[flang] Don't fold SIZE()/SHAPE() into expression referencing optional dummy arguments

[klausler]

Summary

When computing the shape of an expression at compilation time as part of
folding an intrinsic function like SIZE(), don't create an expression that
increases a dependence on the presence of an optional dummy argument.

[vzakhari]

Thank you!

flang/lib/Evaluate/shape.cpp

 auto GetShapeHelper::operator()(const Substring &substring) const -> Result {
   return (*this)(substring.parent());
 }
 
 auto GetShapeHelper::operator()(const ProcedureRef &call) const -> Result {
   if (call.Rank() == 0) {
     return ScalarShape();
   } else if (call.IsElemental()) {
+    // Use the shape of an actual array argument associated with a
+    // non-OPTIONAL dummy object argument.
+    if (context_) {
+      if (auto chars{characteristics::Procedure::FromActuals(
+              call.proc(), call.arguments(), *context_)}) {
+        std::size_t j{0};
         for (const auto &arg : call.arguments()) {
-      if (arg && arg->Rank() > 0) {
+          if (arg && arg->Rank() > 0 && j < chars->dummyArguments.size() &&
+              !chars->dummyArguments[j].IsOptional()) {
             return (*this)(*arg);
           }
+          ++j;
+        }
+      }
     }
     return ScalarShape();
   } else if (const Symbol * symbol{call.proc().GetSymbol()}) {
     return (*this)(*symbol);
   } else if (const auto *intrinsic{call.proc().GetSpecificIntrinsic()}) {
     if (intrinsic->name == "shape" || intrinsic->name == "lbound" ||
         intrinsic->name == "ubound") {
       // For LBOUND/UBOUND, these are the array-valued cases (no DIM=)

flang/test/Evaluate/elem-shape.f90

+! RUN: %flang_fc1 -fdebug-unparse %s 2>&1 | FileCheck %s
+! Ensure that optional arguments aren't used to fold SIZE() or SHAPE()
+module m
+ contains
+  subroutine sub(x,y)
+    real :: x(:), y(:)
+    optional x
+    !CHECK: PRINT *, int(size(y,dim=1,kind=8),kind=4)
+    print *, size(f(x,y))
+  end
+  elemental function f(x,y)
+    real, intent(in) :: x, y
+    optional x
+    f = y
+  end
+end