Don't return unitExpr in dsWhenNoErrs

  - fixes #18149 and #14765

dsWhenNoErrs now returns "runtimeError @ty" when disallowed
representation polymorphism is detected, where ty is the type of the
result CoreExpr. "ty" is passed as an additional argument to
dsWhenNoErrs, and is used only in the case of such an error.

The calls to dsWhenNoErrs must now compute the type of the
CoreExpr they are trying to build, so that an error of the right type
can be used in case of a representation polymorphism failure.

4 files changed, 30 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T12709.stderr b/testsuite/tests/typecheck/should_fail/T12709.stderr
index f8da5ea120..1d3334c6f7 100644
--- a/testsuite/tests/typecheck/should_fail/T12709.stderr
+++ b/testsuite/tests/typecheck/should_fail/T12709.stderr
@@ -5,6 +5,18 @@ T12709.hs:28:13: error:
       Kind: TYPE rep
     In the type of expression: 1
 
+T12709.hs:28:13: error:
+    A representation-polymorphic type is not allowed here:
+      Type: a
+      Kind: TYPE rep
+    In the type of expression: 1 + 2
+
+T12709.hs:28:13: error:
+    A representation-polymorphic type is not allowed here:
+      Type: a
+      Kind: TYPE rep
+    In the type of expression: 1 + 2 + 3
+
 T12709.hs:28:17: error:
     A representation-polymorphic type is not allowed here:
       Type: a
diff --git a/testsuite/tests/typecheck/should_fail/T14765.hs b/testsuite/tests/typecheck/should_fail/T14765.hs
new file mode 100644
index 0000000000..b124252bb4
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T14765.hs
@@ -0,0 +1,11 @@
+{-# LANGUAGE Haskell2010 #-}
+{-# LANGUAGE MagicHash, PolyKinds, ExplicitForAll #-}
+
+module T14765 where
+
+import GHC.Exts
+
+fold :: forall rep a (r :: TYPE rep).
+          (r -> a -> Proxy# r -> r) -> (Proxy# r -> r) -> [a] -> r
+fold f k [] = k proxy#
+fold f k (x : xs) = fold f (f (k proxy#) x) xs
diff --git a/testsuite/tests/typecheck/should_fail/T14765.stderr b/testsuite/tests/typecheck/should_fail/T14765.stderr
new file mode 100644
index 0000000000..c837c0d717
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T14765.stderr
@@ -0,0 +1,6 @@
+
+T14765.hs:11:31: error:
+    A representation-polymorphic type is not allowed here:
+      Type: r
+      Kind: TYPE rep
+    In the type of expression: (k proxy#)
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index b776f1d5dd..8745ea9172 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -480,6 +480,7 @@ test('T14607', normal, compile_fail, [''])
 test('T14605', normal, compile_fail, [''])
 test('T14761a', normal, compile_fail, [''])
 test('T14761b', normal, compile_fail, [''])
+test('T14765', normal, compile_fail, [''])
 test('T14884', normal, compile_fail, [''])
 test('T14904a', normal, compile_fail, [''])
 test('T14904b', normal, compile_fail, [''])