Renumber flags version numbers in 'filterConfigureFlags'.

Previously, the code was inconsistent on whether or not
flags_x_y_z indicated that these flags could be used
up to version x.y.z, or should be used prior to x.y.z.
This commit picks the LATTER and renames everything
consistently this way.  The bonus is that now the names
match up with the conditionals. Yay.
Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

cabal-install/Distribution/Client/Setup.hs

@@ -353,20 +353,34 @@ configureCommand = c
 configureOptions ::  ShowOrParseArgs -> [OptionField ConfigFlags]
 configureOptions = commandOptions configureCommand
 
+-- | Given some 'ConfigFlags' for the version of Cabal that
+-- cabal-install was built with, and a target older 'Version' of
+-- Cabal that we want to pass these flags to, convert the
+-- flags into a form that will be accepted by the older
+-- Setup script.  Generally speaking, this just means filtering
+-- out flags that the old Cabal library doesn't understand, but
+-- in some cases it may also mean "emulating" a feature using
+-- some more legacy flags.
 filterConfigureFlags :: ConfigFlags -> Version -> ConfigFlags
 filterConfigureFlags flags cabalLibVersion
+  -- NB: we expect the latest version to be the most common case,
+  -- so test it first.
   | cabalLibVersion >= Version [1,23,0] [] = flags_latest
-  -- NB: we expect the latest version to be the most common case.
+  -- The naming convention is that flags_version gives flags with
+  -- all flags *introduced* in version eliminated.
+  -- It is NOT the latest version of Cabal library that
+  -- these flags work for; version of introduction is a more
+  -- natural metric.
   | cabalLibVersion <  Version [1,3,10] [] = flags_1_3_10
   | cabalLibVersion <  Version [1,10,0] [] = flags_1_10_0
   | cabalLibVersion <  Version [1,12,0] [] = flags_1_12_0
   | cabalLibVersion <  Version [1,14,0] [] = flags_1_14_0
   | cabalLibVersion <  Version [1,18,0] [] = flags_1_18_0
-  | cabalLibVersion <  Version [1,19,1] [] = flags_1_19_0
-  | cabalLibVersion <  Version [1,19,2] [] = flags_1_19_1
-  | cabalLibVersion <  Version [1,21,1] [] = flags_1_20_0
-  | cabalLibVersion <  Version [1,22,0] [] = flags_1_21_0
-  | cabalLibVersion <  Version [1,23,0] [] = flags_1_22_0
+  | cabalLibVersion <  Version [1,19,1] [] = flags_1_19_1
+  | cabalLibVersion <  Version [1,19,2] [] = flags_1_19_2
+  | cabalLibVersion <  Version [1,21,1] [] = flags_1_21_1
+  | cabalLibVersion <  Version [1,22,0] [] = flags_1_22_0
+  | cabalLibVersion <  Version [1,23,0] [] = flags_1_23_0
   | otherwise = flags_latest
   where
     flags_latest = flags        {
@@ -379,17 +393,17 @@ filterConfigureFlags flags cabalLibVersion
       }
 
     -- Cabal < 1.23 doesn't know about '--profiling-detail'.
-    flags_1_22_0 = flags_latest { configProfDetail    = NoFlag
+    flags_1_23_0 = flags_latest { configProfDetail    = NoFlag
                                 , configProfLibDetail = NoFlag
                                 , configIPID          = NoFlag }
 
     -- Cabal < 1.22 doesn't know about '--disable-debug-info'.
-    flags_1_21_0 = flags_1_22_0 { configDebugInfo = NoFlag }
+    flags_1_22_0 = flags_1_23_0 { configDebugInfo = NoFlag }
 
     -- Cabal < 1.21.1 doesn't know about 'disable-relocatable'
     -- Cabal < 1.21.1 doesn't know about 'enable-profiling'
-    flags_1_20_0 =
-      flags_1_21_0 { configRelocatable = NoFlag
+    flags_1_21_1 =
+      flags_1_22_0 { configRelocatable = NoFlag
                    , configProf = NoFlag
                    , configProfExe = configProf flags
                    , configProfLib =
@@ -399,13 +413,13 @@ filterConfigureFlags flags cabalLibVersion
                    }
     -- Cabal < 1.19.2 doesn't know about '--exact-configuration' and
     -- '--enable-library-stripping'.
-    flags_1_19_1 = flags_1_20_0 { configExactConfiguration = NoFlag
+    flags_1_19_2 = flags_1_21_1 { configExactConfiguration = NoFlag
                                 , configStripLibs = NoFlag }
     -- Cabal < 1.19.1 uses '--constraint' instead of '--dependency'.
-    flags_1_19_0 = flags_1_19_1 { configDependencies = []
+    flags_1_19_1 = flags_1_19_2 { configDependencies = []
                                 , configConstraints  = configConstraints flags }
     -- Cabal < 1.18.0 doesn't know about --extra-prog-path and --sysconfdir.
-    flags_1_18_0 = flags_1_19_0 { configProgramPathExtra = toNubList []
+    flags_1_18_0 = flags_1_19_1 { configProgramPathExtra = toNubList []
                                 , configInstallDirs = configInstallDirs_1_18_0}
     configInstallDirs_1_18_0 = (configInstallDirs flags) { sysconfdir = NoFlag }
     -- Cabal < 1.14.0 doesn't know about '--disable-benchmarks'.