refactor: simplify toAssignable routine

[JLHwung]

Q	A
Tests Pass?	Yes
License	MIT

This PR is a series of parser refactoring and is supposed to review commit-by-commit. It focuses on simplifying the toAssignable routine

babel/packages/babel-parser/src/parser/lval.js

Line 53 in de75dbf

toAssignable(

RemoveisBinding parameter

The first commit removes the isBinding parameter. This parameter is used in toAssignableList in typescript plugin and meant to check if the TSAsExpression or TSTypeAssertion is allowed to casted as type parameters.

babel/packages/babel-parser/src/plugins/typescript/index.js

Line 2540 in de75dbf

if (!isBinding) {

However it suffices to check state.maybeInArrowParameters since toAssignableList is called with true only in

babel/packages/babel-parser/src/parser/expression.js

Line 1879 in de75dbf

setArrowFunctionParameters(

Other than that, this parameter is also used to skip MemberExpression check, which was copied from acorn.

babel/packages/babel-parser/src/parser/lval.js

Line 145 in 2b23c28

case "MemberExpression":

Apparently the check here is now redundant because the default branch is noop.

Remove contextDescription parameter

The contextDescription was introduced in babel/babylon#123 in order to provide better error message for the default case of toAssignable
https://github.com/babel/babylon/pull/123/files#diff-3d279f04a39b1aaba84ac7b20b0f2625R62

However the error message was moved to checkLVal later so the context description is now redundant. It prevails in the calls of toAssignable so it is difficult for linters to detect this variable is unused.

Remove unnecessary nullish check

The nullish check not only contradict with the type annotation of toAssignable

babel/packages/babel-parser/src/parser/lval.js

Line 54 in 2b23c28

node: Node,

where node is indeed a Node, but also can be proved unnecessary here. Among the possible syntax nodes where toAssignable will be called, only array elements can be nullish. However the nullish check of array elements has been implemented in

babel/packages/babel-parser/src/parser/lval.js

Line 210 in 2b23c28

if (elt) {

therefore the nullish check is unecessary.

Simplify on the ParenthesizedExpression

toAssignable will unwrap parenthesized expression again after it is unwrapped in the dedicate ParenthesizedExpression check.

babel/packages/babel-parser/src/parser/lval.js

Line 64 in 2b23c28

const parenthesized = unwrapParenthesizedExpression(node);

The second unwrap is unnecessary. Also added a test case for the SyntaxError: Invalid left-hand side in parenthesized expression (1:1) error.