Docs: datacon eta-expansion, rep-poly checks

The existing notes weren't very clear on how the eta-expansion of
data constructors that occurs in tcInferDataCon/dsConLike interacts
with the representation polymorphism invariants. So we explain with
a few more details how we ensure that the representation-polymorphic
lambdas introduced by tcInferDataCon/dsConLike don't end up causing
problems, by checking they are properly instantiated and then relying
on the simple optimiser to perform beta reduction.

A few additional changes:

  - ConLikeTc just take type variables instead of binders, as we
    never actually used the binders.
  - Removed the FRRApp constructor of FRROrigin; it was no longer used
    now that we use ExpectedFunTyOrigin.
  - Adds a bit of documentation to the constructors
    of ExpectedFunTyOrigin.

3 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/rep-poly/RepPolyLambda.hs b/testsuite/tests/rep-poly/RepPolyLambda.hs
new file mode 100644
index 0000000000..8164a90fbb
--- /dev/null
+++ b/testsuite/tests/rep-poly/RepPolyLambda.hs
@@ -0,0 +1,11 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+
+module RepPolyLambda where
+
+import Data.Kind
+import GHC.Exts
+
+f :: forall r (a :: TYPE r). a -> a
+f = \ x -> x
diff --git a/testsuite/tests/rep-poly/RepPolyLambda.stderr b/testsuite/tests/rep-poly/RepPolyLambda.stderr
new file mode 100644
index 0000000000..d655354548
--- /dev/null
+++ b/testsuite/tests/rep-poly/RepPolyLambda.stderr
@@ -0,0 +1,8 @@
+
+RepPolyLambda.hs:11:5: error:
+    • The binder of the lambda expression
+      does not have a fixed runtime representation.
+      Its type is:
+        a :: TYPE r
+    • In the expression: \ x -> x
+      In an equation for ‘f’: f = \ x -> x
diff --git a/testsuite/tests/rep-poly/all.T b/testsuite/tests/rep-poly/all.T
index e97ae78192..140076e598 100644
--- a/testsuite/tests/rep-poly/all.T
+++ b/testsuite/tests/rep-poly/all.T
@@ -47,6 +47,7 @@ test('RepPolyDoBody1', normal, compile_fail, [''])
 test('RepPolyDoBody2', normal, compile_fail, [''])
 test('RepPolyDoReturn', normal, compile, [''])
 test('RepPolyFFI', normal, compile, [''])
+test('RepPolyLambda', normal, compile_fail, [''])
 test('RepPolyLeftSection1', normal, compile, [''])
 test('RepPolyLeftSection2', normal, compile_fail, [''])
 test('RepPolyMagic', normal, compile_fail, [''])