From e2733c82bd5cacfdcb97dfea44f42b70e8b0182c Mon Sep 17 00:00:00 2001
From: Peter Klausler <pklausler@nvidia.com>
Date: Mon, 10 Mar 2025 13:16:09 -0700
Subject: [PATCH] [flang] Accept useless label on top-level FUNCTION (#129603)

The look-ahead parser for function program units didn't allow for a
useless label on the statement.

Fixes https://github.com/llvm/llvm-project/issues/129456.
---
 flang/lib/Parser/program-parsers.cpp | 2 +-
 flang/test/Parser/func-label.f       | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)
 create mode 100644 flang/test/Parser/func-label.f

diff --git a/flang/lib/Parser/program-parsers.cpp b/flang/lib/Parser/program-parsers.cpp
index e365cd24a6ae..d6e7f1129718 100644
--- a/flang/lib/Parser/program-parsers.cpp
+++ b/flang/lib/Parser/program-parsers.cpp
@@ -64,7 +64,7 @@ static constexpr auto programUnit{
     construct<ProgramUnit>(indirect(subroutineSubprogram)) ||
     construct<ProgramUnit>(indirect(Parser<Submodule>{})) ||
     construct<ProgramUnit>(indirect(Parser<BlockData>{})) ||
-    lookAhead(validFunctionStmt) >>
+    lookAhead(maybe(label) >> validFunctionStmt) >>
         construct<ProgramUnit>(indirect(functionSubprogram)) ||
     construct<ProgramUnit>(indirect(Parser<MainProgram>{}))};
 static constexpr auto normalProgramUnit{StartNewSubprogram{} >> programUnit /
diff --git a/flang/test/Parser/func-label.f b/flang/test/Parser/func-label.f
new file mode 100644
index 000000000000..d78545177f64
--- /dev/null
+++ b/flang/test/Parser/func-label.f
@@ -0,0 +1,7 @@
+! RUN: %flang_fc1 -fsyntax-only -pedantic %s 2>&1 | FileCheck --allow-empty %s
+!CHECK-NOT: error:
+ 1    function fun()
+ 2    end function
+ 3    write(6,*) "pass"
+ 4    end
+