[HW] Extend ElementType parsing to support union types (#9318)

This PR fixes a bug where !hw.union types nested inside !hw.array failed to parse back in after being printed (round-trip failure). When printing nested types, printHWElementType elides the dialect prefix to produce a more readable output (e.g., union<...> instead of !hw.union<...>). However, the corresponding parser helper, parseHWElementType, relies on a hardcoded list of string prefixes to detect when a bare keyword is being used. This list currently includes struct, array, enum, etc., but is missing union. As a result, the parser fails to recognize union<...> as a valid HW type when parsing the output of a valid IR print.

Author: yassinz

lib/Dialect/HW/HWTypes.cpp

@@ -158,7 +158,7 @@ static ParseResult parseHWElementType(AsmParser &p, Type &result) {
   if (typeString.starts_with("array<") || typeString.starts_with("inout<") ||
       typeString.starts_with("uarray<") || typeString.starts_with("struct<") ||
       typeString.starts_with("typealias<") || typeString.starts_with("int<") ||
-      typeString.starts_with("enum<")) {
+      typeString.starts_with("enum<") || typeString.starts_with("union<")) {
     llvm::StringRef mnemonic;
     if (auto parseResult = generatedTypeParser(p, &mnemonic, result);
         parseResult.has_value())

test/Dialect/HW/types.mlir

@@ -51,4 +51,11 @@ module {
     %1 = hw.union_create "a", %i : !hw.typealias<@ns::@bar, !hw.union<a: i1, b: i1>>
     return
   }
+
+  // CHECK-LABEL: func @nestedUnionArray
+  func.func @nestedUnionArray(
+    // CHECK: %arg0: !hw.array<4xunion<u8: i8, u16: i16>>
+    %arg0: !hw.array<4x!hw.union<u8: i8, u16: i16>>) {
+    return
+  }
 }