U Test

Need of U-Test in the presence of T-Test:

The two sample t-test is one of the most used statistical procedures. Its purpose is to test the hypothesis that the means of two groups are the same. The test assumes that the variable in question is normally distributed in two groups. When this assumption is in doubt, the non-parametric Wilcoxon-Mann-Whitney (or rank sum ) test is sometimes suggested as an alternative to the t-test (e.g. the Wikipedia page on the t-test), which doesn't rely on distributional assumptions.
U-Test is to be used as an alternative to the two sample t-test (for example because the normality assumption made by the latter is in doubt), it would seem a reasonable requirement that it ought to be testing the same 'thing'. What null and alternative hypotheses is U-Test testing? Although papers or books may present a single set of hypotheses, it turns out the WMW test is valid under a range of different sets of possible null and alternative hypotheses [see this paper by Fay and Proschan](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2857732/)