[flang] Fix folding of SHAPE(SPREAD(source,dim,ncopies=-1)) (#141146)

The number of copies on the new dimension must be clamped via MAX(0,
ncopies) so that it is no less than zero.

Fixes llvm/llvm-project#141119.

Author: klausler

flang/lib/Evaluate/shape.cpp

@@ -1073,8 +1073,8 @@ auto GetShapeHelper::operator()(const ProcedureRef &call) const -> Result {
         }
       }
     } else if (intrinsic->name == "spread") {
-      // SHAPE(SPREAD(ARRAY,DIM,NCOPIES)) = SHAPE(ARRAY) with NCOPIES inserted
-      // at position DIM.
+      // SHAPE(SPREAD(ARRAY,DIM,NCOPIES)) = SHAPE(ARRAY) with MAX(0,NCOPIES)
+      // inserted at position DIM.
       if (call.arguments().size() == 3) {
         auto arrayShape{
             (*this)(UnwrapExpr<Expr<SomeType>>(call.arguments().at(0)))};
@@ -1086,7 +1086,8 @@ auto GetShapeHelper::operator()(const ProcedureRef &call) const -> Result {
             if (*dim >= 1 &&
                 static_cast<std::size_t>(*dim) <= arrayShape->size() + 1) {
               arrayShape->emplace(arrayShape->begin() + *dim - 1,
-                  ConvertToType<ExtentType>(common::Clone(*nCopies)));
+                  Extremum<SubscriptInteger>{Ordering::Greater, ExtentExpr{0},
+                      ConvertToType<ExtentType>(common::Clone(*nCopies))});
               return std::move(*arrayShape);
             }
           }

flang/test/Evaluate/fold-spread.f90

@@ -12,4 +12,5 @@ module m1
   logical, parameter :: test_log4 = all(any(spread([.false., .true.], 2, 2), dim=2) .eqv. [.false., .true.])
   logical, parameter :: test_m2toa3 = all(spread(reshape([(j,j=1,6)],[2,3]),1,4) == &
                                           reshape([((j,k=1,4),j=1,6)],[4,2,3]))
+  logical, parameter :: test_shape_neg = all(shape(spread(0,1,-1)) == [0])
 end module