8.6 The Context of Overload Resolution
Because declarations can be overloaded, it is possible
for an occurrence of a usage name to have more than one possible interpretation;
in most cases, ambiguity is disallowed. This subclause clause
describes how the possible interpretations resolve to the actual interpretation.
Certain rules of the language
(the Name Resolution Rules) are considered “overloading rules”.
If a possible interpretation violates an overloading rule, it is assumed
not to be the intended interpretation; some other possible interpretation
is assumed to be the actual interpretation. On the other hand, violations
of nonoverloading rules do not affect which interpretation is chosen;
instead, they cause the construct to be illegal. To be legal, there usually
has to be exactly one acceptable interpretation of a construct that is
a “complete context”, not counting any nested complete contexts.
The syntax rules of the language
and the visibility rules given in 8.3
the possible interpretations. Most type checking rules (rules that require
a particular type, or a particular class of types, for example) are overloading
rules. Various rules for the matching of formal and actual parameters
are overloading rules.]
Language Design Principles
The type resolution rules are intended to minimize
the need for implicit declarations and preference rules associated with
implicit conversion and dispatching operations.
Name Resolution Rules
resolution is applied separately to each complete context
counting inner complete contexts.] Each of the following constructs is
a complete context
We would make it the whole pragma
except that certain pragma arguments are allowed to be ambiguous, and
ambiguity applies to a complete context.
This means that the expression
is resolved without looking at the choices.
of a complete context embodies its meaning,
and includes the following information about the constituents of the
complete context, not including constituents of inner complete contexts:
for each constituent of the complete context, to
which syntactic categories it belongs, and by which syntax rules; and
is plural here, because there are lots of trivial productions —
might also be all of the following, in this order: identifier
Basically, we're trying to capture all the information in the parse tree
here, without using compiler-writer's jargon like “parse tree”.
for each usage name, which declaration it denotes
(and, therefore, which view and which entity it denotes); and
In most cases, a usage name denotes the view declared by the denoted
declaration. However, in certain cases, a usage name that denotes a declaration
and appears inside the declarative region of that same declaration, denotes
the current instance of the declaration. For example, within a task_body other than in an access_definition
a usage name that denotes the task_type_declaration
denotes the object containing the currently executing task, and not the
task type declared by the declaration.
for a complete context that is a declarative_item
whether or not it is a completion of a declaration, and (if so) which
declaration it completes.
Ramification: Unfortunately, we are not
confident that the above list is complete. We'll have to live with that.
To be honest: For “possible”
interpretations, the above information is tentative.
Discussion: A possible interpretation
(an input to overload resolution) contains information about what
a usage name might denote, but what it actually does denote
requires overload resolution to determine. Hence the term “tentative”
is needed for possible interpretations; otherwise, the definition would
A possible interpretation
is one that obeys the syntax rules and the visibility rules.
is a possible interpretation that obeys
the overloading rules
[, that is, those rules that specify an expected
type or expected profile, or specify how a construct shall resolve
or be interpreted
To be honest:
One rule that falls into
this category, but does not use the above-mentioned magic words, is the
rule about numbers of parameter associations in a call (see 6.4
Ramification: The Name Resolution Rules
are the ones that appear under the Name Resolution Rules heading. Some
Syntax Rules are written in English, instead of BNF. No rule is a Syntax
Rule or Name Resolution Rule unless it appears under the appropriate
a constituent of a complete context is determined from the overall interpretation
of the complete context as a whole. [Thus, for example, “interpreted
as a function_call
means that the construct's interpretation says that it belongs to the
syntactic category function_call
occurrence of] a usage name denotes
the declaration determined
by its interpretation. It also denotes the view declared by its denoted
declaration, except in the following cases:
Ramification: As explained below, a pragma
argument is allowed to be ambiguous, so it can denote several declarations,
and all of the views declared by those declarations.
If a usage name appears within the declarative region
of a type_declaration
and denotes that same type_declaration
then it denotes the current instance
of the type (rather than
the type itself); the.
current instance of a type is the object or value of the type
that is associated with the execution that evaluates the usage name. Similarly, if a usage name appears within
the declarative region of a subtype_declaration
and denotes that same subtype_declaration,
then it denotes the current instance of the subtype. These rules do This
rule does not apply if the usage
name appears within the subtype_mark
of an access_definition
for an access-to-object type, or within the subtype of a parameter or
result of an access-to-subprogram type.
The phrase “within the subtype_mark”
in the “this rule does not apply” part is intended to cover
a case like access T'Class appearing within the declarative region
of T: here T denotes the type, not the current instance.
