diff --git a/hadrian/src/Settings/Builders/RunTest.hs b/hadrian/src/Settings/Builders/RunTest.hs
index 55a5e5871a3db87c73b5ce845a67bd2044ca9ab8..834cacf03dd5e61b05d35553f182c1a1a5e52e7a 100644
--- a/hadrian/src/Settings/Builders/RunTest.hs
+++ b/hadrian/src/Settings/Builders/RunTest.hs
@@ -46,6 +46,7 @@ runTestGhcFlags = do
         , ifMinGhcVer "711" "-fshow-warning-groups"
         , ifMinGhcVer "801" "-fdiagnostics-color=never"
         , ifMinGhcVer "801" "-fno-diagnostics-show-caret"
+        , pure "-Werror=compat" -- See Trac #15278
         , pure "-dno-debug-output"
         ]