Allow :info for (~) in GHCi

`(~)` is not an identifier according to GHC's parser, which
is why GHCi's `:info` command wouldn't work on it. To rectify this,
we apply the same fix that was put in place for `(->)`: add `(~)` to
GHC's `identifier` parser production.

Test Plan: make test TEST=T10059

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, mpickering, carter

GHC Trac Issues: #10059

Differential Revision: https://phabricator.haskell.org/D4877

compiler/parser/Parser.y

@@ -626,7 +626,9 @@ identifier :: { Located RdrName }
         | qvarop                        { $1 }
         | qconop                        { $1 }
     | '(' '->' ')'      {% ams (sLL $1 $> $ getRdrName funTyCon)
-                               [mj AnnOpenP $1,mu AnnRarrow $2,mj AnnCloseP $3] }
+                               [mop $1,mu AnnRarrow $2,mcp $3] }
+    | '(' '~' ')'       {% ams (sLL $1 $> $ eqTyCon_RDR)
+                               [mop $1,mj AnnTilde $2,mcp $3] }
 
 -----------------------------------------------------------------------------
 -- Backpack stuff

testsuite/tests/ghci/scripts/T10059.script

+:set -XTypeOperators
+:i (~)
+:k (~)
+:set -fprint-equality-relations
+:i (~)

testsuite/tests/ghci/scripts/T10059.stdout

+class (a ~ b) => (~) (a :: k) (b :: k)
+  	-- Defined in ‘Data.Type.Equality’
+instance [incoherent] forall k (a :: k) (b :: k). (a ~ b) => a ~ b
+  -- Defined in ‘Data.Type.Equality’
+(~) :: k -> k -> Constraint
+class (a ~~ b) => (~) (a :: k) (b :: k)
+  	-- Defined in ‘Data.Type.Equality’
+instance [incoherent] forall k (a :: k) (b :: k). (a ~~ b) => a ~ b
+  -- Defined in ‘Data.Type.Equality’

testsuite/tests/ghci/scripts/all.T

@@ -201,6 +201,7 @@ test('T9878', [], ghci_script, ['T9878.script'])
 test('T9878b', [extra_run_opts('-fobject-code')], ghci_script,
      ['T9878b.script'])
 test('T10018', normal, ghci_script, ['T10018.script'])
+test('T10059', normal, ghci_script, ['T10059.script'])
 test('T10122', normal, ghci_script, ['T10122.script'])
 
 test('T10321', normal, ghci_script, ['T10321.script'])