------------------------------------------------------------------------------
--                                                                          --
--                         GNAT COMPILER COMPONENTS                         --
--                                                                          --
--                             S E M _ T Y P E                              --
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--          Copyright (C) 1992-2021, Free Software Foundation, Inc.         --
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-- GNAT is free software;  you can  redistribute it  and/or modify it under --
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------------------------------------------------------------------------------

--  This unit contains the routines used to handle type determination,
--  including the routine used to support overload resolution.

with Types; use Types;

package Sem_Type is

   ---------------------------------------------
   -- Data Structures for Overload Resolution --
   ---------------------------------------------

   --  To determine the unique meaning of an identifier, overload resolution
   --  may have to be performed if the visibility rules alone identify more
   --  than one possible entity as the denotation of a given identifier. When
   --  the visibility rules find such a potential ambiguity, the set of
   --  possible interpretations must be attached to the identifier, and
   --  overload resolution must be performed over the innermost enclosing
   --  complete context. At the end of the resolution, either a single
   --  interpretation is found for all identifiers in the context, or else a
   --  type error (invalid type or ambiguous reference) must be signalled.

   --  The set of interpretations of a given name is stored in a data structure
   --  that is separate from the syntax tree, because it corresponds to
   --  transient information. The interpretations themselves are stored in
   --  table All_Interp. A mapping from tree nodes to sets of interpretations
   --  called Interp_Map, is maintained by the overload resolution routines.
   --  Both these structures are initialized at the beginning of every complete
   --  context.

   --  Corresponding to the set of interpretations for a given overloadable
   --  identifier, there is a set of possible types corresponding to the types
   --  that the overloaded call may return. We keep a 1-to-1 correspondence
   --  between interpretations and types: for user-defined subprograms the type
   --  is the declared return type. For operators, the type is determined by
   --  the type of the arguments. If the arguments themselves are overloaded,
   --  we enter the operator name in the names table for each possible result
   --  type. In most cases, arguments are not overloaded and only one
   --  interpretation is present anyway.

   type Interp is record
      Nam         : Entity_Id;
      Typ         : Entity_Id;
      Abstract_Op : Entity_Id := Empty;
   end record;

   --  Entity Abstract_Op is set to the abstract operation which potentially
   --  disables the interpretation in Ada 2005 mode.

   No_Interp : constant Interp := (Empty, Empty, Empty);

   type Interp_Index is new Int;

   ---------------------
   -- Error Reporting --
   ---------------------

   --  A common error is the use of an operator in infix notation on arguments
   --  of a type that is not directly visible. Rather than diagnosing a type
   --  mismatch, it is better to indicate that the type can be made use-visible
   --  with the appropriate use clause. The global variable Candidate_Type is
   --  set in Add_One_Interp whenever an interpretation might be legal for an
   --  operator if the type were directly visible. This variable is used in
   --  Sem_Ch4 when no legal interpretation is found.

   Candidate_Type : Entity_Id;

   -----------------
   -- Subprograms --
   -----------------

   procedure Init_Interp_Tables;
   --  Initialize data structures for overload resolution

   procedure Collect_Interps (N : Node_Id);
   --  Invoked when the name N has more than one visible interpretation. This
   --  is the high level routine which accumulates the possible interpretations
   --  of the node. The first meaning and type of N have already been stored
   --  in N. If the name is an expanded name, the homonyms are only those that
   --  belong to the same scope.

   function Is_Invisible_Operator (N : Node_Id; T : Entity_Id) return Boolean;
   --  Check whether a predefined operation with universal operands appears in
   --  a context in which the operators of the expected type are not visible.

   procedure List_Interps (Nam : Node_Id; Err : Node_Id);
   --  List candidate interpretations of an overloaded name. Used for various
   --  error reports.

   procedure Add_One_Interp
     (N         : Node_Id;
      E         : Entity_Id;
      T         : Entity_Id;
      Opnd_Type : Entity_Id := Empty);
   --  Add (E, T) to the list of interpretations of the node being resolved.
   --  For calls and operators, i.e. for nodes that have a name field, E is an
   --  overloadable entity, and T is its type. For constructs such as indexed
   --  expressions, the caller sets E equal to T, because the overloading comes
   --  from other fields, and the node itself has no name to resolve. Hidden
   --  denotes whether an interpretation has been disabled by an abstract
   --  operator. Add_One_Interp includes semantic processing to deal with
   --  adding entries that hide one another etc.
   --
   --  For operators, the legality of the operation depends on the visibility
   --  of T and its scope. If the operator is an equality or comparison, T is
   --  always Boolean, and we use Opnd_Type, which is a candidate type for one
   --  of the operands of N, to check visibility.

   procedure Get_First_Interp
     (N  : Node_Id;
      I  : out Interp_Index;
      It : out Interp);
   --  Initialize iteration over set of interpretations for Node N. The first
   --  interpretation is placed in It, and I is initialized for subsequent
   --  calls to Get_Next_Interp.

   procedure Get_Next_Interp (I : in out Interp_Index; It : out Interp);
   --  Iteration step over set of interpretations. Using the value in I, which
   --  was set by a previous call to Get_First_Interp or Get_Next_Interp, the
   --  next interpretation is placed in It, and I is updated for the next call.
   --  The end of the list of interpretations is signalled by It.Nam = Empty.

   procedure Remove_Interp (I : in out Interp_Index);
   --  Remove an interpretation that is hidden by another, or that does not
   --  match the context. The value of I on input was set by a call to either
   --  Get_First_Interp or Get_Next_Interp and references the interpretation
   --  to be removed. The only allowed use of the exit value of I is as input
   --  to a subsequent call to Get_Next_Interp, which yields the interpretation
   --  following the removed one.

