Discrete-spectrum base-change conjecture for complex symmetric spaces

Let $G$ be a real reductive group and let $\pi$ be an irreducible unitary representation of $G(\mathbb{C})$. Discrete-spectrum base-change conjecture. There is at most one pure inner form $G'$ of $G$ over $\mathbb{R}$, necessarily the quasi-split inner form, for which $\pi$ occurs in the discrete spectrum of $L^2(G'(\mathbb{R})\backslash G(\mathbb{C}))$. If it occurs, it has multiplicity one and does not occur in the continuous spectrum for any inner form. The discrete spectrum for $G(\mathbb{R})$ is the base change of the discrete spectrum for $G^{\rm op}$; in particular it is tempered and is nonzero exactly when the discrete spectrum for $G^{\rm op}$ is nonzero, with the final comparison stated in the source when $G^{\rm op}$ is not an inner form of $G$. This conjecture concerns the spectral decomposition of complex symmetric spaces and is reported as largely known in parts, but no complete resolution is supplied here.