[flang] More compile-time error checking for null implied DO loops in array constructors

[klausler]

Summary

An implied DO loop in an array constructor may not have a type (explicit
or otherwise) with a character length that depends on a value of an
implied DO index or a non-constant expression if the implied DO loop
executes no iterations. When the iteration count can be known to be
zero at compilation time, catch the case of a non-constant length
expression correctly.

[vzakhari]

Thank you!

flang/lib/Semantics/expression.cpp

if (type_) {
       auto len{type_->LEN()};
       if (len && IsConstantExpr(*len) && !ContainsAnyImpliedDoIndex(*len)) {
         return len;
       }
     }
     return std::nullopt;
   }
   bool NeedLength() const {
-    return !explicitType_ && type_ &&
-        type_->category() == TypeCategory::Character && !LengthIfGood();
+    return type_ && type_->category() == TypeCategory::Character &&
+        !LengthIfGood();
   }
   void Push(MaybeExpr &&);
   void Add(const parser::AcValue::Triplet &);
   void Add(const parser::Expr &);
   void Add(const parser::AcImpliedDo &);
   void UnrollConstantImpliedDo(const parser::AcImpliedDo &,
       parser::CharBlock name, std::int64_t lower, std::int64_t upper,
       std::int64_t stride);

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

 ! RUN: %python %S/test_errors.py %s %flang_fc1
 ! Confirm enforcement of F'2023 7.8 p5
 subroutine subr(s,n)
   character*(*) s
   !ERROR: Array constructor implied DO loop has no iterations and indeterminate character length
   print *, [(s(1:n),j=1,0)]
   !ERROR: Array constructor implied DO loop has no iterations and indeterminate character length
   print *, [(s(1:n),j=0,1,-1)]
   !ERROR: Array constructor implied DO loop has no iterations and indeterminate character length
   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
-  print *, [character(n)::(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