Be more careful when naming TyCon binders

This patch fixes two rather gnarly test cases:
  * Trac #16342 (mutual recursion)
    See Note [Tricky scoping in generaliseTcTyCon]

  * Trac #16221 (shadowing)
    See Note [Unification variables need fresh Names]

The main changes are:

* Substantial reworking of TcTyClsDecls.generaliseTcTyCon
  This is the big change, and involves the rather tricky
  function TcHsSyn.zonkRecTyVarBndrs.

  See Note [Inferring kinds for type declarations] and
  Note [Tricky scoping in generaliseTcTyCon] for the details.

* bindExplicitTKBndrs_Tv and bindImplicitTKBndrs_Tv both now
  allocate /freshly-named/ unification variables. Indeed, more
  generally, unification variables are always fresh; see
  Note [Unification variables need fresh Names] in TcMType

* Clarify the role of tcTyConScopedTyVars.
  See Note [Scoped tyvars in a TcTyCon] in TyCon

As usual, this dragged in some more refactoring:

* Renamed TcMType.zonkTyCoVarBndr to zonkAndSkolemise

* I renamed checkValidTelescope to checkTyConTelescope;
  it's only used on TyCons, and indeed takes a TyCon as argument.

* I folded the slightly-mysterious reportFloatingKvs into
  checkTyConTelescope. (Previously all its calls immediately
  followed a call to checkTyConTelescope.)  It makes much more
  sense there.

* I inlined some called-once functions to simplify
  checkValidTyFamEqn. It's less spaghetti-like now.

* This patch also fixes Trac #16251.  I'm not quite sure why #16251
  went wrong in the first place, nor how this patch fixes it, but
  hey, it's good, and life is short.

3 files changed, 12 insertions, 12 deletions

diff --git a/testsuite/tests/indexed-types/should_fail/all.T b/testsuite/tests/indexed-types/should_fail/all.T
index 8aef2e7673..4e29910c21 100644
--- a/testsuite/tests/indexed-types/should_fail/all.T
+++ b/testsuite/tests/indexed-types/should_fail/all.T
@@ -69,7 +69,7 @@ test('T2664a', normal, compile, [''])
 test('T2544', normal, compile_fail, [''])
 test('T1897b', normal, compile_fail, [''])
 test('T5439', normal, compile_fail, [''])
-test('T5515', when(compiler_debugged(), expect_broken(16251)), compile_fail, [''])
+test('T5515', normal, compile_fail, [''])
 test('T5934', normal, compile_fail, [''])
 test('T6123', normal, compile_fail, [''])
 test('ExtraTcsUntch', normal, compile_fail, [''])
diff --git a/testsuite/tests/typecheck/should_compile/tc141.stderr b/testsuite/tests/typecheck/should_compile/tc141.stderr
index f0cfdefd03..b0d4ef7f25 100644
--- a/testsuite/tests/typecheck/should_compile/tc141.stderr
+++ b/testsuite/tests/typecheck/should_compile/tc141.stderr
@@ -7,22 +7,22 @@ tc141.hs:11:12: error:
       In a pattern binding: (p :: a, q :: a) = x
 
 tc141.hs:11:31: error:
-    • Couldn't match expected type ‘a2’ with actual type ‘a’
-      ‘a2’ is a rigid type variable bound by
+    • Couldn't match expected type ‘a1’ with actual type ‘b’
+      ‘a1’ is a rigid type variable bound by
         an expression type signature:
-          forall a2. a2
+          forall a1. a1
         at tc141.hs:11:34
-      ‘a’ is a rigid type variable bound by
-        the inferred type of f :: (a, a) -> (a1, a)
+      ‘b’ is a rigid type variable bound by
+        the inferred type of f :: (b, b) -> (a, b)
         at tc141.hs:11:1-37
     • In the expression: q :: a
       In the expression: (q :: a, p)
       In the expression: let (p :: a, q :: a) = x in (q :: a, p)
     • Relevant bindings include
-        p :: a (bound at tc141.hs:11:12)
-        q :: a (bound at tc141.hs:11:17)
-        x :: (a, a) (bound at tc141.hs:11:3)
-        f :: (a, a) -> (a1, a) (bound at tc141.hs:11:1)
+        p :: b (bound at tc141.hs:11:12)
+        q :: b (bound at tc141.hs:11:17)
+        x :: (b, b) (bound at tc141.hs:11:3)
+        f :: (b, b) -> (a, b) (bound at tc141.hs:11:1)
 
 tc141.hs:13:13: error:
     • You cannot bind scoped type variable ‘a’
diff --git a/testsuite/tests/typecheck/should_fail/T2688.stderr b/testsuite/tests/typecheck/should_fail/T2688.stderr
index 63379a03b9..f7980d2734 100644
--- a/testsuite/tests/typecheck/should_fail/T2688.stderr
+++ b/testsuite/tests/typecheck/should_fail/T2688.stderr
@@ -3,10 +3,10 @@ T2688.hs:8:14: error:
     • Couldn't match expected type ‘v’ with actual type ‘s’
       ‘s’ is a rigid type variable bound by
         the class declaration for ‘VectorSpace’
-        at T2688.hs:(5,1)-(8,23)
+        at T2688.hs:5:21
       ‘v’ is a rigid type variable bound by
         the class declaration for ‘VectorSpace’
-        at T2688.hs:(5,1)-(8,23)
+        at T2688.hs:5:19
     • In the expression: v *^ (1 / s)
       In an equation for ‘^/’: v ^/ s = v *^ (1 / s)
     • Relevant bindings include