port bindings
+            if length portNames + 1 == length portBindings
+                && last portBindings == ("", Nil)
+                then init portBindings
+                else portBindings
 
         expandStar :: PortBinding -> [PortBinding]
         expandStar ("*", Nil) =

src/Language/SystemVerilog/Parser/Parse.y

@@ -1672,12 +1672,20 @@ missingToken expected = do
   parseErrorM p $ "missing expected `" ++ expected ++ "`"
 
 checkPortBindings :: [PortBinding] -> ParseState [PortBinding]
-checkPortBindings [] = return []
+-- empty port bindings should stay empty
+checkPortBindings [("", Nil)] = return []
+-- drop the trailing comma in named port bindings
+checkPortBindings bindings
+  | (_ : _, _) <- head bindings
+  , ("", Nil) <- last bindings
+  = checkPortBindings' $ init bindings
+-- a trailing comma is indistinguishable from an explicit unconnected positional
+-- binding, so this is passed-through cleanly and resolved downstream
 checkPortBindings bindings =
-  checkBindings "port connections" $
-  if last bindings == ("", Nil)
-    then init bindings
-    else bindings
+  checkPortBindings' bindings
+
+checkPortBindings' :: [PortBinding] -> ParseState [PortBinding]
+checkPortBindings' = checkBindings "port connections"
 
 checkParamBindings :: [ParamBinding] -> ParseState [ParamBinding]
 checkParamBindings = checkBindings "parameter overrides"

test/error/binding_mix_port_trail.sv

+// pattern: illegal mix of ordered and named port connections
+module example(
+    input a, b, c
+);
+endmodule
+module top;
+    wire a, b, c;
+    example e(.a(1), .b(2), ,);
+endmodule