[flang-commits] [flang] [flang] Fix variable-length character function as actual argument (PR #215719)

[via flang-commits]

https://github.com/ejose02 created https://github.com/llvm/llvm-project/pull/215719

Flang wrongly rejects a fixed-length character function passed to a dummy procedure whose result length depends on dummy arguments, even when the lengths match at the call site.

In FunctionResult::IsCompatibleWith, use IsTkLenCompatibleWith for character result length checks instead of treating all constant vs non-constant length pairs as incompatible. Only reject when both lengths are known constants and differ.

Fixes #192899

>From 02f9ff6e99965137b69befb44e20f13a140fdebe Mon Sep 17 00:00:00 2001
From: ejose <ejose at amd.com>
Date: Wed, 12 Aug 2026 04:20:54 +0000
Subject: [PATCH] [flang] Fix variable-length character function as actual
 argument

When checking whether a character function result is compatible with a
dummy procedure interface, use IsTkLenCompatibleWith instead of rejecting
all constant vs non-constant length pairs. This allows passing a
fixed-length character function to a procedure dummy whose result length
depends on dummy arguments when the lengths match at the call site.

Add call49.f90 regression test.

Co-authored-by: Cursor <cursoragent at cursor.com>
---
 flang/lib/Evaluate/characteristics.cpp | 11 ++++-------
 flang/test/Semantics/call49.f90        | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+), 7 deletions(-)
 create mode 100644 flang/test/Semantics/call49.f90

diff --git a/flang/lib/Evaluate/characteristics.cpp b/flang/lib/Evaluate/characteristics.cpp
index 4b05a25fd8f58..06e8de73b890a 100644
--- a/flang/lib/Evaluate/characteristics.cpp
+++ b/flang/lib/Evaluate/characteristics.cpp
@@ -1230,15 +1230,12 @@ bool FunctionResult::IsCompatibleWith(
             if (IsAssumedLengthCharacter() ||
                 actual.IsAssumedLengthCharacter()) {
               return true;
-            } else {
+            }
+            if (ifaceTypeShape->type().IsTkLenCompatibleWith(
+                    actualTypeShape->type())) {
               auto len{ToInt64(ifaceTypeShape->LEN())};
               auto actualLen{ToInt64(actualTypeShape->LEN())};
-              if (len.has_value() != actualLen.has_value()) {
-                if (whyNot) {
-                  *whyNot = "constant-length vs non-constant-length character "
-                            "results";
-                }
-              } else if (len && *len != *actualLen) {
+              if (len && actualLen && *len != *actualLen) {
                 if (whyNot) {
                   *whyNot = "character results with distinct lengths";
                 }
diff --git a/flang/test/Semantics/call49.f90 b/flang/test/Semantics/call49.f90
new file mode 100644
index 0000000000000..452e620962833
--- /dev/null
+++ b/flang/test/Semantics/call49.f90
@@ -0,0 +1,20 @@
+! RUN: %flang_fc1 -fsyntax-only %s
+! dummy procedure length may depend on dummy arguments
+! actual with fixed length is compatible when lengths match at call
+program test
+  character(4), external :: ext
+  call s(ext, ext, 2)
+contains
+  subroutine s(fun, fun_alt, n)
+    integer :: n
+    character(2 * n), external :: fun
+    character(n * (n + 1) - n**2 + n), external :: fun_alt
+    print *, fun()
+    print *, fun_alt()
+  end subroutine
+end program
+
+function ext()
+  character(4) :: ext
+  ext = 'okko'
+end function