[flang] Substrings with lower bound greater than upper bound

[PeteSteinfeld]

Summary

According to section 9.4.1, paragraph 3,
If the starting point is greater than the ending point, the substring has
length zero

But the compilers code for substring processing was failing a call to CHECK()
in this case. I fixed this by just setting the number of items in the
resulting string to 0 for this situation.

flang/lib/Evaluate/variable.cpp

if (*ubi < 1 || (lbi && *ubi < *lbi)) {
       upper_ = AsExpr(Constant<SubscriptInteger>{*ubi});
     } else if (length && *ubi > *length) {
       context.messages().Say("Upper bound (%jd) on substring is greater "
                              "than character length (%jd)"_en_US,
           *ubi, *length);
       *ubi = *length;
     }
     if (lbi && literal) {
-      CHECK(*ubi >= *lbi);
       auto newStaticData{StaticDataObject::Create()};
-      auto items{*ubi - *lbi + 1};
+      auto items{0}; // If the lower bound is greater, the length is 0
+      if (*ubi >= *lbi) {
+        items = *ubi - *lbi + 1;
+      }
       auto width{(*literal)->itemBytes()};
       auto bytes{items * width};
       auto startByte{(*lbi - 1) * width};
       const auto *from{&(*literal)->data()[0] + startByte};
       for (auto j{0}; j < bytes; ++j) {
         newStaticData->data().push_back(from[j]);
       }
       parent_ = newStaticData;

flang/test/Semantics/resolve49.f90

 ! Test substring
 program p2
   type t1(n1,n2)
     integer,kind :: n1,n2
     integer :: c2(iachar('ABCDEFGHIJ'(n1:n1)))
   end type
   character :: a(10)
   character :: b(5)
+  character :: c(0)
   integer :: n
   n = 3
   b = a(n:7)
   b = a(n+3:)
   b = a(:n+2)
   a(n:7) = b
   a(n+3:) = b
   a(:n+2) = b
   n = iachar(1_'ABCDEFGHIJ'(1:1))
+  c = 'ABCDEFGHIJ'(1:0)
 end
 
 ! Test pointer assignment with bounds
 program p3
   integer, pointer :: a(:,:)
   integer, target :: b(2,2)
   integer :: n
   n = 2