/**
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* Converts destructured parameters with default values to non-shorthand syntax.
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* This fixes the only Tagged Templates-related bug in ES Modules-supporting browsers (Safari 10 & 11).
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* Use this plugin instead of `@babel/plugin-transform-template-literals` when targeting ES Modules.
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*
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* @example
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* // Bug 1: Safari 10/11 doesn't reliably return the same Strings value.
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* // The value changes depending on invocation and function optimization state.
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* function f() { return Object`` }
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* f() === new f() // false, should be true.
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*
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* @example
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* // Bug 2: Safari 10/11 use the same cached strings value when the string parts are the same.
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* // This behavior comes from an earlier version of the spec, and can cause tricky bugs.
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* Object``===Object`` // true, should be false.
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*
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* Benchmarks: https://jsperf.com/compiled-tagged-template-performance
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*/
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export default ({ types: t }) => ({
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name: "transform-tagged-template-caching",
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visitor: {
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TaggedTemplateExpression(path, state) {
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// tagged templates we've already dealt with
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let processed = state.get("processed");
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if (!processed) {
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processed = new WeakSet();
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state.set("processed", processed);
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}
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if (processed.has(path.node)) return path.skip();
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// Grab the expressions from the original tag.
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// tag`a${'hello'}` // ['hello']
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const expressions = path.node.quasi.expressions;
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// Create an identity function helper:
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// identity = t => t
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let identity = state.get("identity");
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if (!identity) {
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identity = path.scope
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.getProgramParent()
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.generateDeclaredUidIdentifier("_");
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state.set("identity", identity);
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const binding = path.scope.getBinding(identity.name);
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binding.path.get("init").replaceWith(
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t.arrowFunctionExpression(
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// re-use the helper identifier for compressability
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[t.identifier("t")],
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t.identifier("t")
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)
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);
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}
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// Use the identity function helper to get a reference to the template's Strings.
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// We replace all expressions with `0` ensure Strings has the same shape.
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// identity`a${0}`
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const template = t.taggedTemplateExpression(
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t.cloneNode(identity),
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t.templateLiteral(
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path.node.quasi.quasis,
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expressions.map(() => t.numericLiteral(0))
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)
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);
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processed.add(template);
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// Install an inline cache at the callsite using the global variable:
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// _t || (_t = identity`a${0}`)
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const ident = path.scope
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.getProgramParent()
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.generateDeclaredUidIdentifier("t");
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path.scope.getBinding(ident.name).path.parent.kind = "let";
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const inlineCache = t.logicalExpression(
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"||",
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ident,
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t.assignmentExpression("=", t.cloneNode(ident), template)
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);
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// The original tag function becomes a plain function call.
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// The expressions omitted from the cached Strings tag are directly applied as arguments.
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// tag(_t || (_t = Object`a${0}`), 'hello')
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const node = t.callExpression(path.node.tag, [
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inlineCache,
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...expressions,
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]);
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path.replaceWith(node);
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},
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},
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});
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