Fix formatter non-convergence with EOL comment before else if

Skip merging end into the else partition when end has a trailing EOL
comment so fit-else-expand does not oscillate on re-format (issue #2540).

verible/verilog/formatting/formatter_test.cc

diff --git a/verible/verilog/formatting/formatter_test.cc b/verible/verilog/formatting/formatter_test.cc
index e885d74..0879182 100644
--- a/verible/verilog/formatting/formatter_test.cc
+++ b/verible/verilog/formatting/formatter_test.cc
@@ -19216,6 +19216,78 @@
   }
 }
 
+// Regression for https://github.com/chipsalliance/verible/issues/2540:
+// Trailing EOL comment after `end` before `else if` must not change whether
+// the else-if assignment stays on one line across re-format (convergence).
+TEST(FormatterEndToEndTest, EndElseIfWithEOLCommentConverges) {
+  static constexpr FormatterTestCase kTestCases[] = {
+      {// Comment on its own line between end and else if
+       "module m;\n"
+       "  always_comb begin\n"
+       "    case (state)\n"
+       "      STATE_A: begin\n"
+       "        if (cond_aaaa) next_state_value = STATE_B;\n"
+       "        else if (cond_bbbb) begin\n"
+       "          next_state_value = STATE_B;\n"
+       "        end\n"
+       "        // xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx\n"
+       "        else if (cond_cccc) next_state_value = STATE_C;\n"
+       "      end\n"
+       "    endcase\n"
+       "  end\n"
+       "endmodule\n",
+       "module m;\n"
+       "  always_comb begin\n"
+       "    case (state)\n"
+       "      STATE_A: begin\n"
+       "        if (cond_aaaa) next_state_value = STATE_B;\n"
+       "        else if (cond_bbbb) begin\n"
+       "          next_state_value = STATE_B;\n"
+       "        end  // xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx\n"
+       "        else if (cond_cccc) next_state_value = STATE_C;\n"
+       "      end\n"
+       "    endcase\n"
+       "  end\n"
+       "endmodule\n"},
+      {// Same construct with comment already on the end line
+       "module m;\n"
+       "  always_comb begin\n"
+       "    case (state)\n"
+       "      STATE_A: begin\n"
+       "        if (cond_aaaa) next_state_value = STATE_B;\n"
+       "        else if (cond_bbbb) begin\n"
+       "          next_state_value = STATE_B;\n"
+       "        end  // xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx\n"
+       "        else if (cond_cccc) next_state_value = STATE_C;\n"
+       "      end\n"
+       "    endcase\n"
+       "  end\n"
+       "endmodule\n",
+       "module m;\n"
+       "  always_comb begin\n"
+       "    case (state)\n"
+       "      STATE_A: begin\n"
+       "        if (cond_aaaa) next_state_value = STATE_B;\n"
+       "        else if (cond_bbbb) begin\n"
+       "          next_state_value = STATE_B;\n"
+       "        end  // xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx\n"
+       "        else if (cond_cccc) next_state_value = STATE_C;\n"
+       "      end\n"
+       "    endcase\n"
+       "  end\n"
+       "endmodule\n"},
+  };
+  FormatStyle style;  // default column_limit (100)
+  for (const auto &test_case : kTestCases) {
+    VLOG(1) << "code-to-format:\n" << test_case.input << "<EOF>";
+    std::ostringstream stream;
+    const auto status =
+        FormatVerilog(test_case.input, "<filename>", style, stream);
+    EXPECT_OK(status) << status.message();
+    EXPECT_EQ(stream.str(), test_case.expected) << "code:\n" << test_case.input;
+  }
+}
+
 }  // namespace
 }  // namespace formatter
 }  // namespace verilog

verible/verilog/formatting/tree-unwrapper.cc

diff --git a/verible/verilog/formatting/tree-unwrapper.cc b/verible/verilog/formatting/tree-unwrapper.cc
index 15a0082..e24aaab 100644
--- a/verible/verilog/formatting/tree-unwrapper.cc
+++ b/verible/verilog/formatting/tree-unwrapper.cc
@@ -1816,6 +1816,14 @@
   }
 }
 
+// True if any token in this leaf partition is an EOL comment.
+static bool PartitionContainsEOLComment(const TokenPartitionTree &partition) {
+  for (const auto &token : partition.Value().TokensRange()) {
+    if (token.TokenEnum() == verilog_tokentype::TK_EOL_COMMENT) return true;
+  }
+  return false;
+}
+
 static void PushEndIntoElsePartition(TokenPartitionTree *partition_ptr) {
   // Then combine 'end' with the following 'else' ...
   // Do not flatten, so that if- and else- clauses can make formatting
@@ -1823,6 +1831,15 @@
   auto &partition = *partition_ptr;
   auto &if_clause_partition = partition.Children().front();
   auto *end_partition = &RightmostDescendant(if_clause_partition);
+  // When 'end' carries a trailing EOL comment, 'else' must start on the next
+  // line (see token annotator: comment before else => MustWrap). Merging
+  // end+comment into the else-if header makes fit-else-expand treat the
+  // header as wider than the eventual formatted line, which wraps the
+  // else-if body on re-format and fails convergence (GitHub issue 2540).
+  if (PartitionContainsEOLComment(*end_partition)) {
+    VLOG(4) << "end has EOL comment, skip merge into else";
+    return;
+  }
   auto *end_parent = verible::MergeLeafIntoNextLeaf(end_partition);
   // if moving leaf results in any singleton partitions, hoist.
   if (end_parent != nullptr) {