[flang] Fixed crash on forward referenced `len` parameter

[PeteSteinfeld]

Summary

It turned out that the root cause of the crash was that the length of the type
of the symbol being passed to SymbolLEN() had a bad expression. So I fixed
it by checking isExplicit() on the ParamValue before calling
GetExplicit(). The case where isExplicit() returned true and
GetExplicit() failed to get an expression indicates a prior error, so I just
returned std::nullopt from SymbolLEN() and avoided the call to the
constructor of DescriptorInquiry.

[klausler]

Please leave the batteries installed in this particular smoke detector and see whether you can stop the annoying noise by extinguishing the fire instead. We don't want to be able to construct erroneous descriptor inquiries into things that aren't descriptors!

[PeteSteinfeld]

1. Updating D80593: [flang] Fixed crash on forward referenced len parameter #
2. Enter a brief description of the changes included in this update.
3. The first line is used as subject, next lines as comment. #
4. If you intended to create a new revision, use:
5. $ arc diff --create

It turned out that the root cause of the crash was that the length of the type
of the symbol being passed to SymbolLEN() had a bad expression. So I fixed
it by checking isExplicit() on the ParamValue before calling
GetExplicit(). The case where isExplicit() returned true and
GetExplicit() failed to get an expression indicates a prior error, so I just
returned std::nullopt from SymbolLEN() and avoided the call to the
constructor of DescriptorInquiry.

I restored the calls to variable.cpp to CHECK(IsDescriptor(last)).

[PeteSteinfeld]

@klausler, you requested changes. Does the current state of this revision look good to you?

[klausler]

If the CHECK() is back, I'm happy.

[sscalpone]

@PeteSteinfeld Please update the summary.

[PeteSteinfeld]

In D80593#2072464, @sscalpone wrote:

@PeteSteinfeld Please update the summary.

Done.

flang/lib/Evaluate/variable.cpp

DescriptorInquiry::DescriptorInquiry(NamedEntity &&base, Field field, int dim)
   CHECK((field == Field::Len && dim == 0) ||
       (field != Field::Len && dim >= 0 && dim < last.Rank()));
 }
 
 // LEN()
 static std::optional<Expr<SubscriptInteger>> SymbolLEN(const Symbol &sym) {
   if (auto dyType{DynamicType::From(sym)}) {
     if (const semantics::ParamValue * len{dyType->charLength()}) {
+      if (len->isExplicit()) {
         if (auto intExpr{len->GetExplicit()}) {
           return ConvertToType<SubscriptInteger>(*std::move(intExpr));
         } else {
+          // There was an error constructing this symbol's type.  It should
+          // have a length expression, but we couldn't retrieve it
+          return std::nullopt;
+        }
+      } else {
         return Expr<SubscriptInteger>{
             DescriptorInquiry{NamedEntity{sym}, DescriptorInquiry::Field::Len}};
       }
     }
   }
   return std::nullopt;
 }
 

flang/test/Semantics/resolve91.f90

 module m4
   real, dimension(:), pointer :: localArray => null()
   type :: t2
     real, dimension(:), pointer :: realArray => null()
     !ERROR: Component 'realarray' is already declared in this derived type
     real, dimension(:), pointer :: realArray => localArray
   end type
 end module m4
+
+module m5
+  !ERROR: Actual argument for 'string=' has bad type 'REAL(4)'
+  character(len=len(a)) :: b
+  !ERROR: The type of 'a' has already been implicitly declared
+  character(len=len(b)) :: a
+end module m5