[flang] Fix crash in CO_REDUCE semantics

[klausler]

A std::optional<> value was being accessed without first ensuring its presence.

[llvmbot]

@llvm/pr-subscribers-flang-semantics

Author: Peter Klausler (klausler)

Changes

A std::optional<> value was being accessed without first ensuring its presence.

---

Full diff: https://github.com/llvm/llvm-project/pull/131211.diff

2 Files Affected:

• (modified) flang/lib/Semantics/check-call.cpp (+4-2)
• (added) flang/test/Semantics/bug396.f90 (+6)

diff --git a/flang/lib/Semantics/check-call.cpp b/flang/lib/Semantics/check-call.cpp
index 205b4a780258c..2991a27612f32 100644
--- a/flang/lib/Semantics/check-call.cpp
+++ b/flang/lib/Semantics/check-call.cpp
@@ -1690,8 +1690,10 @@ static void CheckCoReduce(
           characteristics::FunctionResult::Attr::Allocatable,
           characteristics::FunctionResult::Attr::Pointer,
       };
-  const auto *result{
-      procChars ? procChars->functionResult->GetTypeAndShape() : nullptr};
+  const characteristics::TypeAndShape *result{
+      procChars && procChars->functionResult
+          ? procChars->functionResult->GetTypeAndShape()
+          : nullptr};
   if (!procChars || !procChars->IsPure() ||
       procChars->dummyArguments.size() != 2 || !procChars->functionResult) {
     messages.Say(
diff --git a/flang/test/Semantics/bug396.f90 b/flang/test/Semantics/bug396.f90
new file mode 100644
index 0000000000000..6567f6933c71f
--- /dev/null
+++ b/flang/test/Semantics/bug396.f90
@@ -0,0 +1,6 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1
+external ext
+integer caf[*]
+!ERROR: OPERATION= argument of CO_REDUCE() must be a pure function of two data arguments
+call co_reduce(caf, ext)
+end