Fix #14719 by using the setting the right SrcSpan

Currently, error messages that germane to GADT constructors
put the source span at only the first character in the constructor,
leading to insufficient caret diagnostics. This can be easily fixed
by using a source span that spans the entire constructor, instead of
just the first character.

Test Plan: make test TEST=T14719

Reviewers: alanz, bgamari, simonpj

Reviewed By: alanz, simonpj

Subscribers: simonpj, goldfire, rwbarton, thomie, carter

GHC Trac Issues: #14719

Differential Revision: https://phabricator.haskell.org/D4344

5 files changed, 31 insertions, 4 deletions

diff --git a/compiler/typecheck/TcTyClsDecls.hs b/compiler/typecheck/TcTyClsDecls.hs
index cd08570af6..7436b0d690 100644
--- a/compiler/typecheck/TcTyClsDecls.hs
+++ b/compiler/typecheck/TcTyClsDecls.hs
@@ -2538,8 +2538,8 @@ checkValidTyConTyVars tc
 -------------------------------
 checkValidDataCon :: DynFlags -> Bool -> TyCon -> DataCon -> TcM ()
 checkValidDataCon dflags existential_ok tc con
-  = setSrcSpan (srcLocSpan (getSrcLoc con))     $
-    addErrCtxt (dataConCtxt con)                $
+  = setSrcSpan (getSrcSpan con)  $
+    addErrCtxt (dataConCtxt con) $
     do  { -- Check that the return type of the data constructor
           -- matches the type constructor; eg reject this:
           --   data T a where { MkT :: Bogus a }
diff --git a/testsuite/tests/gadt/T14719.hs b/testsuite/tests/gadt/T14719.hs
new file mode 100644
index 0000000000..004116dcc6
--- /dev/null
+++ b/testsuite/tests/gadt/T14719.hs
@@ -0,0 +1,8 @@
+{-# LANGUAGE GADTs #-}
+module T14719 where
+
+data Foo1 where
+  MkFoo1 :: Bool
+
+newtype Foo2 where
+  MkFoo2 :: Foo2
diff --git a/testsuite/tests/gadt/T14719.stderr b/testsuite/tests/gadt/T14719.stderr
new file mode 100644
index 0000000000..cfac00c0c0
--- /dev/null
+++ b/testsuite/tests/gadt/T14719.stderr
@@ -0,0 +1,18 @@
+
+T14719.hs:5:3: error:
+    • Data constructor ‘MkFoo1’ returns type ‘Bool’
+        instead of an instance of its parent type ‘Foo1’
+    • In the definition of data constructor ‘MkFoo1’
+      In the data type declaration for ‘Foo1’
+  |
+5 |   MkFoo1 :: Bool
+  |   ^^^^^^^^^^^^^^
+
+T14719.hs:8:3: error:
+    • The constructor of a newtype must have exactly one field
+        but ‘MkFoo2’ has none
+    • In the definition of data constructor ‘MkFoo2’
+      In the newtype declaration for ‘Foo2’
+  |
+8 |   MkFoo2 :: Foo2
+  |   ^^^^^^^^^^^^^^
diff --git a/testsuite/tests/gadt/all.T b/testsuite/tests/gadt/all.T
index c81ab80c04..59ec307d58 100644
--- a/testsuite/tests/gadt/all.T
+++ b/testsuite/tests/gadt/all.T
@@ -115,3 +115,4 @@ test('T9380', normal, compile_and_run, [''])
 test('T12087', normal, compile_fail, [''])
 test('T12468', normal, compile_fail, [''])
 test('T14320', normal, compile, [''])
+test('T14719', normal, compile_fail, ['-fdiagnostics-show-caret'])
diff --git a/testsuite/tests/polykinds/T9222.stderr b/testsuite/tests/polykinds/T9222.stderr
index 6e143e0cf9..604cc1b7ec 100644
--- a/testsuite/tests/polykinds/T9222.stderr
+++ b/testsuite/tests/polykinds/T9222.stderr
@@ -5,12 +5,12 @@ T9222.hs:13:3: error:
           inside the constraints: a ~ '(b0, c0)
           bound by the type of the constructor ‘Want’:
                      (a ~ '(b0, c0)) => Proxy b0
-          at T9222.hs:13:3
+          at T9222.hs:13:3-43
       ‘c’ is a rigid type variable bound by
         the type of the constructor ‘Want’:
           forall i1 j1 (a :: (i1, j1)) (b :: i1) (c :: j1).
           ((a ~ '(b, c)) => Proxy b) -> Want a
-        at T9222.hs:13:3
+        at T9222.hs:13:3-43
     • In the ambiguity check for ‘Want’
       To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
       In the definition of data constructor ‘Want’