While looking at PR38747 again I was diving into the alias.c code and tried to confirm that it does what I think it does for VIEW_CONVERT_EXPR. And in fact it basically does.
In general there are odds between what get_alias_set does if you pass it a reference tree compared to what it does if you pass it a type. In particular whether it returns the alias-set of the innermost or the outermost aliased component is not really consistent. But - onto that VIEW_CONVERT_EXPR. VIEW_CONVERT_EXPR is basically ignored / skipped, which means that if you have C++ code that does *(type2 *)&X and translate it to VIEW_CONVERT_EXPR <type2> (X) then you have generated wrong code (this is PR38747, forwprop does that). With VIEW_CONVERT_EXPR you can also easily create the situation where for a reference tree, let it be VIEW_CONVERT_EXPR <T2> (X.a).b like commonly seen in Ada, the alias-set of the outermost component is not a subset of that of the innermost one (the relationship that is usually assured to be true by the record_component_aliases machinery). You are probably safe here if only your frontend generates such conversions and it is very consistent on how it balances V_C_Es with accesses through pointers (because in the above case if .b is an addressable component an access via a pointer to type of b wouldn't necessarily alias the cited reference tree). The alias-oracle tries to use both the outermost and innermost alias-sets for disambiguations, but relies on the outermost alias-set being a subset of the innermost one - a relationship that V_C_E can break. So, what I'd like to know is in which circumstances the Ada frontend uses VIEW_CONVERT_EXPRs and what counter-measures it applies to ensure consistent TBAA. Thanks, Richard.