Fix #14114 by checking for duplicate vars on pattern synonym RHSes

Summary:
Because we weren't checking for duplicate variables on the right-hand
sides of pattern synonyms, bogus definitions like this one passed the renamer:

```lang=haskell
pattern Foo a <- (a,a)
```

Luckily, the fix is simple.

Test Plan: make test TEST=T14114

Reviewers: mpickering, austin, bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, thomie

GHC Trac Issues: #14114

Differential Revision: https://phabricator.haskell.org/D3866

3 files changed, 26 insertions, 0 deletions

diff --git a/testsuite/tests/patsyn/should_fail/T14114.hs b/testsuite/tests/patsyn/should_fail/T14114.hs
new file mode 100644
index 0000000000..b1fb8e6575
--- /dev/null
+++ b/testsuite/tests/patsyn/should_fail/T14114.hs
@@ -0,0 +1,7 @@
+{-# LANGUAGE PatternSynonyms #-}
+module T14114 where
+
+pattern Foo1 a <- (a,a)
+pattern Foo2 a  = (a,a)
+pattern Foo3 a <- (a,a) where
+  Foo3 a = (a,a)
diff --git a/testsuite/tests/patsyn/should_fail/T14114.stderr b/testsuite/tests/patsyn/should_fail/T14114.stderr
new file mode 100644
index 0000000000..a93b51e8f4
--- /dev/null
+++ b/testsuite/tests/patsyn/should_fail/T14114.stderr
@@ -0,0 +1,18 @@
+
+T14114.hs:4:20: error:
+    • Conflicting definitions for ‘a’
+      Bound at: T14114.hs:4:20
+                T14114.hs:4:22
+    • In a pattern synonym declaration
+
+T14114.hs:5:20: error:
+    • Conflicting definitions for ‘a’
+      Bound at: T14114.hs:5:20
+                T14114.hs:5:22
+    • In a pattern synonym declaration
+
+T14114.hs:6:20: error:
+    • Conflicting definitions for ‘a’
+      Bound at: T14114.hs:6:20
+                T14114.hs:6:22
+    • In a pattern synonym declaration
diff --git a/testsuite/tests/patsyn/should_fail/all.T b/testsuite/tests/patsyn/should_fail/all.T
index 86ec79a50c..92989cf060 100644
--- a/testsuite/tests/patsyn/should_fail/all.T
+++ b/testsuite/tests/patsyn/should_fail/all.T
@@ -36,3 +36,4 @@ test('T12819', normal, compile_fail, [''])
 test('UnliftedPSBind', normal, compile_fail, [''])
 test('T13349', normal, compile_fail, [''])
 test('T13470', normal, compile_fail, [''])
+test('T14114', normal, compile_fail, [''])