Fix for issue #3932.

cabal-install/Distribution/Client/ProjectPlanning.hs

@@ -952,6 +952,17 @@ planPackages comp platform solver SolverSettings{..}
           | Dependency name ver <- solverSettingPreferences ]
 
       . addConstraints
+
+          -- If a package has a custom setup then we need to add a setup-depends
+          -- on Cabal. For now it's easier to add this unconditionally.  Once
+          -- qualified constraints land we can turn this into a custom setup
+          -- only constraint.
+          --
+          -- TODO: use a qualified constraint
+            [ LabeledPackageConstraint (PackageConstraintVersion cabalPkgname
+                                       (orLaterVersion (mkVersion [1,20])))
+                                       ConstraintNewBuildCustomSetupLowerBoundCabal
+                                       ] . addConstraints
           -- version constraints from the config file or command line
             [ LabeledPackageConstraint (userToPackageConstraint pc) src
             | (pc, src) <- solverSettingConstraints ]
@@ -2392,6 +2403,17 @@ defaultSetupDeps compiler platform pkg =
           -- * We omit the dep for the Cabal lib itself, since it bootstraps.
           -- * We constrain it to be >= 1.18 < 2
           --
+          -- Note: cabalCompatMinVer only gets applied WHEN WE ARE ADDING a
+          -- default setup build info, i.e., when there is no custom-setup
+          -- stanza. If there is a custom-setup stanza, this codepath never gets
+          -- invoked (that's why there's an error case for
+          -- SetupCustomExplicitDeps).
+          --
+          -- One way we could solve this problem is by also modifying
+          -- custom-setup stanzas when they exist, but we're going to take a
+          -- different approach: add an extra constraint on Cabal globally to
+          -- make sure the solver respects it regardless of whether or not there
+          -- is an explicit setup build info or not. See planPackages.
           cabalConstraint   = orLaterVersion cabalCompatMinVer
                                 `intersectVersionRanges`
                               orLaterVersion (PD.specVersion pkg)
@@ -2421,6 +2443,8 @@ defaultSetupDeps compiler platform pkg =
       -- The internal setup wrapper method has no deps at all.
       SetupNonCustomInternalLib -> Just []
 
+      -- This case gets ruled out by the caller, planPackages, see the note
+      -- above in the SetupCustomIplicitDeps case.
       SetupCustomExplicitDeps ->
         error $ "defaultSetupDeps: called for a package with explicit "
              ++ "setup deps: " ++ display (packageId pkg)

cabal-install/Distribution/Solver/Types/ConstraintSource.hs

@@ -45,6 +45,9 @@ data ConstraintSource =
 
   -- | The source of the constraint is not specified.
   | ConstraintSourceUnknown
+
+  -- | Custom setup requires a minimum lower bound on Cabal
+  | ConstraintNewBuildCustomSetupLowerBoundCabal
   deriving (Eq, Show, Generic)
 
 instance Binary ConstraintSource
@@ -68,3 +71,4 @@ showConstraintSource ConstraintSourceFreeze = "cabal freeze"
 showConstraintSource ConstraintSourceConfigFlagOrTarget =
     "config file, command line flag, or user target"
 showConstraintSource ConstraintSourceUnknown = "unknown source"
+showConstraintSource ConstraintNewBuildCustomSetupLowerBoundCabal = "new-build's support of Custom Setup (issue #3932)"

cabal-install/cabal-install.cabal

@@ -51,6 +51,8 @@ Extra-Source-Files:
   tests/IntegrationTests/custom-setup/Cabal-99998/CabalMessage.hs
   tests/IntegrationTests/custom-setup/Cabal-99999/Cabal.cabal
   tests/IntegrationTests/custom-setup/Cabal-99999/CabalMessage.hs
+  tests/IntegrationTests/custom-setup/custom-setup-old-cabal/Setup.hs
+  tests/IntegrationTests/custom-setup/custom-setup-old-cabal/custom-setup-old-cabal.cabal
   tests/IntegrationTests/custom-setup/custom-setup-without-cabal-defaultMain/Setup.hs
   tests/IntegrationTests/custom-setup/custom-setup-without-cabal-defaultMain/custom-setup-without-cabal-defaultMain.cabal
   tests/IntegrationTests/custom-setup/custom-setup-without-cabal/Setup.hs
@@ -60,6 +62,7 @@ Extra-Source-Files:
   tests/IntegrationTests/custom-setup/custom_setup_without_Cabal_doesnt_allow_Cabal_import.sh
   tests/IntegrationTests/custom-setup/custom_setup_without_Cabal_doesnt_require_Cabal.sh
   tests/IntegrationTests/custom-setup/installs_Cabal_as_setup_dep.sh
+  tests/IntegrationTests/custom-setup/new_build_requires_Cabal_1_20.sh
   tests/IntegrationTests/custom/custom_dep.sh
   tests/IntegrationTests/custom/custom_dep/client/B.hs
   tests/IntegrationTests/custom/custom_dep/client/Setup.hs
@@ -171,11 +174,6 @@ Extra-Source-Files:
   tests/IntegrationTests/regression/t3199/Main.hs
   tests/IntegrationTests/regression/t3199/Setup.hs
   tests/IntegrationTests/regression/t3199/test-3199.cabal
-  tests/IntegrationTests/regression/t3335.sh
-  tests/IntegrationTests/regression/t3335/A.hs
-  tests/IntegrationTests/regression/t3335/Setup.hs
-  tests/IntegrationTests/regression/t3335/cabal.project
-  tests/IntegrationTests/regression/t3335/t3335.cabal
   tests/IntegrationTests/regression/t3827.sh
   tests/IntegrationTests/regression/t3827/cabal.project
   tests/IntegrationTests/regression/t3827/p/P.hs

cabal-install/tests/IntegrationTests/custom-setup/custom-setup-old-cabal/Setup.hs

@@ -0,0 +1,5 @@
+import CabalMessage (message)
+import System.Exit
+import System.IO
+
+main = hPutStrLn stderr message >> exitFailure

cabal-install/tests/IntegrationTests/custom-setup/custom-setup-old-cabal/custom-setup-old-cabal.cabal

@@ -0,0 +1,8 @@
+name: custom-setup
+version: 1.0
+build-type: Custom
+
+custom-setup
+  setup-depends: base, Cabal < 1.20
+
+library

cabal-install/tests/IntegrationTests/custom-setup/new_build_requires_Cabal_1_20.sh

@@ -0,0 +1,9 @@
+# Regression test for issue #3932
+
+. ./common.sh
+
+cd custom-setup-old-cabal
+! cabal new-build > output 2>&1
+
+cat output
+grep -q "(issue #3932) requires >=1.20" output || die "Expect constraint failure"