[flang] Allow explicit '+' in NAMELIST input subscripts

[klausler]

Summary

Array subscripts and substring limits in NAMELIST input are
allowed to bear an explicit plus sign.

[mleair]

LGTM

flang/runtime/namelist.cpp

static bool GetLowerCaseName(
   }
   return false;
 }
 
 static std::optional<SubscriptValue> GetSubscriptValue(IoStatementState &io) {
   std::optional<SubscriptValue> value;
   std::optional<char32_t> ch{io.GetCurrentChar()};
   bool negate{ch && *ch == '-'};
-  if (negate) {
+  if ((ch && *ch == '+') || negate) {
     io.HandleRelativePosition(1);
     ch = io.GetCurrentChar();
   }
   bool overflow{false};
   while (ch && *ch >= '0' && *ch <= '9') {
     SubscriptValue was{value.value_or(0)};
     overflow |= was >= std::numeric_limits<SubscriptValue>::max() / 10;
     value = 10 * was + *ch - '0';

flang/unittests/Runtime/Namelist.cpp

TEST(NamelistTests, Subscripts) {
   // INTEGER :: A(-1:0, -1:1)
   OwningPtr<Descriptor> aDesc{
       MakeArray<TypeCategory::Integer, static_cast<int>(sizeof(int))>(
           std::vector<int>{2, 3}, std::vector<int>(6, 0))};
   aDesc->GetDimension(0).SetBounds(-1, 0);
   aDesc->GetDimension(1).SetBounds(-1, 1);
   const NamelistGroup::Item items[]{{"a", *aDesc}};
   const NamelistGroup group{"justa", 1, items};
-  static char t1[]{"&justa A(0,1:-1:-2)=1 2/"};
+  static char t1[]{"&justa A(0,+1:-1:-2)=1 2/"};
   StaticDescriptor<1, true> statDesc;
   Descriptor &internalDesc{statDesc.descriptor()};
   internalDesc.Establish(TypeCode{CFI_type_char},
       /*elementBytes=*/std::strlen(t1), t1, 0, nullptr, CFI_attribute_pointer);
   auto inCookie{IONAME(BeginInternalArrayListInput)(
       internalDesc, nullptr, 0, __FILE__, __LINE__)};
   ASSERT_TRUE(IONAME(InputNamelist)(inCookie, group));
   auto inStatus{IONAME(EndIoStatement)(inCookie)};

 }
 
 TEST(NamelistTypes, ArraySubstring) {
   OwningPtr<Descriptor> scDesc{
       MakeArray<TypeCategory::Character, 1>(std::vector<int>{2},
           std::vector<std::string>{"abcdefgh", "ijklmnop"}, 8)};
   const NamelistGroup::Item items[]{{"a", *scDesc}};
   const NamelistGroup group{"justa", 1, items};
-  static char t1[]{"&justa A(:)(2:5)='BCDE' 'JKLM'/"};
+  static char t1[]{"&justa A(:)(2:+5)='BCDE' 'JKLM'/"};
   StaticDescriptor<1, true> statDesc;
   Descriptor &internalDesc{statDesc.descriptor()};
   internalDesc.Establish(TypeCode{CFI_type_char},
       /*elementBytes=*/std::strlen(t1), t1, 0, nullptr, CFI_attribute_pointer);
   auto inCookie{IONAME(BeginInternalArrayListInput)(
       internalDesc, nullptr, 0, __FILE__, __LINE__)};
   ASSERT_TRUE(IONAME(InputNamelist)(inCookie, group));
   ASSERT_EQ(IONAME(EndIoStatement)(inCookie), IostatOk)