[flang] Fix bad dereference of NULLIFY pointer object

[PeteSteinfeld]

Summary

When we have a subprogram that has been determined to contain errors, we do not
perform name resolution on its execution part. In this case, if the subprogram
contains a NULLIFY statement, the parser::Name of a pointer object in a NULLIFY
statement will not have had name resolution performed on it. Thus, its symbol
will not have been set. Later, however, we do semantic checking on the NULLIFY
statement. The code that did this assumed that the parser::Name of the
pointer object was non-null.

I fixed this by just removing the null pointer check for the "symbol" member of
the "parser::Name" of the pointer object when doing semantic checking for
NULLIFY statements. I also added a test that will make the compiler crash
without this change.

flang/lib/Semantics/check-nullify.cpp

void NullifyChecker::Leave(const parser::NullifyStmt &nullifyStmt) {
   const Scope &scope{context_.FindScope(*context_.location())};
   const Scope *pure{FindPureProcedureContaining(scope)};
   parser::ContextualMessages messages{
       *context_.location(), &context_.messages()};
   for (const parser::PointerObject &pointerObject : nullifyStmt.v) {
     std::visit(
         common::visitors{
             [&](const parser::Name &name) {
-              const Symbol &symbol{DEREF(name.symbol)};
-              if (context_.HasError(&symbol)) {
+              const Symbol *symbol{name.symbol};
+              if (context_.HasError(symbol)) {
                 // already reported an error
-              } else if (!IsVariableName(symbol) && !IsProcName(symbol)) {
+              } else if (!IsVariableName(*symbol) && !IsProcName(*symbol)) {
                 messages.Say(name.source,
                     "name in NULLIFY statement must be a variable or procedure pointer name"_err_en_US);
-              } else if (!IsPointer(symbol)) { // C951
+              } else if (!IsPointer(*symbol)) { // C951
                 messages.Say(name.source,
                     "name in NULLIFY statement must have the POINTER attribute"_err_en_US);
               } else if (pure) {
-                CheckDefinabilityInPureScope(messages, symbol, scope, *pure);
+                CheckDefinabilityInPureScope(messages, *symbol, scope, *pure);
               }
             },
             [&](const parser::StructureComponent &structureComponent) {
               evaluate::ExpressionAnalyzer analyzer{context_};
               if (MaybeExpr checked{analyzer.Analyze(structureComponent)}) {
                 if (!IsPointer(*structureComponent.component.symbol)) { // C951
                   messages.Say(structureComponent.component.source,
                       "component in NULLIFY statement must have the POINTER attribute"_err_en_US);

flang/test/Semantics/nullify02.f90

 
 !ERROR: name in NULLIFY statement must have the POINTER attribute
 Nullify(prp)
 
 !ERROR: name in NULLIFY statement must be a variable or procedure pointer name
 Nullify(maxvalue)
 
 End Program
+
+! Make sure that the compiler doesn't crash when NULLIFY is used in a context
+! that has reported errors
+module badNullify
+  interface
+    module function ptrFun()
+      integer, pointer :: ptrFun
+    end function
+  end interface
+contains
+  !ERROR: 'ptrfun' was not declared a separate module procedure
+  module function ptrFun()
+    integer, pointer :: ptrFun
+    real :: realVar
+    nullify(ptrFun)
+    nullify(realVar)
+  end function
+end module