Descriptor extents must be nonnegative (#1025)

Author: vdonaldson

flang/lib/Lower/BoxAnalyzer.h

@@ -393,7 +393,8 @@ class BoxAnalyzer : public fir::details::matcher<BoxAnalyzer> {
             lbounds.push_back(*lb); // origin for this dim
             if (auto high = subs.ubound().GetExplicit()) {
               if (auto ub = Fortran::evaluate::ToInt64(*high)) {
-                shapes.push_back(*ub - *lb + 1); // extent for this dim
+                auto extent = *ub - *lb + 1;
+                shapes.push_back(extent < 0 ? 0 : extent);
                 continue;
               }
             } else if (subs.ubound().isAssumed()) {

flang/test/Lower/array-constructor-1.f90

@@ -5,8 +5,7 @@ module units
 contains
   ! CHECK-LABEL: _QMunitsPis_preconnected_unit
   logical function is_preconnected_unit(u)
-  ! CHECK: [[units_ssa:%[0-9]+]] = fir.address_of([[units_value:.*]]) :
-  ! !fir.ref<!fir.array<3xi32>>
+  ! CHECK: [[units_ssa:%[0-9]+]] = fir.address_of([[units_value:.*]]) : !fir.ref<!fir.array<3xi32>>
     integer :: u
     integer :: i
     is_preconnected_unit = .true.

flang/test/Lower/zero-size.f90

@@ -0,0 +1,20 @@
+! RUN: bbc -o - %s | FileCheck %s
+
+! CHECK-LABEL: _QPzero
+subroutine zero(aa)
+  real, dimension(:) :: aa
+  print*, size(aa)
+end
+
+! CHECK-LABEL: _QQmain
+program prog
+  real nada(2:-1)
+  interface
+    subroutine zero(aa)
+      real, dimension(:) :: aa
+    end
+  end interface
+  ! CHECK: %[[shape:[0-9]*]] = fir.shape_shift %c2, %c0 : (index, index) -> !fir.shapeshift<1>
+  ! CHECK: %2 = fir.embox %0(%[[shape]]) : (!fir.ref<!fir.array<0xf32>>, !fir.shapeshift<1>) -> !fir.box<!fir.array<0xf32>>
+  call zero(nada)
+end