* [Ada] ICE handling discriminant-dependent index constraint for access component
@ 2022-09-06 7:15 Marc Poulhiès
0 siblings, 0 replies; only message in thread
From: Marc Poulhiès @ 2022-09-06 7:15 UTC (permalink / raw)
To: gcc-patches; +Cc: Steve Baird
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The compiler would fail with an internal error in some cases involving
a discriminated record type that provides a discriminant-dependent index
constraint for the subtype of a component of an access-to-array type when a
dereference of that component of some object is mentioned in a pre- or
postcondition expression.
Tested on x86_64-pc-linux-gnu, committed on trunk
gcc/ada/
* sem_ch4.adb
(Analyze_Selected_Component): Define new Boolean-valued function,
Constraint_Has_Unprefixed_Discriminant_Reference, which takes a
subtype that is subject to a discriminant-dependent constraint and
returns True if any of the constraint values are unprefixed
discriminant names. Usually, the Etype of a selected component
node is set to Etype of the component. However, in the case of an
access-to-array component for which this predicate returns True,
we instead use the base type of the Etype of the component.
Normally such problematic discriminant references are addressed by
calling Build_Actual_Subtype_Of_Component, but that doesn't work
if Full_Analyze is False.
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diff --git a/gcc/ada/sem_ch4.adb b/gcc/ada/sem_ch4.adb
--- a/gcc/ada/sem_ch4.adb
+++ b/gcc/ada/sem_ch4.adb
@@ -4814,6 +4814,14 @@ package body Sem_Ch4 is
Is_Single_Concurrent_Object : Boolean;
-- Set True if the prefix is a single task or a single protected object
+ function Constraint_Has_Unprefixed_Discriminant_Reference
+ (Typ : Entity_Id) return Boolean;
+ -- Given a subtype that is subject to a discriminant-dependent
+ -- constraint, returns True if any of the values of the constraint
+ -- (i.e., any of the index values for an index constraint, any of
+ -- the discriminant values for a discriminant constraint)
+ -- are unprefixed discriminant names.
+
procedure Find_Component_In_Instance (Rec : Entity_Id);
-- In an instance, a component of a private extension may not be visible
-- while it was visible in the generic. Search candidate scope for a
@@ -4842,6 +4850,56 @@ package body Sem_Ch4 is
-- _Procedure, and collect all its interpretations (since it may be an
-- overloaded interface primitive); otherwise return False.
+ ------------------------------------------------------
+ -- Constraint_Has_Unprefixed_Discriminant_Reference --
+ ------------------------------------------------------
+
+ function Constraint_Has_Unprefixed_Discriminant_Reference
+ (Typ : Entity_Id) return Boolean
+ is
+
+ function Is_Discriminant_Name (N : Node_Id) return Boolean is
+ ((Nkind (N) = N_Identifier)
+ and then (Ekind (Entity (N)) = E_Discriminant));
+ begin
+ if Is_Array_Type (Typ) then
+ declare
+ Index : Node_Id := First_Index (Typ);
+ Rng : Node_Id;
+ begin
+ while Present (Index) loop
+ Rng := Index;
+ if Nkind (Rng) = N_Subtype_Indication then
+ Rng := Range_Expression (Constraint (Rng));
+ end if;
+
+ if Nkind (Rng) = N_Range then
+ if Is_Discriminant_Name (Low_Bound (Rng))
+ or else Is_Discriminant_Name (High_Bound (Rng))
+ then
+ return True;
+ end if;
+ end if;
+
+ Next_Index (Index);
+ end loop;
+ end;
+ else
+ declare
+ Elmt : Elmt_Id := First_Elmt (Discriminant_Constraint (Typ));
+ begin
+ while Present (Elmt) loop
+ if Is_Discriminant_Name (Node (Elmt)) then
+ return True;
+ end if;
+ Next_Elmt (Elmt);
+ end loop;
+ end;
+ end if;
+
+ return False;
+ end Constraint_Has_Unprefixed_Discriminant_Reference;
+
--------------------------------
-- Find_Component_In_Instance --
--------------------------------
@@ -5289,6 +5347,33 @@ package body Sem_Ch4 is
end;
end if;
+ -- If Etype (Comp) is an access type whose designated subtype
+ -- is constrained by an unprefixed discriminant value,
+ -- then ideally we would build a new subtype with an
+ -- appropriately prefixed discriminant value and use that
+ -- instead, as is done in Build_Actual_Subtype_Of_Component.
+ -- That turns out to be difficult in this context (with
+ -- Full_Analysis = False, we could be processing a selected
+ -- component that occurs in a Postcondition pragma;
+ -- PPC pragmas are odd because they can contain references
+ -- to formal parameters that occur outside the subprogram).
+ -- So instead we punt on building a new subtype and we
+ -- use the base type instead. This might introduce
+ -- correctness problems if N were the target of an
+ -- assignment (because a required check might be omitted);
+ -- fortunately, that's impossible because a reference to the
+ -- current instance of a type does not denote a variable view
+ -- when the reference occurs within an aspect_specification.
+ -- GNAT's Precondition and Postcondition pragmas follow the
+ -- same rules as a Pre or Post aspect_specification.
+
+ elsif Has_Discriminant_Dependent_Constraint (Comp)
+ and then Ekind (Etype (Comp)) = E_Access_Subtype
+ and then Constraint_Has_Unprefixed_Discriminant_Reference
+ (Designated_Type (Etype (Comp)))
+ then
+ Set_Etype (N, Base_Type (Etype (Comp)));
+
-- If Full_Analysis not enabled, just set the Etype
else
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