Template parser can't handle let declaration with syntactic placeholder variable

Bug Report

• I would like to work on a fix!

Current behavior

When template is called with a let statement containing a syntactic placeholder for the variable (e.g. let %%foo%% = 1), the parser rejects it and mysteriously complains about Unexpected reserved word 'let'.

Welcome to Node.js v15.6.0.
Type ".help" for more information.
> require("@babel/template/package.json").version
'7.12.7'
> template = require("@babel/template").default
> template("let %%foo%% = 1")()
Uncaught [SyntaxError: Unexpected reserved word 'let' (2:0)
  1 | /* @babel/template */;
> 2 | let %%foo%% = 1
    =============
] {
  loc: Position { line: 2, column: 0 },
  pos: 23,
  code: 'BABEL_TEMPLATE_PARSE_ERROR'
}

Input Code

template("let %%foo%% = 1")({ foo: "x" });

These also give the same syntax error:

template.statement("let %%foo%% = 1")({ foo: "x" });
template.statements("let %%foo%% = 1")({ foo: "x" });
template.program("let %%foo%% = 1")({ foo: "x" });

Expected behavior

The template invocation should return an AST for let x = 1.

These invocations return an AST as expected:

// No placeholder
template("let x = 1")();

// Identifier placeholder
template("let FOO = 1;")({ FOO: "x" });

// const or var instead of let
template("const %%foo%% = 1")({ foo: "x" });
template("var %%foo%% = 1")({ foo: "x" });

// Ordinary identifier appears first
template("let a = 0, %%foo%% = 1")({ foo: "x" });

// Array destructuring
template("let [%%foo%%] = []")({ foo: "x" });

Babel Configuration (babel.config.js, .babelrc, package.json#babel, cli command, .eslintrc)

• Importing @babel/template directly from the Node repl; no Babel config.

Environment

$ npx envinfo --preset babel

  System:
    OS: macOS High Sierra 10.13.6
  Binaries:
    Node: 15.6.0 - /usr/local/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 7.4.0 - /usr/local/bin/npm
  npmPackages:
    @babel/template: ^7.12.7 => 7.12.7 

I can also reproduce this in AST Explorer and in RunKit, so it’s unlikely to be environment-specific.

Possible Solution

Additional context

[Zalathar]

I already have a slightly different solution, but either way the change is small, so we can worry about the details after I write some more tests and post a PR.

[Zalathar]

While writing tests I noticed that let %%lhs%%; and var %%lhs%%; (without an initializer) also don’t work, for reasons that are similar to this issue but not quite the same.

I’ll leave those alone for now since they’re easier to work around (just initialize to undefined) and I don’t want to deal with scope-creep.