Require ghc >= 8.6, base >= 4.12.

This allows us to get rid of the old custom prelude and
some crufty cpp.  But the primary reason for this is that
conduit has bumped its base lower bound to 4.12, making it
impossible for us to support lower base versions.

.github/workflows/ci.yml

@@ -43,20 +43,10 @@ jobs:
       fail-fast: true
       matrix:
         versions:
-          - ghc: '8.2.2'
-            cabal: '2.4'
-            prefix: 'v2-'
-            cabalopts: ''
-            testopts: ''
-          - ghc: '8.4.4'
-            cabal: '2.4'
-            prefix: 'v2-'
-            cabalopts: '-f-embed_data_files'
-            testopts: ''
           - ghc: '8.6.5'
             cabal: '3.2'
             prefix: ''
-            cabalopts: ''
+            cabalopts: '-f-embed_data_files'
             testopts: '--test-option=--hide-successes --test-option=--ansi-tricks=false'
           - ghc: '8.8.4'
             cabal: '3.2'

cabal.project

@@ -1,5 +1,5 @@
 packages: pandoc.cabal
 tests: True
 flags: +embed_data_files
-constraints: aeson >= 2.0.1.0, transformers >= 0.5.6
+constraints: aeson >= 2.0.1.0
 

pandoc.cabal

@@ -409,31 +409,27 @@ flag trypandoc
 
 common common-options
   default-language: Haskell2010
-  build-depends:    base         >= 4.9 && < 5
+  build-depends:    base         >= 4.12 && < 5
   ghc-options:      -Wall -fno-warn-unused-do-bind
                     -Wincomplete-record-updates
                     -Wnoncanonical-monad-instances
+                    -Wcpp-undef
+                    -Wincomplete-uni-patterns
+                    -Widentities
+                    -Wpartial-fields
+                    -Wmissing-signatures
+                    -fhide-source-paths
+                    -- -Wmissing-export-lists
 
-  if impl(ghc < 8.6)
-    hs-source-dirs:   prelude
-    other-modules:    Prelude
-    build-depends:    base-compat >= 0.10.5
-    other-extensions: NoImplicitPrelude
+  if impl(ghc >= 8.10)
+    ghc-options:    -Wunused-packages
+
+  if impl(ghc >= 9.0)
+    ghc-options:    -Winvalid-haddock
 
   if os(windows)
     cpp-options:      -D_WINDOWS
 
-  -- Later:
-  -- -Wpartial-fields        (currently used in Powerpoint writer)
-  -- -Wmissing-export-lists  (currently some Odt modules violate this)
-  -- -Wredundant-constraints (problematic if we want to support older base)
-  if impl(ghc >= 8.2)
-    ghc-options:      -Wcpp-undef
-  if impl(ghc >= 8.4)
-    ghc-options:      -Wincomplete-uni-patterns
-                      -Widentities
-                      -fhide-source-paths
-
 common common-executable
   import:           common-options
   build-depends:    pandoc

prelude/Prelude.hs

@@ -1,15 +0,0 @@
-{-# LANGUAGE NoImplicitPrelude #-}
-
--- The intent is that this Prelude provide the API of
--- the base 4.11 Prelude in a way that is portable for
--- all base versions.
-
-module Prelude
-(
-  module Prelude.Compat
-, Semigroup(..)
-)
-where
-
-import Prelude.Compat
-import Data.Semigroup (Semigroup(..))  -- includes (<>)

src/Text/Pandoc/App/CommandLineOptions.hs

@@ -33,10 +33,8 @@ import Data.Bifunctor (second)
 import Data.Char (toLower)
 import Data.List (intercalate, sort, foldl')
 #ifdef _WINDOWS
-#if MIN_VERSION_base(4,12,0)
 import Data.List (isPrefixOf)
 #endif
-#endif
 import Data.Maybe (fromMaybe, isJust)
 import Data.Text (Text)
 import Safe (tailDef)
@@ -1078,7 +1076,6 @@ readMetaValue s
 -- beginning with \\ to \\?\UNC\. -- See #5127.
 normalizePath :: FilePath -> FilePath
 #ifdef _WINDOWS
-#if MIN_VERSION_base(4,12,0)
 normalizePath fp =
   if "\\\\" `isPrefixOf` fp && not ("\\\\?\\" `isPrefixOf` fp)
     then "\\\\?\\UNC\\" ++ drop 2 fp
@@ -1086,6 +1083,3 @@ normalizePath fp =
 #else
 normalizePath = id
 #endif
-#else
-normalizePath = id
-#endif