To be honest:
The current instance of
a generic unit is the instance created by whichever generic_instantiation
is of interest at any given time.
A usage name that denotes a view also denotes the
entity of that view.
Ramification: Usually, a usage name denotes
only one declaration, and therefore one view and one entity.
The expected type
for a given expression
, or other
construct determines, according to the type resolution rules
below, the types considered for the construct during overload resolution.
[ The type resolution rules provide support for class-wide
programming, universal numeric
dispatching operations, and anonymous access types:]
Ramification: Expected types are defined
throughout the RM95. The most important definition is that, for a subprogram,
the expected type for the actual parameter is the type of the formal
The type resolution rules are trivial unless
either the actual or expected type is universal, class-wide, or of an
anonymous access type.
If a construct
is expected to be of any type in a class of types, or of the universal
or class-wide type for a class, then the type of the construct shall
resolve to a type in that class or to a universal type that covers the
Ramification: This matching rule handles
(among other things) cases like the Val attribute, which denotes a function
that takes a parameter of type universal_integer.
The last part of the rule, “or to a universal
type that covers includes
the class” implies that if the expected type for an expression
is universal_fixed, then an expression whose type is universal_real
(such as a real literal) is OK.
the expected type for a construct is a specific type T
, then the
type of the construct shall resolve either to T
rule is not
intended to create a preference for the specific type
— such a preference would cause Beaujolais effects.
to T'Class; or
to a universal type that covers
Because it says “access-to-variable”
instead of “access-to-object,” two subprograms that differ
only in that one has a parameter of an access-to-constant type, and the
other has an access parameter, are distinguishable during overload resolution.
The case where the actual is access-to-D
will only be legal as part of a call on a dispatching operation; see
Operations of Tagged Types
”. Note that that rule is not a Name
when T is a named general access-to-object
type (see 3.10) with designated type D,
to an anonymous access-to-object type whose designated type covers or
is covered by D; or
when T is an anonymous access-to-subprogram
type (see 3.10), to an access-to-subprogram
type whose designated profile is type conformant type-conformant with that of T.
In certain contexts, [such as
in a subprogram_renaming_declaration
the Name Resolution Rules define an expected profile
for a given
such cases, the name
shall resolve to the name of a callable entity whose profile is type
conformant with the expected profile.
The parameter and result sub
types are not used in overload resolution.
Only type conformance of profiles is considered during overload resolution.
Legality rules generally require at least mode
but those rules are not used in overload resolution.
When the expected type for
a construct is one that requires that its
expected type required to
be a single
type in a given class, the type of expected
the construct shall be determinable solely from the context
in which the construct appears, excluding the construct itself, but using
the requirement that it be in the given class;
the type of the construct is then this single expected type
the context shall not be one that expects any type in some class that
contains types of the given class; in particular, the construct shall
not be the operand of a type_conversion
For example, the expected type for a string literal the
is required to be a single string access
type. But the expected type for the operand of a type_conversion
is any type. Therefore, a string literal the
is not allowed as the operand of a type_conversion
This is true even if there is only one string access
type in scope (which is never the case)
The reason for these rules is so that the compiler will not have to search
“everywhere” to see if there is exactly one type in a class
The first sentence is carefully worded so that
it only mentions “expected type” as part of identifying the
interesting case, but doesn't require that the context actually provide
such an expected type. This allows such constructs to be used inside
of constructs that don't provide an expected type (like qualified expressions
and renames). Otherwise, such constructs wouldn't allow aggregates,
'Access, and so on.
Other than for the simple_expression
of a membership test, if the expected type for a name
is not the same as the actual type of the name
the actual type shall be convertible to the expected type (see 4.6);
further, if the expected type is a named access-to-object type with designated
type D1 and the actual type is an anonymous access-to-object type
with designated type D2, then D1 shall cover D2,
and the name
shall denote a view with an accessibility level for which the statically
deeper relationship applies[; in particular it shall not denote an access
parameter nor a stand-alone access object].
Reason: This rule
prevents an implicit conversion that would be illegal if it was an explicit
conversion. For instance, this prevents assigning an access-to-constant
value into a stand-alone anonymous access-to-variable object. It also
covers convertibility of the designated type and accessibility checks.
The rule also minimizes
cases of implicit conversions when the tag check or the accessibility
check might fail. We word it this way because access discriminants should
also be disallowed if their enclosing object is designated by an access
rule does not apply to expressions that don't have expected types (such
as the operand of a qualified expression or the expression of a renames).
We don't need a rule like this in those cases, as the type needs to be
the same; there is no implicit conversion.