   procedure Save_Interps (Old_N : Node_Id; New_N : Node_Id);
   --  If an overloaded node is rewritten during semantic analysis, its
   --  possible interpretations must be linked to the copy. This procedure
   --  transfers the overload information (Is_Overloaded flag, and list of
   --  interpretations) from Old_N, the old node, to New_N, its new copy.
   --  It has no effect in the non-overloaded case.

   function Covers (T1, T2 : Entity_Id) return Boolean;
   --  This is the basic type compatibility routine. T1 is the expected type,
   --  imposed by context, and T2 is the actual type. The processing reflects
   --  both the definition of type coverage and the rules for operand matching;
   --  that is, this does not exactly match the RM definition of "covers".

   function Disambiguate
     (N      : Node_Id;
      I1, I2 : Interp_Index;
      Typ    : Entity_Id) return Interp;
   --  If more than one interpretation of a name in a call is legal, apply
   --  preference rules (universal types first) and operator visibility in
   --  order to remove ambiguity. I1 and I2 are the first two interpretations
   --  that are compatible with the context, but there may be others.

   function Entity_Matches_Spec (Old_S,  New_S : Entity_Id) return Boolean;
   --  To resolve subprogram renaming and default formal subprograms in generic
   --  definitions. Old_S is a possible interpretation of the entity being
   --  renamed, New_S has an explicit signature. If Old_S is a subprogram, as
   --  opposed to an operator, type and mode conformance are required.

   function Find_Unique_Type (L : Node_Id; R : Node_Id) return Entity_Id;
   --  Used in second pass of resolution, for equality and comparison nodes. L
   --  is the left operand, whose type is known to be correct, and R is the
   --  right operand, which has one interpretation compatible with that of L.
   --  Return the type intersection of the two.

   function Has_Compatible_Type (N : Node_Id; Typ : Entity_Id) return Boolean;
   --  Verify that some interpretation of the node N has a type compatible with
   --  Typ. If N is not overloaded, then its unique type must be compatible
   --  with Typ. Otherwise iterate through the interpretations of N looking for
   --  a compatible one.

   function Hides_Op (F : Entity_Id; Op : Entity_Id) return Boolean;
   --  A user-defined function hides a predefined operator if it matches the
   --  signature of the operator, and is declared in an open scope, or in the
   --  scope of the result type.

   function Interface_Present_In_Ancestor
     (Typ   : Entity_Id;
      Iface : Entity_Id) return Boolean;
   --  Ada 2005 (AI-251): Typ must be a tagged record type/subtype and Iface
   --  must be an abstract interface type (or a class-wide abstract interface).
   --  This function is used to check if Typ or some ancestor of Typ implements
   --  Iface (returning True only if so).

   function Intersect_Types (L, R : Node_Id) return Entity_Id;
   --  Find the common interpretation to two analyzed nodes. If one of the
   --  interpretations is universal, choose the non-universal one. If either
   --  node is overloaded, find single common interpretation.

   function In_Generic_Actual (Exp : Node_Id) return Boolean;
   --  Determine whether the expression is part of a generic actual. At the
   --  time the actual is resolved the scope is already that of the instance,
   --  but conceptually the resolution of the actual takes place in the
   --  enclosing context and no special disambiguation rules should be applied.

   function Is_Ancestor
     (T1            : Entity_Id;
      T2            : Entity_Id;
      Use_Full_View : Boolean := False) return Boolean;
   --  T1 is a tagged type (not class-wide). Verify that it is one of the
   --  ancestors of type T2 (which may or not be class-wide). If Use_Full_View
   --  is True then the full-view of private parents is used when climbing
   --  through the parents of T2.
   --
   --  Note: For analysis purposes the flag Use_Full_View must be set to False
   --  (otherwise we break the privacy contract since this routine returns true
   --  for hidden ancestors of private types). For expansion purposes this flag
   --  is generally set to True since the expander must know with precision the
   --  ancestors of a tagged type. For example, if a private type derives from
   --  an interface type then the interface may not be an ancestor of its full
   --  view since the full-view is only required to cover the interface (RM 7.3
   --  (7.3/2))) and this knowledge affects construction of dispatch tables.

   function Is_Progenitor
     (Iface : Entity_Id;
      Typ   : Entity_Id) return Boolean;
   --  Determine whether the interface Iface is implemented by Typ. It requires
   --  traversing the list of abstract interfaces of the type, as well as that
   --  of the ancestor types. The predicate is used to determine when a formal
   --  in the signature of an inherited operation must carry the derived type.

   function Is_Subtype_Of (T1 : Entity_Id; T2 : Entity_Id) return Boolean;
   --  Checks whether T1 is any subtype of T2 directly or indirectly

   function Operator_Matches_Spec (Op, New_S : Entity_Id) return Boolean;
   --  Used to resolve subprograms renaming operators, and calls to user
   --  defined operators. Determines whether a given operator Op, matches
   --  a specification, New_S.

   procedure Set_Abstract_Op (I : Interp_Index; V : Entity_Id);
   --  Set the abstract operation field of an interpretation

   function Valid_Comparison_Arg (T : Entity_Id) return Boolean;
   --  A valid argument to an ordering operator must be a discrete type, a
   --  real type, or a one dimensional array with a discrete component type.

   function Valid_Boolean_Arg (T : Entity_Id) return Boolean;
   --  A valid argument of a boolean operator is either some boolean type, or a
   --  one-dimensional array of boolean type.

   procedure Write_Interp (It : Interp);
   --  Debugging procedure to display an Interp

   procedure Write_Overloads (N : Node_Id);
   --  Debugging procedure to output info on possibly overloaded entities for
   --  specified node.

end Sem_Type;