From 515eb6b45bb705a715abc6ab64fdb69ca5799bda Mon Sep 17 00:00:00 2001
From: Geoff Romer <gromer@google.com>
Date: Tue, 20 May 2025 09:38:08 -0700
Subject: [PATCH] Better diagnostic message in CHECK (#5504)

---
 toolchain/check/pattern_match.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/toolchain/check/pattern_match.cpp b/toolchain/check/pattern_match.cpp
index 58c5581cbc50..8fb915182ef2 100644
--- a/toolchain/check/pattern_match.cpp
+++ b/toolchain/check/pattern_match.cpp
@@ -303,7 +303,8 @@ auto MatchContext::DoEmitPatternMatch(Context& context,
       break;
     }
     case MatchKind::Callee: {
-      CARBON_CHECK(!param_pattern.index.has_value());
+      CARBON_CHECK(!param_pattern.index.has_value(),
+                   "ValueParamPattern index set before callee pattern match");
       param_pattern.index = NextRuntimeIndex();
       ReplaceInstBeforeConstantUse(context, entry.pattern_id, param_pattern);
       auto param_id = AddInst<SemIR::ValueParam>(