A complete context shall have at least one acceptable
interpretation; if there is exactly one, then that one is chosen.
Ramification: This, and the rule below
about ambiguity, are the ones that suck in all the Syntax Rules and Name
Resolution Rules as compile-time rules. Note that this and the ambiguity
rule have to be Legality Rules.
There is a preference
for the primitive operators (and range
of the root numeric types root_integer
particular, if two acceptable interpretations of a constituent of a complete
context differ only in that one is for a primitive operator (or range
of the type root_integer
, and the other is
not, the interpretation using the primitive operator (or range
of the root numeric type is preferred
reason for this preference is so that expressions involving literals
and named numbers can be unambiguous. For example, without the preference
rule, the following would be ambiguous:
N : constant := 123;
if N > 100 then -- Preference for root_integer "> <" operator.
Similarly, there is a preference for the equality
operators of the universal_access type (see 4.5.2).
If two acceptable interpretations of a constituent of a complete context
differ only in that one is for an equality operator of the universal_access
type, and the other is not, the interpretation using the equality operator
of the universal_access type is preferred.
Reason: This preference
is necessary because of implicit conversion from an anonymous access
type to a named access type, which would allow the equality operator
of any named access type to be used to compare anonymous access values
(and that way lies madness).
For a complete context, if there is exactly one overall
acceptable interpretation where each constituent's interpretation is
the same as or preferred (in the above sense) over those in all other
overall acceptable interpretations, then that one overall acceptable
interpretation is chosen.
Otherwise, the complete
context is ambiguous
A complete context that is a pragma_argument_association
is allowed to be ambiguous (unless otherwise specified for the particular
pragma), but only if every acceptable interpretation of the pragma argument
is as a name
that statically denotes a callable entity.
all of the declarations determined by its interpretations, and all of
the views declared by these declarations.
This applies to Inline, Suppress, Import, Export, and Convention pragma
For example, it is OK to say “pragma Export(C,
=> P.Q);”, even if there are two directly visible
P's, and there are two Q's declared in the visible part of each P. In
this case, P.Q denotes four different declarations. This rule also applies
to certain pragmas defined in the Specialized Needs Annexes. It almost
applies to Pure, Elaborate_Body, and Elaborate_All pragma
but those can't have overloading for other reasons. Note that almost all of these pragmas are obsolescent (see J.10
and J.15), and a major reason is that this
rule has proven to be too broad in practice (it is common to want to
specify something on a single subprogram of an overloaded set, that can't
be done easily with this rule). Aspect_specifications,
which are given on individual declarations, are preferred in Ada 2012.
Note that if a pragma argument denotes a call
to a callable entity, rather than the entity itself, this exception does
not apply, and ambiguity is disallowed.
Note that we need to carefully define which
pragma-related rules are Name Resolution Rules, so that, for example,
does not pick up subprograms declared in enclosing declarative regions,
and therefore make itself illegal.
We say “statically denotes” in the
above rule in order to avoid having to worry about how many times the
in case it denotes more than one callable entity.
17 If a usage name has only one acceptable
interpretation, then it denotes the corresponding entity. However, this
does not mean that the usage name is necessarily legal since other requirements
exist which are not considered for overload resolution; for example,
the fact that an expression is static, whether an object is constant,
mode and subtype conformance rules, freezing rules, order of elaboration,
and so on.
Similarly, subtypes are not considered for overload
resolution (the violation of a constraint does not make a program illegal
but raises an exception during program execution).
Incompatibilities With Ada 83
new preference rule for operators of root numeric types is upward incompatible,
but only in cases that involved Beaujolais
effects in Ada 83.
Such cases are ambiguous in Ada 95.
Extensions to Ada 83
The rule that allows an
expected type to match an actual expression of a universal type, in combination
with the new preference rule for operators of root numeric types, subsumes
the Ada 83 "implicit conversion" rules for universal types.
Wording Changes from Ada 83
In Ada 83, it is not clear what the “syntax
rules” are. AI83-00157 states that a certain textual rule is a
syntax rule, but it's still not clear how one tells in general which
textual rules are syntax rules. We have solved the problem by stating
exactly which rules are syntax rules — the ones that appear under
the “Syntax” heading.
RM83 has a long list of the “forms”
of rules that are to be used in overload resolution (in addition to the
syntax rules). It is not clear exactly which rules fall under each form.
We have solved the problem by explicitly marking all rules that are used
in overload resolution. Thus, the list of kinds of rules is unnecessary.
It is replaced with some introductory (intentionally vague) text explaining
the basic idea of what sorts of rules are overloading rules.
