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Update declaring constructors to allow the new constructor syntax #4543

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merged 3 commits into from
Oct 30, 2025
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Update declaring constructors to allow the new constructor syntax #4543

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0883d64
Update the declaring constructors feature specification to allow the …
eernstg Oct 22, 2025
19471aa
Make factory a reselved word
eernstg Oct 22, 2025
e2ffa48
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eernstg Oct 30, 2025
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89 changes: 57 additions & 32 deletions accepted/future-releases/primary-constructors/feature-specification.md
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Original file line number Diff line number Diff line change
Expand Up @@ -575,64 +575,81 @@ constructors as well.
```ebnf
<classDeclaration> ::= // First alternative modified.
(<classModifiers> | <mixinClassModifiers>)
'class' <classNamePart> <superclass>? <interfaces>? <classBody>
'class' <classNameMaybePrimary> <superclass>? <interfaces>? <classBody>
| ...;

<primaryConstructorNoConst> ::= // New rule.
<typeIdentifier> <typeParameters>?
('.' <identifierOrNew>)? <declaringParameterList>
<primaryConstructor> ::= // New rule.
'const'? <typeWithParameters> ('.' <identifierOrNew>)?
<declaringParameterList>;

<classNamePart> ::= // New rule.
'const'? <primaryConstructorNoConst>
<classNameMaybePrimary> ::= // New rule.
<primaryConstructor>
| <typeWithParameters>;

<typeWithParameters> ::= <typeIdentifier> <typeParameters>?

<classBody> ::= // New rule.
'{' (<metadata> <classMemberDeclaration>)* '}'
'{' (<metadata> <memberDeclaration>)* '}'
| ';';

<extensionTypeDeclaration> ::= // Modified rule.
'extension' 'type' <classNamePart> <interfaces>?
'extension' 'type' <classNameMaybePrimary> <interfaces>?
<extensionTypeBody>;

<extensionTypeMemberDeclaration> ::= <classMemberDeclaration>;

<extensionTypeBody> ::=
'{' (<metadata> <extensionTypeMemberDeclaration>)* '}'
'{' (<metadata> <memberDeclaration>)* '}'
| ';';

<enumType> ::= // Modified rule.
'enum' <classNamePart> <mixins>? <interfaces>? '{'
<enumEntry> (',' <enumEntry>)* (',')?
(';' (<metadata> <classMemberDeclaration>)*)?
'enum' <classNameMaybePrimary> <mixins>? <interfaces>? '{'
<enumEntry> (',' <enumEntry>)* ','?
(';' (<metadata> <memberDeclaration>)*)?
'}';

<constructorSignature> ::= // Modified rule.
<constructorName> <formalParameterList>
<constructorName> <formalParameterList> // Old form.
| <constructorHead> <formalParameterList> // New form.
| <declaringConstructorSignature>;

<declaringConstructorSignature> ::= // New rule.
'this' ('.' <identifierOrNew>)? <declaringParameterList>?;
'this' <identifier>? <declaringParameterList>?;

<constantConstructorSignature> ::= // Modified rule.
'const' <constructorName> <formalParameterList>
'const' <constructorName> <formalParameterList> // Old form.
| 'const' <constructorHead> <formalParameterList> // New form.
| <declaringConstantConstructorSignature>;

<constructorName> ::= // Modified rule.
<typeIdentifierOrNew> ('.' identifierOrNew)?;

<typeIdentifierOrNew> ::= // New rule.
<typeIdentifier>
| 'new';
<typeIdentifier> ('.' identifierOrNew)?;

<declaringConstantConstructorSignature> ::= // New rule.
'const' 'this' ('.' <identifierOrNew>)? <declaringParameterList>;
'const' 'this' <identifier>? <declaringParameterList>;

<constructorHead> ::= // New rule.
'new' <identifier>?;

<factoryConstructorHead> ::= // New rule.
'factory' <identifier>?;

<identifierOrNew> ::=
<identifier>
| 'new'

<factoryConstructorSignature> ::= // Modified rule.
'const'? 'factory' <constructorName> <formalParameterList> // Old form.
| 'const'? <factoryConstructorHead> <formalParameterList>; // New form.

<redirectingFactoryConstructorSignature> ::= // Modified rule.
'const'? 'factory' <constructorName> <formalParameterList> '='
<constructorDesignation> // Old form.
| 'CONST'? <factoryConstructorHead> <formalParameterList> '='
<constructorDesignation>; // New form.

<constantConstructorSignature> ::= // Modified rule.
: 'const' <constructorName> <formalParameterList> // Old form.
| 'const' <constructorHead> <formalParameterList> // New form.
| <declaringConstantConstructorSignature>;

<simpleFormalParameter> ::= // Modified rule.
'covariant'? <type>? <identifier>;

Expand Down Expand Up @@ -685,31 +702,39 @@ constructors as well.
('=' <expression>)?;
```

The word `factory` is removed from the set of built-in identifiers and
added to the set of reserved words.

*This allows the new constructor declaration syntax such as `factory();` to
be unambiguous. It is also a breaking change because `factory` can no
longer be the name of a declaration. However, this occurs only infrequently
today.*

A _declaring constructor_ declaration is a declaration that contains a
`<declaringConstructorSignature>` with a `<declaringParameterList>`, or a
declaration that contains a `<declaringConstantConstructorSignature>`, or
it is a `<primaryConstructorNoConst>` in the header of a class, enum, or
extension type declaration, together with a declaration in the body that
contains a `<declaringConstructorSignature>` *(which does not contain a
it is a `<primaryConstructor>` in the header of a class, enum, or extension
type declaration, together with a declaration in the body that contains a
`<declaringConstructorSignature>` *(which does not contain a
`<declaringParameterList>`, because that's an error)*.

A class or extension type declaration whose class body is `;` is treated as
a declaration whose body is `{}`.

Let _D_ be a class, extension type, or enum declaration.

A compile-time error occurs if _D_ includes a `<classNamePart>` that
contains a `<primaryConstructorNoConst>`, and the body of _D_ contains a
A compile-time error occurs if _D_ includes a `<classNameMaybePrimary>`
that contains a `<primaryConstructor>`, and the body of _D_ contains a
`<declaringConstructorSignature>` that contains a
`<declaringParameterList>`.

*It is an error to have a declaring parameter list both in the header and
in the body.*

A compile-time error occurs if _D_ includes a `<classNamePart>` that
does not contain a `<primaryConstructorNoConst>`, and the body of _D_
contains a `<declaringConstructorSignature>` that does not
contain a `<declaringParameterList>`.
A compile-time error occurs if _D_ includes a `<classNameMaybePrimary>`
that does not contain a `<primaryConstructor>`, and the body of _D_
contains a `<declaringConstructorSignature>` that does not contain a
`<declaringParameterList>`.

*It is an error to have a declaring constructor in the class body, but
no declaring parameter list, neither in the header nor in the body.*
Expand Down