Ada Reference Manual (Ada 2022) — Legal Information

# 8.6 The Context of Overload Resolution

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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 describes how the possible interpretations resolve to the actual interpretation.
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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.
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The syntax rules of the language and the visibility rules given in 8.3 determine 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.

#### Name Resolution Rules

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Overload resolution is applied separately to each complete context, not counting inner complete contexts. Each of the following constructs is a complete context:
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A context_item.
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A declarative_item or declaration.
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A statement.
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A pragma_argument_association.
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The selecting_expression of a case_statement or case_expression.
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The variable_name of an assignment_statement A, if the expression of A contains one or more target_names.
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An (overall) interpretation 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:
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for each constituent of the complete context, to which syntactic categories it belongs, and by which syntax rules; and
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for each usage name, which declaration it denotes (and, therefore, which view and which entity it denotes); and
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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.
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A possible interpretation is one that obeys the syntax rules and the visibility rules. An acceptable interpretation 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.
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The interpretation of 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.
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Each 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:
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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 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.
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Within an aspect_specification for a type or subtype, the current instance represents a value of the type; it is not an object. Unless otherwise specified, the nominal subtype of this value is given by the subtype itself (the first subtype in the case of a type_declaration), prior to applying any predicate specified directly on the type or subtype. If the type or subtype is by-reference, the associated object of the value is the object associated (see 6.2) with the evaluation of the usage name.
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If a usage name appears within the declarative region of a generic_declaration (but not within its generic_formal_part) and it denotes that same generic_declaration, then it denotes the current instance of the generic unit (rather than the generic unit itself). See also 12.3.
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A usage name that denotes a view also denotes the entity of that view.
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The expected type for a given expression, name, or other construct determines, according to the type resolution rules given below, the types considered for the construct during overload resolution. The type resolution rules provide support for class-wide programming, universal literals, dispatching operations, and anonymous access types:
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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 class.
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If the expected type for a construct is a specific type T, then the type of the construct shall resolve either to T, or:
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to T'Class; or
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to a universal type that covers T; or
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when T is a specific anonymous access-to-object type (see 3.10) with designated type D, to an access-to-object type whose designated type is D'Class or is covered by D; or
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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
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when T is an anonymous access-to-subprogram type (see 3.10), to an access-to-subprogram type whose designated profile is type conformant with that of T.
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In certain contexts, such as in a subprogram_renaming_declaration, the Name Resolution Rules define an expected profile for a given name; in such cases, the name shall resolve to the name of a callable entity whose profile is type conformant with the expected profile.

#### Legality Rules

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When a construct is one that requires that its expected type be a single type in a given class, the type of 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. Furthermore, 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.
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Other than for the tested_simple_expression of a membership test, if the expected type for a name or expression is not the same as the actual type of the name or expression, 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 or expression 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.
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A complete context shall have at least one acceptable interpretation; if there is exactly one, then that one is chosen.
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There is a preference for the primitive operators (and ranges) of the root numeric types root_integer and root_real. In 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 or root_real, and the other is not, the interpretation using the primitive operator (or range) of the root numeric type is preferred.
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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.
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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.
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A complete context other than a pragma_argument_association shall not be ambiguous.
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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. Such a name denotes all of the declarations determined by its interpretations, and all of the views declared by these declarations.
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NOTE 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.
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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).

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