[flang] Improve length information in character transformational (#65771)

Intrinsic resolution currently does not resolve constant length
information for character transformational (with "sameChar") where the
argument has constant length but is not a variable or a constant
expression.

It is not required to fold those expressions (only inquiry on constant
expression or variable with constant length is required to be a constant
expression).

But constant length information for character is valuable for lowering,
so I think this is a nice and easy to have.

Author: jeanPerier

flang/lib/Evaluate/intrinsics.cpp

@@ -2093,7 +2093,15 @@ std::optional<SpecificCall> IntrinsicInterface::Match(
       CHECK(sameArg);
       if (std::optional<DynamicType> aType{sameArg->GetType()}) {
         if (result.categorySet.test(aType->category())) {
-          resultType = *aType;
+          if (const auto *sameChar{UnwrapExpr<Expr<SomeCharacter>>(*sameArg)}) {
+            if (auto len{ToInt64(Fold(context, sameChar->LEN()))}) {
+              resultType = DynamicType{aType->kind(), *len};
+            } else {
+              resultType = *aType;
+            }
+          } else {
+            resultType = *aType;
+          }
         } else {
           resultType = DynamicType{*category, aType->kind()};
         }

flang/test/Evaluate/fold-substr.f90

@@ -20,3 +20,11 @@ module m
   logical, parameter :: test_07d = ca(1)(5:) == ""
   logical, parameter :: test_07e = ca(1)(:) == "abcd"
 end module
+
+subroutine foo(x)
+  character(4) :: x(:, :)
+  logical, parameter :: test_01 = len(transpose(x(:, :)(:))) == 4
+  logical, parameter :: test_02 = len(transpose(x(:, :)(1:2))) == 2
+  logical, parameter :: test_03 = len(maxval(x(:, :)(:))) == 4
+  logical, parameter :: test_04 = len(maxval(x(:, :)(1:2))) == 2
+end subroutine