Corollary 2
Tag 01GF, permanent. Cite it as [Bourbaki, Tag 01GF].
Lie Groups and Lie Algebras, chapter IX, § 7. Irreducible representations of connected compact Lie groups
For all and every regular element of T,
where the two sums are over the elements of W.
Indeed, is non-zero, since is regular (formula (4)).
If is a central function on G, the restriction of to T is invariant under W, so is anti-invariant under W. Further, by Spectral Theories and Th. 1, the family is an algebraic basis of the space of central representative functions on G and an orthonormal basis of the space ZL of classes of square-integrable central functions on G.
Thus, from Prop. 3 and Th. 2 we deduce:
Bourbaki sets the proof directly after the statement, and the corpus does not separate them, so what is above is the statement and its proof together.