Vendor filepath inside template-haskell

Adding filepath as a dependency of template-haskell means that it can't
be reinstalled if any build-plan depends on template-haskell.

This is a temporary solution for the 9.4 release.

A longer term solution is to split-up the template-haskell package into
the wired-in part and a non-wired-in part which can be reinstalled. This
was deemed quite risky on the 9.4 release timescale.

Fixes #21738

1 files changed, 20 insertions, 1 deletions

diff --git a/libraries/template-haskell/template-haskell.cabal.in b/libraries/template-haskell/template-haskell.cabal.in
index 9ecbd81613..af678ceab6 100644
--- a/libraries/template-haskell/template-haskell.cabal.in
+++ b/libraries/template-haskell/template-haskell.cabal.in
@@ -27,6 +27,14 @@ source-repository head
     location: https://gitlab.haskell.org/ghc/ghc.git
     subdir:   libraries/template-haskell
 
+
+-- We give the option to vendor filepath to avoid making filepath non-reinstallable..
+-- see #21738 for why we are doing this now and what the plan is for the future.
+Flag vendor-filepath
+    Description: Vendor the dependency on filepath
+    Default: False
+    Manual: True
+
 Library
     default-language: Haskell2010
     other-extensions:
@@ -58,9 +66,20 @@ Library
         base        >= 4.11 && < 4.18,
         ghc-boot-th == @ProjectVersionMunged@,
         ghc-prim,
-        filepath,
         pretty      == 1.1.*
 
+    if flag(vendor-filepath)
+      other-modules:
+        System.FilePath
+        System.FilePath.Posix
+        System.FilePath.Windows
+      hs-source-dirs: ../filepath .
+      default-extensions:
+        ImplicitPrelude
+    else
+      build-depends: filepath
+      hs-source-dirs: .
+
     ghc-options: -Wall
 
     -- We need to set the unit ID to template-haskell (without a