Add Mikolaj’s test

i.e. fail on build. This means the error is no more “just” a
PackageDistInexcusable (Hackage will refuse it), *but* a
PackageBuildImpossible.

Cabal/src/Distribution/PackageDescription/Check/Target.hs

@@ -657,7 +657,9 @@ checkAutogenModules ams bi = do
   checkSpecVer
     CabalSpecV3_12
     (elem autoInfoModuleName allModsForAuto)
-    (PackageDistInexcusable CVAutogenPackageInfoGuard)
+    (PackageBuildImpossible CVAutogenPackageInfoGuard)
+      -- See Mikolaj’s comments on #9481 on why this has to be
+      -- PackageBuildImpossible and not merely PackageDistInexcusable.
   where
     allModsForAuto :: [ModuleName]
     allModsForAuto = ams ++ otherModules bi

cabal-testsuite/PackageTests/BuildAutogenPackageGuard/cabal.out

@@ -0,0 +1,10 @@
+# cabal v2-build
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following will be built:
+ - pkg-0 (lib) (first run)
+Configuring library for pkg-0...
+Error: [Cabal-5559]
+To use the autogenerated module PackageInfo_* you need to specify `cabal-version: 3.12` or higher.
+Error: [Cabal-7125]
+Failed to build pkg-0-inplace. The failure occurred during the configure step.

cabal-testsuite/PackageTests/BuildAutogenPackageGuard/cabal.project

@@ -0,0 +1,9 @@
+packages: .
+
+source-repository-package
+    type: git
+-- A Sample repo to test post-checkout-command
+    location: https://github.com/haskell/bytestring
+    post-checkout-command: true
+    tag: 0.10.9.0
+-- https://en.wikipedia.org/wiki/True_and_false_(commands)

cabal-testsuite/PackageTests/BuildAutogenPackageGuard/cabal.test.hs

@@ -0,0 +1,8 @@
+import Test.Cabal.Prelude
+
+-- #9331, guard PackageInfo functionality behind 3.12: make it a
+-- build failure.
+main = cabalTest $ do
+  withProjectFile "cabal.project" $ do
+    fails $ cabal "v2-build" ["pkg"]
+

cabal-testsuite/PackageTests/BuildAutogenPackageGuard/pkg.cabal

@@ -0,0 +1,17 @@
+cabal-version: 2.4
+name: pkg
+version: 0
+license: GPL-3.0-or-later
+maintainer: Someone
+category: Example
+synopsis: Foo
+description: FooBar
+build-type: Simple
+
+library
+    default-language: Haskell2010
+    build-depends: base == 4.*
+    -- ☞ N.B.: PackageInfo packages must contain the same name of
+    -- of the package! (In this example: `prova`).
+    autogen-modules: PackageInfo_pkg
+    exposed-modules: PackageInfo_pkg

cabal-testsuite/PackageTests/Check/ConfiguredPackage/CabalVersion/AutoGenMods/cabal.out

@@ -1,6 +1,7 @@
 # cabal check
+The package will not build sanely due to these errors:
+Error: To use the autogenerated module PackageInfo_* you need to specify `cabal-version: 3.12` or higher.
 The following errors will cause portability problems on other environments:
 Error: Packages using 'cabal-version: 2.0' and the autogenerated module Paths_* must include it also on the 'autogen-modules' field besides 'exposed-modules' and 'other-modules'. This specifies that the module does not come with the package and is generated on setup. Modules built with a custom Setup.hs script also go here to ensure that commands like sdist don't fail.
 Error: Packages using 'cabal-version: 2.0' and the autogenerated module PackageInfo_* must include it in 'autogen-modules' as well as 'exposed-modules' and 'other-modules'. This specifies that the module does not come with the package and is generated on setup. Modules built with a custom Setup.hs script also go here to ensure that commands like sdist don't fail.
-Error: To use the autogenerated module PackageInfo_* you need to specify `cabal-version: 3.12` or higher.
 Error: Hackage would reject this package.

cabal-testsuite/PackageTests/Check/ConfiguredPackage/Sanity/AutogenVersion/cabal.out

@@ -1,4 +1,4 @@
 # cabal check
-The following errors will cause portability problems on other environments:
+The package will not build sanely due to these errors:
 Error: To use the autogenerated module PackageInfo_* you need to specify `cabal-version: 3.12` or higher.
 Error: Hackage would reject this package.

cabal-testsuite/PackageTests/Check/NonConfCheck/PathsExtensions/cabal.out

@@ -1,7 +1,6 @@
 # cabal check
 The package will not build sanely due to these errors:
+Error: To use the autogenerated module PackageInfo_* you need to specify `cabal-version: 3.12` or higher.
 Error: Packages using RebindableSyntax with OverloadedStrings or OverloadedLists in default-extensions, in conjunction with the autogenerated module Paths_*, are known to cause compile failures with Cabal < 2.2. To use these default-extensions with a Paths_* autogen module, specify at least 'cabal-version: 2.2'.
 Error: Packages using RebindableSyntax with OverloadedStrings or OverloadedLists in default-extensions, in conjunction with the autogenerated module PackageInfo_*, are known to cause compile failures with Cabal < 2.2. To use these default-extensions with a PackageInfo_* autogen module, specify at least 'cabal-version: 2.2'.
-The following errors will cause portability problems on other environments:
-Error: To use the autogenerated module PackageInfo_* you need to specify `cabal-version: 3.12` or higher.
 Error: Hackage would reject this package.