Add a regression test for #21130

This problem was due to a bug in cloneWanted, which was incorrectly
creating a coercion hole to hold an evidence variable.

This bug was introduced by 8bb52d91 and fixed in 81740ce8.

Fixes #21130

3 files changed, 45 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T21130.hs b/testsuite/tests/typecheck/should_fail/T21130.hs
new file mode 100644
index 0000000000..abeafdb50a
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T21130.hs
@@ -0,0 +1,10 @@
+{-# LANGUAGE TypeFamilies, FlexibleContexts #-}
+module T21130 where
+
+type family Item l
+type instance Item [a] = a
+
+f :: Enum (Item l) => l
+f = f
+
+x = (_ f) :: Int
diff --git a/testsuite/tests/typecheck/should_fail/T21130.stderr b/testsuite/tests/typecheck/should_fail/T21130.stderr
new file mode 100644
index 0000000000..d1d018bc9f
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T21130.stderr
@@ -0,0 +1,34 @@
+
+T21130.hs:10:6:
+     Found hole: _ :: t0 -> Int
+      Where: ‘t0’ is an ambiguous type variable
+     In the expression: (_ f) :: Int
+      In an equation for ‘x’: x = (_ f) :: Int
+     Relevant bindings include x :: Int (bound at T21130.hs:10:1)
+       Valid hole fits include
+         head :: forall a. GHC.Stack.Types.HasCallStack => [a] -> a
+           with head @Int
+          (imported from ‘Prelude’ at T21130.hs:2:8-13
+           (and originally defined in ‘GHC.List’))
+         last :: forall a. GHC.Stack.Types.HasCallStack => [a] -> a
+           with last @Int
+          (imported from ‘Prelude’ at T21130.hs:2:8-13
+           (and originally defined in ‘GHC.List’))
+         read :: forall a. Read a => String -> a
+           with read @Int
+          (imported from ‘Prelude’ at T21130.hs:2:8-13
+           (and originally defined in ‘Text.Read’))
+
+ T21130.hs:10:8:
+     Ambiguous type variable ‘t0’ arising from a use of ‘f’
+      prevents the constraint ‘(Enum (Item t0))’ from being solved.
+      Probable fix: use a type annotation to specify what ‘t0’ should be.
+       Potentially matching instances:
+        instance Enum Ordering -- Defined in ‘GHC.Enum’
+        instance Enum Integer -- Defined in ‘GHC.Enum’
+         ...plus N others
+         ...plus N instances involving out-of-scope types
+         (use -fprint-potential-instances to see them all)
+     In the first argument of ‘_’, namely ‘f’
+       In the expression: (_ f) :: Int
+      In an equation for ‘x’: x = (_ f) :: Int
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index 2febea800d..a8a4e4d3b5 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -650,3 +650,4 @@ test('T18398', normal, compile_fail, [''])
 test('T18406', normal, compile_fail, [''])
 test('AmbigFDs', normal, compile_fail, [''])
 test('T20064', normal, compile_fail, [''])
+test('T21130', normal, compile_fail, [''])