`--allow-boot-library-installs` fails on GHC < 9.14 with "unbuildable library from ghc-internal"

[andreabedini]

Describe the bug

On master, passing --allow-boot-library-installs turns an otherwise working
plan into a failure, on any GHC that does not report wired-in unit ids (GHC < 9.14).
The error names ghc-internal, a package the user never mentioned:

Error:
    Dependency on unbuildable library from ghc-internal
    In the stanza 'library'
    In the package 'base-4.22.0.0'

Note that the solver does not report an unsolvable plan. It succeeds, having
selected base-4.22.0.0 from Hackage; the failure happens afterwards, during
elaboration.

This is a regression on master only. Released cabal-install 3.18.1.0 is not
affected.

To Reproduce

#!/usr/bin/env bash
# Usage:  ./repro.sh            # uses `cabal` and `ghc` from PATH
#         CABAL=/path/to/cabal GHC=ghc-9.12.2 ./repro.sh
#
# Exits 0 if cabal behaves correctly, non-zero if the bug is present.
set -eu

CABAL=${CABAL:-cabal}
GHC=${GHC:-ghc}

dir=$(mktemp -d)
trap 'rm -rf "$dir"' EXIT
cd "$dir"

cat > repro.cabal <<'EOF'
cabal-version: 2.4
name:          repro
version:       0
build-type:    Simple

library
  exposed-modules:  Lib
  build-depends:    base
  default-language: Haskell2010
EOF
printf 'module Lib where\n' > Lib.hs

echo "cabal-install $("$CABAL" --numeric-version), ghc $("$GHC" --numeric-version)"
echo

echo "=== 1. without --allow-boot-library-installs (expected: succeeds) ==="
"$CABAL" build --dry-run -w "$GHC"
echo

echo "=== 2. with --allow-boot-library-installs (expected: succeeds) ==="
if "$CABAL" build --dry-run -w "$GHC" --allow-boot-library-installs; then
  echo
  echo "RESULT: ok -- no bug on this cabal/ghc combination."
else
  echo
  echo "RESULT: BUG -- the same plan fails once --allow-boot-library-installs is passed."
  exit 1
fi

Step 1 establishes that the plan is fine; step 2 differs only by the flag.

GHC	result
9.10.3	fails
9.12.2	fails
9.14.1	succeeds (reports wired-in unit ids)

Without the flag, all three succeed.

Expected behavior

--allow-boot-library-installs should not make a plan fail. It relaxes a
restriction; it should never turn a solvable plan into an unsolvable one.

System information

• Linux
• cabal-install built from master (a0c8112), version 3.19.0.0
• GHC 9.10.3, 9.12.2, 9.14.1

Additional context

dependOnWiredIns adds two kinds of constraint: one installed-unit-id
constraint per wired-in unit of the compiler, and one base >= 4.22 version
constraint added in #12055.

cabal/cabal-install/src/Distribution/Client/Dependency.hs

Lines 447 to 463 in a0c8112

	dependOnWiredIns :: CompilerInfo -> DepResolverParams -> DepResolverParams
	dependOnWiredIns compiler params = addConstraints extraConstraints params
	where
	extraConstraints =
	[ LabeledPackageConstraint
	(PackageConstraint (ScopeAnyQualifier pkgName) (PackagePropertyInstalledSpecificUnitId unitId))
	ConstraintSourceNonReinstallablePackage
	| (pkgName, unitId) <- fromMaybe [] $ compilerInfoWiredInUnitIds compiler
	]
	++
	-- Old versions of `base` must be excluded from build plans still as they do not depend on any version of a wired-in unit.
	-- If we do not do this then we will get confusing error messages about old versions of `base` being unbuildable.
	-- Newer versions of `base` will be handled gracefully as they were designed to be reinstallable.
	[ LabeledPackageConstraint
	(PackageConstraint (ScopeAnyQualifier $ mkPackageName "base") (PackagePropertyVersion (orLaterVersion (mkVersion [4, 22]))))
	ConstraintSourceNonReinstallablePackage
	]

The second constraint only makes sense next to the first. The unit-id
constraints pin the new, reinstallable base; the version constraint then
excludes the old, non-reinstallable one. But it is added unconditionally, while
the unit-id constraints are empty for a compiler that reports no wired-in units.

That path is reachable for such a compiler, because the caller enters it on
either of two conditions:

cabal/cabal-install/src/Distribution/Client/Dependency.hs

Lines 919 to 921 in a0c8112

	if isJust (compilerInfoWiredInUnitIds comp) || asBool (depResolverAllowBootLibInstalls params)
	then dependOnWiredIns comp params
	else dontInstallNonReinstallablePackages params

--allow-boot-library-installs alone is enough. So on GHC 9.12, the installed
base-4.21.0.0 is excluded by the version constraint, base-4.22.0.0 is
selected from Hackage instead, and it cannot be built because ghc-internal is
not available there.

Fixed by #12301, which guards the version constraint on the compiler actually
reporting wired-in unit ids.