13.2.1.1 Static Semantics: Early Errors:
> LexicalBinding : BindingIdentifier
> It is a Syntax Error if the BoundNames of BindingIdentifier contains "let".
> It is a Syntax Error if IsConstantDeclaration of the LexicalDeclaration containing this production is true.
The early error description mixes up two different rules, one with and one without the optional Initialiser.
1.
> It is a Syntax Error if IsConstantDeclaration of the LexicalDeclaration containing this production is true.
This early error only applies when there is no Initialiser.
2.
> It is a Syntax Error if the BoundNames of BindingIdentifier contains "let".
Whereas this rule applies independent of the presence of an initialiser.
fixed in rev21 editor's draft
I've just noticed there is a missing restriction for "let" in BindingPatterns, but maybe you've already noticed this when fixing this bug report. That means in addition to "LexicalBinding : BindingIdentifier", "LexicalBinding : BindingPattern" must also not contain "let" in the BoundNames of BindingPattern. Was that issue already fixed in this bug report?
fixed in rev21 draft