It is not clear from RM83 what information is embodied in a “meaning”
or an “interpretation.” “Meaning” and “interpretation”
were intended to be synonymous; we now use the latter only in defining
the rules about overload resolution. “Meaning” is used only
informally. This subclause clause
attempts to clarify what is meant by “interpretation.”
For example, RM83 does not make it clear that
overload resolution is required in order to match subprogram_bodies
with their corresponding declarations (and even to tell whether a given
is the completion of a previous declaration). Clearly, the information
needed to do this is part of the “interpretation” of a subprogram_body
The resolution of such things is defined in terms of the “expected
profile” concept. Ada 95 has some new cases where expected profiles
are needed — the resolution of P'Access, where P might denote a
subprogram, is an example.
seem to imply that an interpretation embodies information about what
is denoted by each usage name, but not information about which syntactic
category each construct belongs to. However, it seems necessary to include
such information, since the Ada grammar is highly ambiguous. For example,
X(Y) might be a function_call
or an indexed_component
and no context-free/syntactic information can tell the difference. It
seems like we should view X(Y) as being, for example, “interpreted
as a function_call
(if that's what overload resolution decides it is). Note that there are
examples where the denotation of each usage name does not imply the syntactic
category. However, even if that were not true, it seems that intuitively,
the interpretation includes that information. Here's an example:
type A is access T;
type T is array(Integer range 1..10) of A;
I : Integer := 3;
function F(X : Integer := 7) return A;
Y : A := F(I); -- Ambiguous? (We hope so.)
Consider the declaration of Y (a complete context).
In the above example, overload resolution can easily determine the declaration,
and therefore the entity, denoted by Y, A, F, and I. However, given all
of that information, we still don't know whether F(I) is a function_call
or an indexed_component
whose prefix prefix
is a function_call
(In the latter case, it is equivalent to F(7).all
It seems clear that the declaration of Y ought
to be considered ambiguous. We describe that by saying that there are
two interpretations, one as a function_call
and one as an indexed_component
These interpretations are both acceptable to the overloading rules. Therefore,
the complete context is ambiguous, and therefore illegal.
It is the intent that the
Ada 95 preference rule for root numeric operators is more locally enforceable
than that of RM83-4.6(15). It should also eliminate interpretation shifts
due to the addition or removal of a use_clause
(the so called Beaujolais
Cases like the Val attribute are now handled
using the normal type resolution rules, instead of having special cases
that explicitly allow things like “any integer type.”
Incompatibilities With Ada 95
Ada 95 allowed name resolution
to distinguish between anonymous access-to-variable and access-to-constant
types. This is similar to distinguishing between subprograms with in
and in out parameters, which is known to be bad. Thus, that part
of the rule was dropped as we now have anonymous access-to-constant types,
making this much more likely.
type Cacc is access constant Integer;
procedure Proc (Acc : access Integer) ...
procedure Proc (Acc : Cacc) ...
List : Cacc := ...;
Proc (List); -- OK in Ada 95, ambiguous in Ada 2005.
If there is any code like
this (such code should be rare), it will be ambiguous in Ada 2005.
Extensions to Ada 95
Generalized the anonymous access
resolution rules to support the new capabilities of anonymous access
types (that is, access-to-subprogram and access-to-constant).
We now allow the creation of self-referencing types
via anonymous access types. This is an extension in unusual cases involving
task and protected types. For example:
task type T;
task body T is
procedure P (X : access T) is -- Illegal in Ada 95, legal in Ada 2005
Wording Changes from Ada 95
Corrected the “single expected type”
so that it works in contexts that don't have expected types (like object
renames and qualified expressions). This fixes a hole in Ada 95 that
appears to prohibit using aggregates,
'Access, character literals, string literals, and allocators
in qualified expressions.
Incompatibilities With Ada 2005
Implicit conversion is now
allowed from anonymous access-to-object types to general access-to-object
types. Such conversions can make calls ambiguous. That can only happen
when there are two visible subprograms with the same name and have profiles
that differ only by a parameter that is of a named or anonymous access
type, and the actual argument is of an anonymous access type. This should
be rare, as many possible calls would be ambiguous even in Ada 2005 (including
and any actual of a named access type if the designated types are the
Extensions to Ada 2005
Implicit conversion is allowed
from anonymous access-to-object types to general access-to-object types
if the designated type is convertible and runtime checks are minimized.
See also the incompatibilities section.
Wording Changes from Ada 2005
Added a requirement here that implicit conversions
are convertible to the appropriate type. This rule was scattered about
the Standard, we moved a single generalized version here.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe