[flang][prescanner] fix invalid check

[akuhlens]

TokenSequence::pop_back() had a check assumed that tokens are never empty. Loosen this check since isn't true.

towards #146362

[llvmbot]

@llvm/pr-subscribers-flang-parser

Author: Andre Kuhlenschmidt (akuhlens)

Changes

TokenSequence::pop_back() had a check assumed that tokens are never empty. Loosen this check since isn't true.

---

Full diff: https://github.com/llvm/llvm-project/pull/146613.diff

2 Files Affected:

• (modified) flang/lib/Parser/token-sequence.cpp (+2-1)
• (added) flang/test/Parser/issue-146362.1.f90 (+5)

diff --git a/flang/lib/Parser/token-sequence.cpp b/flang/lib/Parser/token-sequence.cpp
index 40a074eaf0a47..9deb513e4f64f 100644
--- a/flang/lib/Parser/token-sequence.cpp
+++ b/flang/lib/Parser/token-sequence.cpp
@@ -30,7 +30,8 @@ void TokenSequence::clear() {
 
 void TokenSequence::pop_back() {
   CHECK(!start_.empty());
-  CHECK(nextStart_ > start_.back());
+  // If the last token is empty then `nextStart_ == start_.back()`.
+  CHECK(nextStart_ >= start_.back());
   std::size_t bytes{nextStart_ - start_.back()};
   nextStart_ = start_.back();
   start_.pop_back();
diff --git a/flang/test/Parser/issue-146362.1.f90 b/flang/test/Parser/issue-146362.1.f90
new file mode 100644
index 0000000000000..a37fd4b182a7f
--- /dev/null
+++ b/flang/test/Parser/issue-146362.1.f90
@@ -0,0 +1,5 @@
+! RUN: %flang_fc1 -fsyntax-only -cpp %s 2>&1
+#define UNITY(k) 1_ ## k
+PROGRAM REPRODUCER
+WRITE(*,*) UNITY(4)
+END PROGRAM REPRODUCER
\ No newline at end of file