From adfc1c0896b74767d7504411f8d3c38050272fb0 Mon Sep 17 00:00:00 2001
From: Gregory Becker <becker33@llnl.gov>
Date: Tue, 13 Dec 2022 13:27:25 -0800
Subject: [PATCH] error messages: add causation for conflicting variants error

---
 lib/spack/spack/solver/concretize.lp | 12 ++++++++++++
 lib/spack/spack/solver/display.lp    |  1 +
 2 files changed, 13 insertions(+)

diff --git a/lib/spack/spack/solver/concretize.lp b/lib/spack/spack/solver/concretize.lp
index c18b99b8f1d..cdfea9883ae 100644
--- a/lib/spack/spack/solver/concretize.lp
+++ b/lib/spack/spack/solver/concretize.lp
@@ -613,6 +613,18 @@ attr("variant_value", Package, Variant, Value) :-
     variant(Package, Variant),
     build(Package).
 
+error(0, "'{0}' required multiple values for single-valued variant '{1}'\n    Requested 'Spec({1}={2})' and 'Spec({1}={3})'", Package, Variant, Value1, Value2, startcauses, Cause1, Cause2)
+  :- attr("node", Package),
+     variant(Package, Variant),
+     variant_single_value(Package, Variant),
+     build(Package),
+     attr("variant_value", Package, Variant, Value1),
+     imposed_constraint(Cause1, "variant_set", Package, Variant, Value1),
+     condition_holds(Cause1),
+     attr("variant_value", Package, Variant, Value2),
+     imposed_constraint(Cause2, "variant_set", Package, Variant, Value2),
+     condition_holds(Cause2),
+     Value1 < Value2. % see[1]
 
 error(100, "'{0}' required multiple values for single-valued variant '{1}'", Package, Variant)
   :- attr("node", Package),
diff --git a/lib/spack/spack/solver/display.lp b/lib/spack/spack/solver/display.lp
index ff7c8f987fc..868062b3f48 100644
--- a/lib/spack/spack/solver/display.lp
+++ b/lib/spack/spack/solver/display.lp
@@ -25,6 +25,7 @@
 #show error/6.
 #show error/7.
 #show error/8.
+#show error/9.
 
 % show cause -> effect data for errors
 #show condition_cause/2.