From ec6b2c63d93d8f8edeafcc7330d0b2349463d73d Mon Sep 17 00:00:00 2001
From: Peter Klausler <pklausler@nvidia.com>
Date: Mon, 27 Jan 2025 08:56:58 -0800
Subject: [PATCH] [flang] Fold character array constructor with unknown length
 (#123983)

When a character array constructor does not have an explicit type with a
constant length, the compiler can still fold it if all of its elements
are constants. These array constructors will have been wrapped up in the
internal %SET_LENGTH operation, which will determine the final length of
the folded value, so use the maximum length of the constant elements as
the length of the folded array constructor.

Fixes https://github.com/llvm/llvm-project/issues/123766.
---
 flang/lib/Evaluate/fold-implementation.h | 22 ++++++++++++++++------
 flang/test/Evaluate/bug123766.f90        |  5 +++++
 2 files changed, 21 insertions(+), 6 deletions(-)
 create mode 100644 flang/test/Evaluate/bug123766.f90

diff --git a/flang/lib/Evaluate/fold-implementation.h b/flang/lib/Evaluate/fold-implementation.h
index 4dcc737688ca..b0f39e63d094 100644
--- a/flang/lib/Evaluate/fold-implementation.h
+++ b/flang/lib/Evaluate/fold-implementation.h
@@ -1267,6 +1267,12 @@ public:
   explicit ArrayConstructorFolder(FoldingContext &c) : context_{c} {}
 
   Expr<T> FoldArray(ArrayConstructor<T> &&array) {
+    if constexpr (T::category == TypeCategory::Character) {
+      if (const auto *len{array.LEN()}) {
+        charLength_ = ToInt64(Fold(context_, common::Clone(*len)));
+        knownCharLength_ = charLength_.has_value();
+      }
+    }
     // Calls FoldArray(const ArrayConstructorValues<T> &) below
     if (FoldArray(array)) {
       auto n{static_cast<ConstantSubscript>(elements_.size())};
@@ -1274,12 +1280,9 @@ public:
         return Expr<T>{Constant<T>{array.GetType().GetDerivedTypeSpec(),
             std::move(elements_), ConstantSubscripts{n}}};
       } else if constexpr (T::category == TypeCategory::Character) {
-        if (const auto *len{array.LEN()}) {
-          auto length{Fold(context_, common::Clone(*len))};
-          if (std::optional<ConstantSubscript> lengthValue{ToInt64(length)}) {
-            return Expr<T>{Constant<T>{
-                *lengthValue, std::move(elements_), ConstantSubscripts{n}}};
-          }
+        if (charLength_) {
+          return Expr<T>{Constant<T>{
+              *charLength_, std::move(elements_), ConstantSubscripts{n}}};
         }
       } else {
         return Expr<T>{
@@ -1300,6 +1303,11 @@ private:
           elements_.emplace_back(c->At(index));
         } while (c->IncrementSubscripts(index));
       }
+      if constexpr (T::category == TypeCategory::Character) {
+        if (!knownCharLength_) {
+          charLength_ = std::max(c->LEN(), charLength_.value_or(-1));
+        }
+      }
       return true;
     } else {
       return false;
@@ -1349,6 +1357,8 @@ private:
 
   FoldingContext &context_;
   std::vector<Scalar<T>> elements_;
+  std::optional<ConstantSubscript> charLength_;
+  bool knownCharLength_{false};
 };
 
 template <typename T>
diff --git a/flang/test/Evaluate/bug123766.f90 b/flang/test/Evaluate/bug123766.f90
new file mode 100644
index 000000000000..b58989e6c26d
--- /dev/null
+++ b/flang/test/Evaluate/bug123766.f90
@@ -0,0 +1,5 @@
+! RUN: %python %S/test_folding.py %s %flang_fc1
+character(10), parameter :: a = '0123456789'
+character(3), parameter :: arr(3) = [(a(1:i), i=1,3)]
+logical, parameter :: test1 = all(arr == ["0", "01", "012"])
+end