What is the test for a defendant to prove that he would not have entered into a plea bargain if properly advised?

In order to satisfy his burden to prove prejudice, "the defendant must provide a declaration or testimony stating that he or she would not have entered into the plea bargain if properly advised. It is up to the trial court to determine whether the defendant's assertion is credible, and the court may reject an assertion that is not supported by an explanation or other corroborating circumstances." (People v. Martinez, supra, 57 Cal.4th at p. 565.) In determining whether a defendant meets this burden, "[c]ourts should not upset a plea solely because of post hoc assertions from a defendant

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about how he would have pleaded but for his attorney's deficiencies. Judges should instead look to contemporaneous evidence to substantiate a defendant's expressed preferences." (Lee v. United States, supra, ___ U.S. at p. ___ [137 S.Ct. at p. 1967].)