[flang] Don't crash on some bogus expressions

[klausler]

Summary

Recover more gracefully from user errors in expressions.

[PeteSteinfeld]

All builds, tests, and looks good.

flang/lib/Semantics/expression.cpp

 // Wraps a data reference in a typed Designator<>, and a procedure
 // or procedure pointer reference in a ProcedureDesignator.
 MaybeExpr ExpressionAnalyzer::Designate(DataRef &&ref) {
-  const Symbol &symbol{ref.GetLastSymbol().GetUltimate()};
+  const Symbol &last{ref.GetLastSymbol()};
+  const Symbol &symbol{last.GetUltimate()};
   if (semantics::IsProcedure(symbol)) {
     if (auto *component{std::get_if<Component>(&ref.u)}) {
       return Expr<SomeType>{ProcedureDesignator{std::move(*component)}};

           symbol.name());
     }
     return std::nullopt;
+  } else if (MaybeExpr result{AsGenericExpr(std::move(ref))}) {
+    return result;
   } else {
-    return AsGenericExpr(std::move(ref));
+    if (!context_.HasError(last) && !context_.HasError(symbol)) {
+      AttachDeclaration(
+          Say("'%s' is not an object that can appear in an expression"_err_en_US,
+              last.name()),
+          symbol);
+      context_.SetError(last);
+    }
+    return std::nullopt;
   }
 }
 

flang/lib/Semantics/resolve-names.cpp

 }
 
 bool DeclarationVisitor::CheckUseError(const parser::Name &name) {
-  const auto *details{name.symbol->detailsIf<UseErrorDetails>()};
+  const auto *details{
+      name.symbol ? name.symbol->detailsIf<UseErrorDetails>() : nullptr};
   if (!details) {
     return false;
   }

     msg.Attach(location, "'%s' was use-associated from module '%s'"_en_US,
         name.source, module->GetName().value());
   }
+  context().SetError(*name.symbol);
   return true;
 }
 

         !prevRoot.has<EntityDetails>()) {
       Say2(name, "Index name '%s' conflicts with existing identifier"_err_en_US,
           *prev, "Previous declaration of '%s'"_en_US);
+      context().SetError(symbol);
       return;
     } else {
       if (const auto *type{prevRoot.GetType()}) {

flang/test/Semantics/select-rank.f90

     SELECT RANK(ptr=>x)
     RANK (3)
       PRINT *, "PRINT RANK 3"
+      !ERROR: 'ptr' is not an object that can appear in an expression
       j = INT(0, KIND=MERGE(KIND(0), -1, RANK(ptr) == 0))
     RANK (1)
       PRINT *, "PRINT RANK 1"
+      !ERROR: 'ptr' is not an object that can appear in an expression
       j = INT(0, KIND=MERGE(KIND(0), -1, RANK(ptr) == 1))
     END SELECT
    end subroutine