[flang] Adjust semantics of the char length of an array constructor (#97337)

An implied DO loop with no trips in an array constructor does not have a
well-defined character length unless its data items have a length that
is constant expression. That works, but the implementation is too
broadly applied. An array constructor with an explicit type-spec always
has a well-defined length.

Author: klausler

flang/lib/Semantics/expression.cpp

@@ -1584,7 +1584,8 @@ class ArrayConstructorContext {
   std::optional<Expr<SubscriptInteger>> LengthIfGood() const {
     if (type_) {
       auto len{type_->LEN()};
-      if (len && IsConstantExpr(*len) && !ContainsAnyImpliedDoIndex(*len)) {
+      if (explicitType_ ||
+          (len && IsConstantExpr(*len) && !ContainsAnyImpliedDoIndex(*len))) {
         return len;
       }
     }

flang/test/Lower/HLFIR/array-ctor-character.f90

@@ -93,10 +93,11 @@ subroutine test_set_length_sanitize(i, c1)
   call takes_char([character(len=i):: c1])
 end subroutine
 ! CHECK-LABEL:   func.func @_QPtest_set_length_sanitize(
-! CHECK:   %[[VAL_6:.*]]:2 = hlfir.declare {{.*}}Ec1
-! CHECK:   %[[VAL_9:.*]]:2 = hlfir.declare {{.*}}Ei
-! CHECK:   %[[VAL_25:.*]] = fir.load %[[VAL_9]]#0 : !fir.ref<i64>
+! CHECK:   %[[VAL_2:.*]]:2 = hlfir.declare {{.*}}Ec1
+! CHECK:   %[[VAL_3:.*]]:2 = hlfir.declare %arg0
+! CHECK:   %[[VAL_4:.*]] = fir.load %[[VAL_3]]#0 : !fir.ref<i64>
+! CHECK:   %[[VAL_25:.*]] = fir.load %[[VAL_3]]#0 : !fir.ref<i64>
 ! CHECK:   %[[VAL_26:.*]] = arith.constant 0 : i64
 ! CHECK:   %[[VAL_27:.*]] = arith.cmpi sgt, %[[VAL_25]], %[[VAL_26]] : i64
 ! CHECK:   %[[VAL_28:.*]] = arith.select %[[VAL_27]], %[[VAL_25]], %[[VAL_26]] : i64
-! CHECK:   %[[VAL_29:.*]] = hlfir.set_length %[[VAL_6]]#0 len %[[VAL_28]] : (!fir.boxchar<1>, i64) -> !hlfir.expr<!fir.char<1,?>>
+! CHECK:   %[[VAL_29:.*]] = hlfir.set_length %[[VAL_2]]#0 len %[[VAL_28]] : (!fir.boxchar<1>, i64) -> !hlfir.expr<!fir.char<1,?>>

flang/test/Semantics/array-constr-len.f90

@@ -10,6 +10,5 @@ subroutine subr(s,n)
   print *, [(s(1:j),j=1,0)]
   print *, [(s(1:1),j=1,0)] ! ok
   print *, [character(2)::(s(1:n),j=1,0)] ! ok
-  !ERROR: Array constructor implied DO loop has no iterations and indeterminate character length
   print *, [character(n)::(s(1:n),j=1,0)]
 end