From 3ff1bf0028085f5bf3ff9dab1aa7b1dcd5b7b033 Mon Sep 17 00:00:00 2001
From: Andrea Bedini <andrea@bedini.au>
Date: Mon, 3 Aug 2026 15:25:50 +0800
Subject: [PATCH] cabal-install: only add base>=4.22 constraint with wired-in
 unit ids

`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.

The second 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 was added unconditionally, while
the unit-id constraints are empty for a compiler that reports no wired-in
units. Such a compiler has no installed `base` satisfying the bound, so
plans that reach this code fail, and `--allow-boot-library-installs`
alone is enough to reach it.

Fixes #12328.
---
 cabal-install/src/Distribution/Client/Dependency.hs    |  9 ++++++---
 .../UnitTests/Distribution/Solver/Modular/Solver.hs    |  7 +++++++
 changelog.d/base-422-wired-in-guard.md                 | 10 ++++++++++
 3 files changed, 23 insertions(+), 3 deletions(-)
 create mode 100644 changelog.d/base-422-wired-in-guard.md

diff --git a/cabal-install/src/Distribution/Client/Dependency.hs b/cabal-install/src/Distribution/Client/Dependency.hs
index afc77c032aa..04815cea7d5 100644
--- a/cabal-install/src/Distribution/Client/Dependency.hs
+++ b/cabal-install/src/Distribution/Client/Dependency.hs
@@ -445,13 +445,16 @@ setSolverVerbosity verbosity params =
     }
 
 dependOnWiredIns :: CompilerInfo -> DepResolverParams -> DepResolverParams
-dependOnWiredIns compiler params = addConstraints extraConstraints params
+dependOnWiredIns compiler params =
+  case compilerInfoWiredInUnitIds compiler of
+    Nothing -> params
+    Just wiredInUnitIds -> addConstraints (extraConstraints wiredInUnitIds) params
   where
-    extraConstraints =
+    extraConstraints wiredInUnitIds =
       [ LabeledPackageConstraint
         (PackageConstraint (ScopeAnyQualifier pkgName) (PackagePropertyInstalledSpecificUnitId unitId))
         ConstraintSourceNonReinstallablePackage
-      | (pkgName, unitId) <- fromMaybe [] $ compilerInfoWiredInUnitIds compiler
+      | (pkgName, unitId) <- wiredInUnitIds
       ]
         ++
         -- Old versions of `base` must be excluded from build plans still as they do not depend on any version of a wired-in unit.
diff --git a/cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs b/cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs
index d76928e982f..cdacc2ee44b 100644
--- a/cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs
+++ b/cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs
@@ -218,6 +218,13 @@ tests =
           allowBootLibInstalls $
             mkTest dbBase "Install base with --allow-boot-library-installs" ["base"] $
               solverSuccess [("base", 5), ("ghc-prim", 1), ("integer-gmp", 1), ("integer-simple", 1)]
+      , -- The base >= 4.22 constraint only makes sense alongside the wired-in
+        -- unit id constraints. A compiler that reports none has no installed
+        -- base satisfying it, so it must not be added here.
+        runTest $
+          allowBootLibInstalls $
+            mkTest dbBaseOld "Install old base with --allow-boot-library-installs" ["base"] $
+              solverSuccess [("base", 1)]
       ]
   , testGroup
       "Reinstallable base, template-haskell, but not ghc{,-internal} (GHC with wiredInUnitIds)"
diff --git a/changelog.d/base-422-wired-in-guard.md b/changelog.d/base-422-wired-in-guard.md
new file mode 100644
index 00000000000..cdcb654fd59
--- /dev/null
+++ b/changelog.d/base-422-wired-in-guard.md
@@ -0,0 +1,10 @@
+---
+synopsis: Fix `--allow-boot-library-installs` on GHC < 9.14
+packages: [cabal-install]
+prs: 12301
+issues: 12328
+---
+
+`--allow-boot-library-installs` made builds fail on compilers that report no
+wired-in unit ids, because `base` was constrained to version 4.22 or later
+regardless of the compiler.