From 3e8fc5122fed2baee3a4d27d51575f6dd5174ea8 Mon Sep 17 00:00:00 2001
From: Adam Chlipala <adam@chlipala.net>
Date: Sun, 20 May 2018 19:28:17 -0400
Subject: Proper error message when the body of a 'val' declaration fails to
 check against the type annotation

---
 src/elaborate.sml  | 3 ++-
 tests/pairUnify.ur | 6 ++++++
 2 files changed, 8 insertions(+), 1 deletion(-)
 create mode 100644 tests/pairUnify.ur

diff --git a/src/elaborate.sml b/src/elaborate.sml
index 4a04d4bf..51d00bd8 100644
--- a/src/elaborate.sml
+++ b/src/elaborate.sml
@@ -4046,7 +4046,8 @@ and elabDecl (dAll as (d, loc), (env, denv, gs)) =
                           | L.PAnnot (p', _) => singleVar p'
                           | _ => NONE
                 in
-                    unifyCons env loc et pt;
+                    (unifyCons env loc et pt
+                     handle CUnify (c1, c2, env', err) => expError env (Unify (e', c1, c2, env', err)));
 
                     (case exhaustive (env, et, [p'], loc) of
                          NONE => ()
diff --git a/tests/pairUnify.ur b/tests/pairUnify.ur
new file mode 100644
index 00000000..1c9f9759
--- /dev/null
+++ b/tests/pairUnify.ur
@@ -0,0 +1,6 @@
+datatype a = A
+datatype b = B
+
+val x : a * b = (A, B)
+
+val y : b = x
-- 
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