[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 ↗

(On Diff #291278)

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

[PeteSteinfeld]

flang/lib/Semantics/symbol.cpp

544 ↗

(On Diff #291278)

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/expression.cpp

if (auto *proc{std::get_if<ProcedureDesignator>(&callee->u)}) {
           call.source, std::move(*proc), std::move(callee->arguments));
     } else if (structureConstructor) {
       // Structure constructor misparsed as function reference?
       CHECK(std::holds_alternative<semantics::SymbolRef>(callee->u));
       const Symbol &derivedType{*std::get<semantics::SymbolRef>(callee->u)};
       const auto &designator{std::get<parser::ProcedureDesignator>(call.t)};
       if (const auto *name{std::get_if<parser::Name>(&designator.u)}) {
         semantics::Scope &scope{context_.FindScope(name->source)};
+        semantics::DerivedTypeSpec dtSpec{
+            name->source, derivedType.GetUltimate()};
+        if (dtSpec.IsForwardReferenced()) {
+          Say(call.source,
+              "Cannot construct value for derived type '%s' "
+              "before it is defined"_err_en_US,
+              name->source);
+          return std::nullopt;
+        }
         const semantics::DeclTypeSpec &type{
-            semantics::FindOrInstantiateDerivedType(scope,
-                semantics::DerivedTypeSpec{
-                    name->source, derivedType.GetUltimate()},
-                context_)};
+            semantics::FindOrInstantiateDerivedType(
+                scope, std::move(dtSpec), context_)};
         auto &mutableRef{const_cast<parser::FunctionReference &>(funcRef)};
         *structureConstructor =
             mutableRef.ConvertToStructureConstructor(type.derivedTypeSpec());
         return Analyze(structureConstructor->value());
       }
     }
   }
   return std::nullopt;

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

 
 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)
+  !ERROR: Cannot construct value for derived type 't2' before it is defined
   parameter(y=t2(12.3))
   type t2
     real :: c
   end type
 end subroutine