[flang-commits] [flang] [Flang] bug: preprocessor increases backslash to double backslash (PR #191512)

[via flang-commits]

https://github.com/SunilKuravinakop created https://github.com/llvm/llvm-project/pull/191512

Correcting the preprocessing (-E) output when '\' is used in a string.
Prior to this fix it was being replaced with "\\".

Issue is in [191463](https://github.com/llvm/llvm-project/issues/191463)

>From 3d5df863b8b0e85a5965b17b1d2199cf2418b7f0 Mon Sep 17 00:00:00 2001
From: Sunil Kuravinakop <kuravina at pe31.hpc.amslabs.hpecorp.net>
Date: Fri, 10 Apr 2026 12:36:14 -0500
Subject: [PATCH] Correcting the preprocessing (-E) output when '\' is used in
 a string. Prior to this fix it was being replaced with "\\".

---
 flang/lib/Parser/prescan.cpp            | 6 +++---
 flang/test/Driver/escaped-backslash.f90 | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/flang/lib/Parser/prescan.cpp b/flang/lib/Parser/prescan.cpp
index 78537aef1fdfe..c1dd17400012a 100644
--- a/flang/lib/Parser/prescan.cpp
+++ b/flang/lib/Parser/prescan.cpp
@@ -1014,9 +1014,9 @@ void Prescanner::QuotedCharacterLiteral(
     if (*at_ == '\\') {
       if (escapesEnabled) {
         isEscaped = !isEscaped;
-      } else {
-        // The parser always processes escape sequences, so don't confuse it
-        // when escapes are disabled.
+      } else if (!preprocessingOnly_) {
+        // Except when -E is used, the parser always processes escape sequences,
+        // so don't confuse it when escapes are disabled.
         insert('\\');
       }
     } else {
diff --git a/flang/test/Driver/escaped-backslash.f90 b/flang/test/Driver/escaped-backslash.f90
index 90dd1783dd115..75e4ff3beacdc 100644
--- a/flang/test/Driver/escaped-backslash.f90
+++ b/flang/test/Driver/escaped-backslash.f90
@@ -15,8 +15,8 @@
 ! RUN: %flang_fc1 -E -fbackslash %s  2>&1 | FileCheck %s --check-prefix=UNESCAPED
 
 ! ESCAPED:program Backslash
-! ESCAPED-NEXT:New\\nline
-! ESCAPED-NOT:New\nline
+! ESCAPED-NEXT:New\nline
+! ESCAPED-NOT:New\\nline
 
 ! UNESCAPED:program Backslash
 ! UNESCAPED-NEXT:New\nline