Merge 'flags' stanzas in cabal.project (fix haskell#10767)

Add helpers monoidField[Parsec] as they might prove useful for other
fields too.

Author: hsyl20

cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs

@@ -1563,7 +1563,7 @@ legacyPackageConfigFieldDescrs =
             parseTokenQ
             configConfigureArgs
             (\v conf -> conf{configConfigureArgs = v})
-        , simpleFieldParsec
+        , monoidFieldParsec
             "flags"
             dispFlagAssignment
             parsecFlagAssignment

cabal-install/src/Distribution/Deprecated/ParseUtils.hs

@@ -44,6 +44,7 @@ module Distribution.Deprecated.ParseUtils
   , showFreeText
   , field
   , simpleField
+  , monoidField
   , listField
   , listFieldWithSep
   , spaceListField
@@ -52,6 +53,7 @@ module Distribution.Deprecated.ParseUtils
   , readPToMaybe
   , fieldParsec
   , simpleFieldParsec
+  , monoidFieldParsec
   , listFieldParsec
   , commaListFieldParsec
   , commaNewLineListFieldParsec
@@ -254,6 +256,32 @@ simpleFieldParsec
 simpleFieldParsec name showF readF get set =
   liftField get set $ fieldParsec name showF readF
 
+monoidField
+  :: Semigroup a
+  => String
+  -> (a -> Doc)
+  -> ReadP a a
+  -> (b -> a)
+  -> (a -> b -> b)
+  -> FieldDescr b
+monoidField name showF readF get set =
+  liftField get set' $ field name showF readF
+  where
+    set' xs b = set (get b <> xs) b
+
+monoidFieldParsec
+  :: Semigroup a
+  => String
+  -> (a -> Doc)
+  -> ParsecParser a
+  -> (b -> a)
+  -> (a -> b -> b)
+  -> FieldDescr b
+monoidFieldParsec name showF readF get set =
+  liftField get set' $ fieldParsec name showF readF
+  where
+    set' xs b = set (get b <> xs) b
+
 commaListFieldWithSepParsec
   :: Separator
   -> String

cabal-testsuite/PackageTests/ProjectConfig/FlagsSemigroup/cabal.out

@@ -0,0 +1,10 @@
+# cabal build
+Configuration is affected by the following files:
+- cabal.project
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following will be built:
+ - test-0.1 (lib) (first run)
+Configuring library for test-0.1...
+Preprocessing library for test-0.1...
+Building library for test-0.1...

cabal-testsuite/PackageTests/ProjectConfig/FlagsSemigroup/cabal.project

@@ -0,0 +1,7 @@
+packages: .
+
+package test
+  -- test for #10767
+  flags: -bar
+  flags: -baz
+  -- flags: -bar -baz -- this works

cabal-testsuite/PackageTests/ProjectConfig/FlagsSemigroup/cabal.test.hs

@@ -0,0 +1,4 @@
+import Test.Cabal.Prelude
+
+main = cabalTest $ do
+  cabal "build" []

cabal-testsuite/PackageTests/ProjectConfig/FlagsSemigroup/test.cabal

@@ -0,0 +1,18 @@
+cabal-version:  3.0
+name:           test
+version:        0.1
+license:        BSD-3-Clause
+build-type:     Simple
+
+flag bar
+  default: True
+  manual: True
+
+flag baz
+  default: True
+  manual: True
+
+Library
+  default-language: Haskell2010
+  if flag(bar) || flag (baz)
+    buildable: False

changelog.d/pr-10805.md

@@ -0,0 +1,17 @@
+---
+synopsis: Merge 'flags' stanzas in cabal.project files
+packages: [cabal-install]
+prs: 10805
+issues: [10767]
+---
+
+Merge 'flags' stanzas in cabal.project files. Now
+
+```
+flags: -foo
+flags: -bar
+```
+
+is equivalent to `flags: -foo -bar` while before it was equivalent to `flags: -bar`
+(only the latest stanza was taken into account, the other ones were silently
+ignored).