Avoid inadvertent circumventing of the check for under-qualified array ellipsis.

Fixes #3212

PiperOrigin-RevId: 820820986

Author: richmckeever

xls/dslx/type_system_v2/typecheck_module_v2_test.cc

@@ -1124,6 +1124,29 @@ TEST(TypecheckV2Test, GlobalArrayConstantWithEllipsisAndNoElementsFails) {
                                        "without an element to repeat")));
 }
 
+TEST(TypecheckV2Test, EllipsisArrayWithNoDeclaredSizeFails) {
+  EXPECT_THAT(
+      R"(
+fn bar(x: u32[5]) -> u32 {
+  x[4]
+}
+
+fn foo() -> u32 {
+  let a = u32:1;
+  let b = u32:2;
+  let y = [a, b, a, ...];
+  bar(y)
+}
+
+#[test]
+fn foo_test() {
+  assert_eq(foo(), u32:1);
+}
+)",
+      TypecheckFails(HasSubstr(
+          "Array has ellipsis (`...`) but does not have a type annotation.")));
+}
+
 TEST(TypecheckV2Test, GlobalArrayConstantEmptyWithoutAnnotationFails) {
   EXPECT_THAT(
       "const X = [];",

xls/dslx/type_system_v2/unify_type_annotations.cc

@@ -48,7 +48,6 @@
 namespace xls::dslx {
 namespace {
 
-
 bool operator==(const SignednessAndSize& x, const SignednessAndSize& y) {
   return x.flag == y.flag && x.is_signed == y.is_signed && x.size == y.size;
 }
@@ -71,6 +70,11 @@ std::optional<const TypeAnnotation*> GetSliceContainerSize(
   return std::nullopt;
 }
 
+bool IsArrayTypeWithMinSize(const TypeAnnotation* annotation) {
+  return annotation->annotation_kind() == TypeAnnotationKind::kArray &&
+         down_cast<const ArrayTypeAnnotation*>(annotation)->dim_is_min();
+}
+
 absl::Status MinSizeLargerThanStandardSizeError(
     const TypeAnnotation* min_annotation, const SignednessAndSize& min,
     const SignednessAndSize& standard, const FileTable& file_table) {
@@ -121,7 +125,7 @@ class Unifier {
     if (annotations.empty()) {
       return module_.Make<AnyTypeAnnotation>(/*multiple=*/true);
     }
-    if (annotations.size() == 1 &&
+    if (annotations.size() == 1 && !IsArrayTypeWithMinSize(annotations[0]) &&
         !table_.GetAnnotationFlag(annotations[0])
              .HasFlag(TypeInferenceFlag::kFormalMemberType)) {
       XLS_ASSIGN_OR_RETURN(std::optional<StructOrProcRef> struct_or_proc_ref,