[flang] Fix bug for forward referenced type

[PeteSteinfeld]

Summary

A type name in an IMPLICIT declaration that was later used in a PARAMETER
statement caused problems because the default symbol scope had not yet been
initialized. I avoided dereferencing in the situation where the default scope
was uninitialized and added a test that triggers the problem.

[klausler]

flang/lib/Semantics/symbol.cpp

544–545

if (const Scope *localScope{scope ? scope : scope_}) { return dtDetails->GetParentComponent(localScope); }

[PeteSteinfeld]

flang/lib/Semantics/symbol.cpp

544–545

Thanks, Peter. I'll redo this section of code.

[PeteSteinfeld]

Reworked some code as recommended by Peter Klausler.

[tskeith]

flang/test/Semantics/bad-forward-type.f90

76

These messages both seem to be wrong: type t2 is defined later. And they are not associated with the line that has the error.

Sorry about the late comment.

[PeteSteinfeld]

Reopening to fix Tim's comments.

flang/test/Semantics/bad-forward-type.f90

76

I agree. I was focused on fixing the crash, but I should clean this up while I'm at it. I'll reopen the revision. Stay tuned for more changes.

[PeteSteinfeld]

I added code to detect the actual error and associate it with the correct
source line. Note that the git commit message is changed with a more
detailed explanation.

[tskeith]

Looks good.

flang/lib/Semantics/symbol.cpp

if (const Symbol * parentComponent{GetParentComponent(scope)}) {
     return &object.type()->derivedTypeSpec();
   } else {
     return nullptr;
   }
 }
 
 const Symbol *Symbol::GetParentComponent(const Scope *scope) const {
   if (const auto *dtDetails{detailsIf<DerivedTypeDetails>()}) {
-    if (!scope) {
-      scope = scope_;
+    if (const Scope * localScope{scope ? scope : scope_}) {
+      return dtDetails->GetParentComponent(DEREF(localScope));
     }
-    return dtDetails->GetParentComponent(DEREF(scope));
-  } else {
-    return nullptr;
   }
+  return nullptr;
 }
 
 void DerivedTypeDetails::add_component(const Symbol &symbol) {
   if (symbol.test(Symbol::Flag::ParentComp)) {
     CHECK(componentNames_.empty());
   }
   componentNames_.push_back(symbol.name());
 }

flang/test/Semantics/bad-forward-type.f90

subroutine s6(x)
   type(undefpdt(1)) :: x
 end subroutine
 
 subroutine s7(x)
   !ERROR: Derived type 'undef' not found
   type, extends(undef) :: t
   end type
 end subroutine
+
+subroutine s8
+  !ERROR: Derived type 't2' was used but never defined
+  !ERROR: The derived type 't2' was forward-referenced but not defined
+  implicit type(t2)(x)
+  parameter(y=t2(12.3))
+  type t2
+    real :: c
+  end type
+end subroutine