From 3bc3cac7acbf42982dce2e75d4c281dda2dbcce6 Mon Sep 17 00:00:00 2001
From: Peter Klausler <pklausler@nvidia.com>
Date: Wed, 15 Oct 2025 16:12:40 -0700
Subject: [PATCH] [flang] Fix character length checking in ALLOCATE

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 823aa4e795e35..d7772da210917 100644
--- a/flang/lib/Semantics/check-allocate.cpp
+++ b/flang/lib/Semantics/check-allocate.cpp
@@ -439,7 +439,7 @@ 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;
   }
@@ -452,7 +452,7 @@ 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 0000000000000..5e020aeb4dc0d
--- /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