From 36c9b4fd6de9cb5665e98b816e54e8aec5592f05 Mon Sep 17 00:00:00 2001
From: Peter Klausler <pklausler@nvidia.com>
Date: Thu, 16 Oct 2025 12:21:14 -0700
Subject: [PATCH] [flang] Fix character length checking in ALLOCATE (#163657)

The known character length compatibility check for ALLOCATE statements
needs to allow for negative lengths, which are effectively zero.

Fixes https://github.com/llvm/llvm-project/issues/163242.
---
 flang/lib/Semantics/check-allocate.cpp | 4 ++--
 flang/test/Semantics/bug163242.f90     | 5 +++++
 2 files changed, 7 insertions(+), 2 deletions(-)
 create mode 100644 flang/test/Semantics/bug163242.f90

diff --git a/flang/lib/Semantics/check-allocate.cpp b/flang/lib/Semantics/check-allocate.cpp
index 0779419e6272..e019bbdfa27f 100644
--- a/flang/lib/Semantics/check-allocate.cpp
+++ b/flang/lib/Semantics/check-allocate.cpp
@@ -441,7 +441,7 @@ static bool HaveCompatibleLengths(
         evaluate::ToInt64(type1.characterTypeSpec().length().GetExplicit())};
     auto v2{
         evaluate::ToInt64(type2.characterTypeSpec().length().GetExplicit())};
-    return !v1 || !v2 || *v1 == *v2;
+    return !v1 || !v2 || (*v1 >= 0 ? *v1 : 0) == (*v2 >= 0 ? *v2 : 0);
   } else {
     return true;
   }
@@ -454,7 +454,7 @@ static bool HaveCompatibleLengths(
     auto v1{
         evaluate::ToInt64(type1.characterTypeSpec().length().GetExplicit())};
     auto v2{type2.knownLength()};
-    return !v1 || !v2 || *v1 == *v2;
+    return !v1 || !v2 || (*v1 >= 0 ? *v1 : 0) == (*v2 >= 0 ? *v2 : 0);
   } else {
     return true;
   }
diff --git a/flang/test/Semantics/bug163242.f90 b/flang/test/Semantics/bug163242.f90
new file mode 100644
index 000000000000..5e020aeb4dc0
--- /dev/null
+++ b/flang/test/Semantics/bug163242.f90
@@ -0,0 +1,5 @@
+!RUN: %flang -fc1 -fsyntax-only %s | FileCheck --allow-empty %s
+!CHECK-NOT: error:
+character(0), allocatable :: ch
+allocate(character(-1) :: ch)
+end