united-states
Lying and making unduly optimistic statements about the future are two different things.
A legally actionable fraudulent misstatement of fact (either at common law or on a securities fraud theory) must be about a presently existing fact that is not an opinion, or must fairly imply a presently existing fact. See, e.g., Investopedia ("Misrepresentation applies only to statements of fact, not to opinions or predictions.").
A statement that a technological breakthrough will be made is a "forward looking statement" about a fact that does not presently exist. It can only be the basis of a lawsuit if the person making it had no plausible reason to believe that it would become true (e.g. the firm isn't working on, or having someone else work on, or expecting to buy from someone else, this breakthrough). This is defined is U.S. Securities law as follows:
(1) Forward-looking statement The term “forward-looking statement”
means— (A) a statement containing a projection of revenues, income
(including income loss), earnings (including earnings loss) per share,
capital expenditures, dividends, capital structure, or other financial
items; (B) a statement of the plans and objectives of management for
future operations, including plans or objectives relating to the
products or services of the issuer; (C) a statement of future economic
performance, including any such statement contained in a discussion
and analysis of financial condition by the management or in the
results of operations included pursuant to the rules and regulations
of the Commission; (D) any statement of the assumptions underlying or
relating to any statement described in subparagraph (A), (B), or (C);
(E) any report issued by an outside reviewer retained by an issuer, to
the extent that the report assesses a forward-looking statement made
by the issuer; or (F) a statement containing a projection or estimate
of such other items as may be specified by rule or regulation of the
Commission.
15 USC § 78u-5(i)(1).
Part (1)(B) of this definition would include expected technological breakthroughs.
Someone can make a contractual promise that something will be done by a particular date. But a statement that a technological breakthrough will happen, which is made to investors and the general public, is not a promise and is not legally binding as a contract.
Generally, a prospectus (i.e. a formal communication made to investors) will specifically disclaim this liability. In the case of Elon Musk's company, the prospectus says:
We may fail to meet our publicly announced guidance or other
expectations about our business, which could cause our stock price to
decline.
We provide guidance regarding our expected financial and business
performance, such as projections regarding sales and production, as
well as anticipated future revenues, gross margins, profitability and
cash flows. Correctly identifying key factors affecting business
conditions and predicting future events is inherently an uncertain
process, and our guidance may not ultimately be accurate and has in
the past been inaccurate in certain respects, such as the timing of
new product manufacturing ramps. Our guidance is based on certain
assumptions such as those relating to global and local economic
conditions, anticipated production and sales volumes (which generally
are not linear throughout a given period), average sales prices,
supplier and commodity costs, and planned cost reductions. If our
guidance is not accurate or varies from actual results due to our
inability to meet our assumptions or the impact on our financial
performance that could occur as a result of various risks and
uncertainties, the market value of our common stock could decline
significantly.
Disclaimers of this kind are ubiquitous in statements made by publicly held companies which are subject to U.S. securities law regulation.