[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

@@ -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>

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