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

… array constructors

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.

Differential Revision: https://reviews.llvm.org/D156753

flang/lib/Semantics/expression.cpp

@@ -1510,8 +1510,8 @@ class ArrayConstructorContext {
     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 &);

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

@@ -10,5 +10,6 @@ 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
-  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