Fix #15039 by pretty-printing equalities more systematically

GHC previously had a handful of special cases for
pretty-printing equalities in a more user-friendly manner, but they
were far from comprehensive (see #15039 for an example of where this
fell apart).

This patch makes the pretty-printing of equalities much more
systematic. I've adopted the approach laid out in
https://ghc.haskell.org/trac/ghc/ticket/15039#comment:4, and updated
`Note [Equality predicates in IfaceType]` accordingly. We are now
more careful to respect the properties of the
`-fprint-explicit-kinds` and `-fprint-equality-relations` flags,
which led to some improvements in error message outputs.

Along the way, I also tweaked the error-reporting machinery not to
print out the type of a typed hole when the type is an unlifted
equality, since it's kind (`TYPE ('TupleRep '[])`) was more
confusing than anything.

Test Plan: make test TEST="T15039a T15039b T15039c T15039d"

Reviewers: simonpj, goldfire, bgamari

Reviewed By: simonpj

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15039

Differential Revision: https://phabricator.haskell.org/D4696

1 files changed, 5 insertions, 1 deletions

diff --git a/testsuite/tests/partial-sigs/should_compile/all.T b/testsuite/tests/partial-sigs/should_compile/all.T
index 0a0483b139..bdfc13d284 100644
--- a/testsuite/tests/partial-sigs/should_compile/all.T
+++ b/testsuite/tests/partial-sigs/should_compile/all.T
@@ -75,4 +75,8 @@ test('T14217', normal, compile_fail, [''])
 test('T14643', normal, compile, [''])
 test('T14643a', normal, compile, [''])
 test('T14715', normal, compile, [''])
-
+test('T15039a', normal, compile, [''])
+test('T15039b', normal, compile, ['-fprint-explicit-kinds'])
+test('T15039c', normal, compile, ['-fprint-equality-relations'])
+test('T15039d', normal, compile,
+                ['-fprint-explicit-kinds -fprint-equality-relations'])