Install binutils-gold for ghc < 9.6 on Linux

src/installer.ts

@@ -146,6 +146,21 @@ export async function installTool(
   os: OS,
   arch: Arch
 ): Promise<void> {
+  // GHC < 9.6 bindists ship an hsc2hs wrapper with a hardcoded
+  // "--lflag=-fuse-ld=gold", so ld.gold has to be present whenever hsc2hs
+  // runs, even if GHC itself is already installed (preinstalled or cached).
+  // Hence, do this before the isInstalled check.
+  // “version” may not be a semantic version (e.g. “latest-nightly”),
+  // so guard “compareVersions” with “validate”.
+  if (
+    os === 'linux' &&
+    tool === 'ghc' &&
+    validate(version) &&
+    compareVersions(version, '9.6') < 0 // meaning version < 9.6
+  ) {
+    await aptBinutilsGold();
+  }
+
   if (await isInstalled(tool, version, os, arch)) return;
   warn(tool, version);
 
@@ -281,6 +296,35 @@ async function aptBuildEssential(): Promise<boolean> {
   return returnCode === 0;
 }
 
+async function aptBinutilsGold(): Promise<boolean> {
+  // Up to ubuntu-24.04, ld.gold is part of binutils. Since binutils 2.44
+  // deprecated gold, ubuntu-25.10 and later ship it in a separate
+  // binutils-gold package instead.
+  if (await which('ld.gold', false)) {
+    core.debug('ld.gold is already installed');
+    return true;
+  }
+  if (!(await which('apt-get', false))) {
+    core.warning(
+      'ld.gold not found and apt-get is unavailable; hsc2hs of ghc < 9.6 may fail to link'
+    );
+    return false;
+  }
+
+  core.info(`Installing binutils-gold using apt-get (for ghc < 9.6)`);
+  // exec throws (instead of returning non-zero) if e.g. sudo is not
+  // available; this must not fail the action, as hsc2hs may not be needed.
+  const returnCode = await exec(
+    `sudo -- sh -c "apt-get update && apt-get -y install binutils-gold"`
+  ).catch(() => -1);
+  if (returnCode !== 0) {
+    core.warning(
+      'Failed to install binutils-gold; hsc2hs of ghc < 9.6 may fail to link'
+    );
+  }
+  return returnCode === 0;
+}
+
 async function aptLibNCurses5(): Promise<boolean> {
   core.info(
     `Installing libcurses5 and libtinfo5 using apt-get (for ghc < 8.3)`