Remove properties that are used by invalid code

[bradzacher]

Currently we have the following AST properties declared:

typescript-eslint/packages/ast-spec/src/declaration/TSEnumDeclaration/spec.ts

Line 13 in 8329498

modifiers?: Modifier[];

typescript-eslint/packages/ast-spec/src/declaration/TSModuleDeclaration/spec.ts

Line 25 in 8329498

modifiers?: Modifier[];

typescript-eslint/packages/ast-spec/src/declaration/TSInterfaceDeclaration/spec.ts

Line 14 in 8329498

implements?: TSInterfaceHeritage[];

typescript-eslint/packages/ast-spec/src/declaration/TSInterfaceDeclaration/spec.ts

Line 15 in 8329498

abstract?: boolean;

These all represent bad code that would cause errors if one attempted to build in TS:

public enum Foo {}
// 'public' modifier cannot appear on a module or namespace element.(1044)

abstract namespace Bar {}
// 'abstract' modifier can only appear on a class, method, or property declaration.(1242)

interface Baz implements Bam {}
// Interface declaration cannot have 'implements' clause.(1176)

abstract interface Bam {}
// 'abstract' modifier can only appear on a class, method, or property declaration.

TS parses these fine and emits a semantic error, not a syntactic error.
Our parser currently does not error on syntactically bad code, thus we also don't error on it (#1852).

We should remove these properties from the AST as it is non-sensical to include them - nobody should ever be inspecting the code that is produced on them.

[DMartens]

I intented to create an PR for this, but each of the properties seem to be intended as they are handled explicitly:

• TSEnumDeclaration and TSModuleDeclaration have an own private function in the converter to the modifiers
• TSInterfaceDeclaration explicitly sets implements if it is available

[bradzacher]

Yes it's implemented. But that doesn't mean it should be there!

Some of this code was written by non-TS folks back in the day - who worked to just convert the AST 1-for-1 without know what was right or wrong.

We should remove properties and the code that populates them!

[juank1809]

I was about to submit a PR to this issue, but I stumbled upon a realization: there are still valid modifiers, more specifically const and declare, to handle.

I think the best approach to this problem is to modify the code in order to restrict and forbid invalid modifiers within these nodes instead of removing it.

[bradzacher]

more specifically const and declare

These are handled via boolean flags.

typescript-eslint/packages/ast-spec/src/declaration/TSEnumDeclaration/spec.ts

Lines 11 to 12 in 18a81cb

	const?: boolean;
	declare?: boolean;

typescript-eslint/packages/ast-spec/src/declaration/TSModuleDeclaration/spec.ts

Line 24 in 18a81cb

declare?: boolean;

typescript-eslint/packages/ast-spec/src/declaration/TSInterfaceDeclaration/spec.ts

Line 16 in 18a81cb

declare?: boolean;

[bradzacher]

Another set of cases to be removed:

typescript-eslint/packages/ast-spec/src/element/TSIndexSignature/spec.ts

Line 13 in 8ed7219

export?: boolean;

typescript-eslint/packages/ast-spec/src/element/TSMethodSignature/spec.ts

Line 23 in 8ed7219

export?: boolean;

typescript-eslint/packages/ast-spec/src/element/TSPropertySignature/spec.ts

Line 21 in 8ed7219

export?: boolean;

typescript-eslint/packages/ast-spec/src/parameter/TSParameterProperty/spec.ts

Line 14 in 8ed7219

export?: boolean;

Syntactically the TS parser allows the export modifier, but semantically it disallows it.

[bradzacher]

Another case to be removed:

typescript-eslint/packages/ast-spec/src/element/TSPropertySignature/spec.ts

Line 18 in 8ed7219

initializer?: Expression;

Syntactically the TS parser allows an initialiser (likely due to it reusing parsing logic for class properties), but semantically it is disallowed.

[bradzacher]

Another case to be removed:

typescript-eslint/packages/ast-spec/src/base/MethodDefinitionBase.ts

Line 24 in 8ed7219

typeParameters?: TSTypeParameterDeclaration;

typeParameters is placed on the .value node, not the MethodDefinition

[JoshuaKGoldberg]

Fixed by #4863 and will be available in v6! 🚀