Check for uninferrable variables in tcInferPatSynDecl

This fixes #21479

See Note [Unquantified tyvars in a pattern synonym]

While doing this, I found that some error messages pointed at the
pattern synonym /name/, rather than the /declaration/ so I widened the
SrcSpan to encompass the declaration.

(cherry picked from commit bdc99cc22c903d15eb7f4cd8da4b37d307179808)

8 files changed, 24 insertions, 5 deletions

diff --git a/testsuite/tests/linear/should_fail/LinearPatSyn2.stderr b/testsuite/tests/linear/should_fail/LinearPatSyn2.stderr
index 1360983907..78b3ee52f6 100644
--- a/testsuite/tests/linear/should_fail/LinearPatSyn2.stderr
+++ b/testsuite/tests/linear/should_fail/LinearPatSyn2.stderr
@@ -1,5 +1,5 @@
 
-LinearPatSyn2.hs:6:9: error:
+LinearPatSyn2.hs:6:1: error:
     • Pattern synonyms do not support linear fields (GHC #18806):
         x %1 -> Maybe x
     • In the declaration for pattern synonym ‘J’
diff --git a/testsuite/tests/patsyn/should_fail/T11667.stderr b/testsuite/tests/patsyn/should_fail/T11667.stderr
index f4dafc0f28..e08299b235 100644
--- a/testsuite/tests/patsyn/should_fail/T11667.stderr
+++ b/testsuite/tests/patsyn/should_fail/T11667.stderr
@@ -3,7 +3,7 @@ T11667.hs:12:22: error:
     • Could not deduce (Num a) arising from the literal ‘42’
       from the context: Eq a
         bound by the signature for pattern synonym ‘Pat1’
-        at T11667.hs:12:9-12
+        at T11667.hs:12:1-23
       Possible fix:
         add (Num a) to the "required" context of
           the signature for pattern synonym ‘Pat1’
diff --git a/testsuite/tests/patsyn/should_fail/T14507.stderr b/testsuite/tests/patsyn/should_fail/T14507.stderr
index beeb4de685..d3dfa0a04f 100644
--- a/testsuite/tests/patsyn/should_fail/T14507.stderr
+++ b/testsuite/tests/patsyn/should_fail/T14507.stderr
@@ -1,5 +1,5 @@
 
-T14507.hs:21:9: error:
+T14507.hs:21:1: error:
     • Iceland Jack!  Iceland Jack! Stop torturing me!
       Pattern-bound variable x :: TypeRep a
         has a type that mentions pattern-bound coercion: co
diff --git a/testsuite/tests/patsyn/should_fail/T14552.stderr b/testsuite/tests/patsyn/should_fail/T14552.stderr
index 34ee266cdd..92c1adb57b 100644
--- a/testsuite/tests/patsyn/should_fail/T14552.stderr
+++ b/testsuite/tests/patsyn/should_fail/T14552.stderr
@@ -1,5 +1,5 @@
 
-T14552.hs:22:9: error:
+T14552.hs:22:1: error:
     • Cannot generalise type; skolem ‘k’ would escape its scope
       if I tried to quantify (t0 :: k) in this type:
         forall k (w :: k --> *). Exp a0 (F @k @(*) w t0)
diff --git a/testsuite/tests/patsyn/should_fail/T15685.stderr b/testsuite/tests/patsyn/should_fail/T15685.stderr
index afc6d45de4..204260a9e4 100644
--- a/testsuite/tests/patsyn/should_fail/T15685.stderr
+++ b/testsuite/tests/patsyn/should_fail/T15685.stderr
@@ -9,7 +9,7 @@ T15685.hs:13:24: error:
         at T15685.hs:13:19-26
       ‘k’ is a rigid type variable bound by
         the inferred type of HereNil :: NS f as
-        at T15685.hs:13:9-15
+        at T15685.hs:13:1-26
     • In the pattern: Nil
       In the pattern: Here Nil
       In the declaration for pattern synonym ‘HereNil’
diff --git a/testsuite/tests/patsyn/should_fail/T21479.hs b/testsuite/tests/patsyn/should_fail/T21479.hs
new file mode 100644
index 0000000000..889e811e88
--- /dev/null
+++ b/testsuite/tests/patsyn/should_fail/T21479.hs
@@ -0,0 +1,13 @@
+{-# Language PatternSynonyms #-}
+{-# Language ViewPatterns #-}
+{-# Language GADTs #-}
+
+module T21479 where
+
+data T a where
+  MkT :: T Int
+
+foo :: () -> T a
+foo = foo
+
+pattern T1 <- (foo -> MkT)
\ No newline at end of file
diff --git a/testsuite/tests/patsyn/should_fail/T21479.stderr b/testsuite/tests/patsyn/should_fail/T21479.stderr
new file mode 100644
index 0000000000..07601492a8
--- /dev/null
+++ b/testsuite/tests/patsyn/should_fail/T21479.stderr
@@ -0,0 +1,5 @@
+
+T21479.hs:13:1: error:
+    • Uninferrable type variable a0 in
+      the provided context: (a0 :: *) ~ (Int :: *)
+    • In the declaration for pattern synonym ‘T1’
diff --git a/testsuite/tests/patsyn/should_fail/all.T b/testsuite/tests/patsyn/should_fail/all.T
index 8993e5c4bf..2db852a35b 100644
--- a/testsuite/tests/patsyn/should_fail/all.T
+++ b/testsuite/tests/patsyn/should_fail/all.T
@@ -48,3 +48,4 @@ test('T15694', normal, compile_fail, [''])
 test('T16900', normal, compile_fail, ['-fdiagnostics-show-caret'])
 test('T14552', normal, compile_fail, [''])
 test('T18856', normal, compile_fail, ['-fdiagnostics-show-caret'])
+test('T21479', normal, compile_fail, [''])