Have the solver reject packages with a too-new/unsupported spec-version

cabal-install/Distribution/Solver/Modular/IndexConversion.hs

@@ -12,6 +12,7 @@ import Distribution.Compiler
 import Distribution.InstalledPackageInfo as IPI
 import Distribution.Package                          -- from Cabal
 import Distribution.Simple.BuildToolDepends          -- from Cabal
+import Distribution.Simple.Utils (cabalVersion)      -- from Cabal
 import Distribution.Types.ExeDependency              -- from Cabal
 import Distribution.Types.PkgconfigDependency        -- from Cabal
 import Distribution.Types.ComponentName              -- from Cabal
@@ -191,8 +192,37 @@ convGPD os arch cinfo strfl solveExes pn
 
     addStanza :: Stanza -> DependencyReason pn -> DependencyReason pn
     addStanza s (DependencyReason pn' fs ss) = DependencyReason pn' fs (s : ss)
+
+    -- | We infer the maximally supported spec-version from @lib:Cabal@'s version
+    --
+    -- As we cannot predict the future, we can only properly support
+    -- spec-versions predating (and including) the @lib:Cabal@ version
+    -- used by @cabal-install@.
+    --
+    -- This relies on 'cabalVersion' having always at least 3 components to avoid
+    -- comparisons like @2.0.0 > 2.0@ which would result in confusing results.
+    --
+    -- NOTE: Before we can switch to a /normalised/ spec-version
+    -- comparison (e.g. by truncating to 3 components, and removing
+    -- trailing zeroes) we'd have to make sure all other places where
+    -- the spec-version is compared against a bound do it
+    -- consistently.
+    maxSpecVer = cabalVersion
+
+    -- | Required/declared spec-version of the package
+    --
+    -- We don't truncate patch-levels, as specifying a patch-level
+    -- spec-version is discouraged and not supported anymore starting
+    -- with spec-version 2.2.
+    reqSpecVer = specVersion pkg
+
+    -- | A too-new specVersion is turned into a global 'FailReason'
+    -- which prevents the solver from selecting this release (and if
+    -- forced to, emit a meaningful solver error message).
+    fr | reqSpecVer > maxSpecVer = Just (UnsupportedSpecVer reqSpecVer)
+       | otherwise               = Nothing
   in
-    PInfo flagged_deps (L.map fst exes) fds Nothing
+    PInfo flagged_deps (L.map fst exes) fds fr
 
 -- | Create a flagged dependency tree from a list @fds@ of flagged
 -- dependencies, using @f@ to form the tree node (@f@ will be

cabal-install/Distribution/Solver/Modular/Message.hs

@@ -146,6 +146,7 @@ showFR c Backjump                         = " (backjumping, conflict set: " ++ s
 showFR _ MultipleInstances                = " (multiple instances)"
 showFR c (DependenciesNotLinked msg)      = " (dependencies not linked: " ++ msg ++ "; conflict set: " ++ showConflictSet c ++ ")"
 showFR c CyclicDependencies               = " (cyclic dependencies; conflict set: " ++ showConflictSet c ++ ")"
+showFR _ (UnsupportedSpecVer ver)         = " (unsupported spec-version " ++ display ver ++ ")"
 -- The following are internal failures. They should not occur. In the
 -- interest of not crashing unnecessarily, we still just print an error
 -- message though.

cabal-install/Distribution/Solver/Modular/Tree.hs

@@ -118,6 +118,7 @@ data FailReason = UnsupportedExtension Extension
                 | MultipleInstances
                 | DependenciesNotLinked String
                 | CyclicDependencies
+                | UnsupportedSpecVer Ver
   deriving (Eq, Show)
 
 -- | Information about a dependency involved in a conflict, for error messages.

cabal-testsuite/PackageTests/CustomWithoutCabal/custom-setup-without-cabal.cabal

@@ -1,7 +1,7 @@
+cabal-version: 2.0
 name: custom-setup-without-cabal
 version: 1.0
 build-type: Custom
-cabal-version: >= 99999
 
 custom-setup
   setup-depends: base

cabal-testsuite/PackageTests/Regression/T3436/custom-setup/custom-setup.cabal

@@ -1,7 +1,7 @@
+cabal-version: 2.0
 name: custom-setup
 version: 1.0
 build-type: Custom
-cabal-version: >= 99999
 
 custom-setup
   setup-depends: base, Cabal >= 99999