gistfile1.txt

commit fd9986790de790e3bec7e5a7ac19fc6b2541b199
Author: Krzysztof Gogolewski <[email protected]>
Date:   Mon Jun 9 12:44:07 2025 +0200

    Fix EPT enforcement when mixing unboxed tuples and non-tuples
    
    The code was assuming that an alternative cannot be returning a normal
    datacon and an unboxed tuple at the same time. However, as seen in #26107,
    this can happen when using a GADT to refine the representation type.
    
    The solution is just to conservatively return TagDunno.

diff --git a/compiler/GHC/Stg/EnforceEpt/Types.hs b/compiler/GHC/Stg/EnforceEpt/Types.hs
index dc022ecd48d..318e7f984c2 100644
--- a/compiler/GHC/Stg/EnforceEpt/Types.hs
+++ b/compiler/GHC/Stg/EnforceEpt/Types.hs
@@ -39,8 +39,8 @@ type InferStgAlt        = GenStgAlt        'InferTaggedBinders
 combineAltInfo :: TagInfo -> TagInfo -> TagInfo
 combineAltInfo TagDunno         _              = TagDunno
 combineAltInfo _                TagDunno       = TagDunno
-combineAltInfo (TagTuple {})    TagProper      = panic "Combining unboxed tuple with non-tuple result"
-combineAltInfo TagProper       (TagTuple {})   = panic "Combining unboxed tuple with non-tuple result"
+combineAltInfo (TagTuple {})    TagProper      = TagDunno  -- This can happen with rep-polymorphic result, see #26107
+combineAltInfo TagProper       (TagTuple {})   = TagDunno  -- This can happen with rep-polymorphic result, see #26107
 combineAltInfo TagProper        TagProper      = TagProper
 combineAltInfo (TagTuple is1)  (TagTuple is2)  = TagTuple (zipWithEqual combineAltInfo is1 is2)
 combineAltInfo (TagTagged)      ti             = ti
diff --git a/testsuite/tests/rep-poly/T26107.hs b/testsuite/tests/rep-poly/T26107.hs
new file mode 100644
index 00000000000..b045f3a6f0c
--- /dev/null
+++ b/testsuite/tests/rep-poly/T26107.hs
@@ -0,0 +1,14 @@
+{-# LANGUAGE GADTs, UnboxedTuples #-}
+module T26107 where
+
+import Data.Kind
+import GHC.Exts
+
+type T :: TYPE rep -> Type
+data T a where
+  A :: T Bool
+  B :: T (# #)
+
+f :: forall rep (a :: TYPE rep). T a -> a
+f A = True
+f B = (# #)
diff --git a/testsuite/tests/rep-poly/all.T b/testsuite/tests/rep-poly/all.T
index c80111962a5..fcedb6d666f 100644
--- a/testsuite/tests/rep-poly/all.T
+++ b/testsuite/tests/rep-poly/all.T
@@ -41,6 +41,7 @@ test('T23883a', normal, compile_fail, [''])
 test('T23883b', normal, compile_fail, [''])
 test('T23883c', normal, compile_fail, [''])
 test('T23903', normal, compile_fail, [''])
+test('T26107', js_broken(22364), compile, ['-O'])
 
 test('EtaExpandDataCon', normal, compile, ['-O'])
 test('EtaExpandStupid1', normal, compile, ['-Wno-deprecated-flags'])