Fix #16895 by checking whether infix expression operator is a variable

2 files changed, 13 insertions, 6 deletions

diff --git a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
index 6eaadd648e..792a78b606 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
@@ -123,7 +123,10 @@ isSymOcc n
 pprInfixExp :: Exp -> Doc
 pprInfixExp (VarE v) = pprName' Infix v
 pprInfixExp (ConE v) = pprName' Infix v
-pprInfixExp _        = text "<<Non-variable/constructor in infix context>>"
+pprInfixExp (UnboundVarE v) = pprName' Infix v
+-- This case will only ever be reached in exceptional circumstances.
+-- For example, when printing an error message in case of a malformed expression.
+pprInfixExp e = text "`" <> ppr e <> text "`"
 
 pprExp :: Precedence -> Exp -> Doc
 pprExp _ (VarE v)     = pprName' Applied v
diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
index 2d79d5a28f..f79a8e2b0c 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
@@ -1895,11 +1895,15 @@ data Exp
 
   | InfixE (Maybe Exp) Exp (Maybe Exp) -- ^ @{x + y} or {(x+)} or {(+ x)} or {(+)}@
 
-    -- It's a bit gruesome to use an Exp as the
-    -- operator, but how else can we distinguish
-    -- constructors from non-constructors?
-    -- Maybe there should be a var-or-con type?
-    -- Or maybe we should leave it to the String itself?
+    -- It's a bit gruesome to use an Exp as the operator when a Name
+    -- would suffice. Historically, Exp was used to make it easier to
+    -- distinguish between infix constructors and non-constructors.
+    -- This is a bit overkill, since one could just as well call
+    -- `startsConId` or `startsConSym` (from `GHC.Lexeme`) on a Name.
+    -- Unfortunately, changing this design now would involve lots of
+    -- code churn for consumers of the TH API, so we continue to use
+    -- an Exp as the operator and perform an extra check during conversion
+    -- to ensure that the Exp is a constructor or a variable (#16895).
 
   | UInfixE Exp Exp Exp                -- ^ @{x + y}@
                                        --