[flang] Fix assert on character literal substrings as arguments

[PeteSteinfeld]

Summary

Character literal substrings used as arguments were causing asserts. This
happened when the code was trying to get the DynamicType of the substring. We
were only recording the DynamicType of the Designator on which the substring
was based. For character literal substrings, the Designator was a character
literal, and we weren't handling getting its type.

I fixed this by changing the GetType() method for DynamicType to check to
see if we were getting the type of a Substring and calculating the type of
the substring by getting the number of bytes in an element of the string.

I also changed the test resolve49.f90 with some tests, one of which causes
the original crash.

[klausler]

Thanks! Ship it.

flang/lib/Evaluate/variable.cpp

#endif
           },
       },
       u);
 }
 
 template <typename T>
 std::optional<DynamicType> Designator<T>::GetType() const {
   if constexpr (IsLengthlessIntrinsicType<Result>) {
-    return {Result::GetType()};
-  } else {
-    return DynamicType::From(GetLastSymbol());
+    return Result::GetType();
+  } else if (const Symbol * symbol{GetLastSymbol()}) {
+    return DynamicType::From(*symbol);
+  } else if constexpr (Result::category == TypeCategory::Character) {
+    if (const Substring * substring{std::get_if<Substring>(&u)}) {
+      const auto *parent{substring->GetParentIf<StaticDataObject::Pointer>()};
+      CHECK(parent);
+      return DynamicType{TypeCategory::Character, (*parent)->itemBytes()};
     }
   }
+  return std::nullopt;
+}
 
 static NamedEntity AsNamedEntity(const SymbolVector &x) {
   CHECK(!x.empty());
   NamedEntity result{x.front()};
   int j{0};
   for (const Symbol &symbol : x) {
     if (j++ != 0) {
       DataRef base{result.IsSymbol() ? DataRef{result.GetLastSymbol()}

flang/test/Semantics/resolve49.f90

 ! RUN: %S/test_errors.sh %s %t %f18
 ! Test section subscript
 program p1
   real :: a(10,10)
   real :: b(5,5)
   real :: c
   integer :: n
   n = 2
   b = a(1:10:n,1:n+3)
 end
 
 ! 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)
   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))
 end
 
 ! Test pointer assignment with bounds
 program p3
   integer, pointer :: a(:,:)
   integer, target :: b(2,2)
   integer :: n
   n = 2