The crash is is caused by the double template instantiation.
See the added test. Here is what happens:
- Template arguments for the partial specialization get instantiated.
- This causes instantitation into the corrensponding requires expression.
- TemplateInsantiator correctly handles instantiation of parameters inside RequiresExprBody and instantiates the constraint expression inside the NestedRequirement.
- To build the substituted NestedRequirement, TemplateInsantiator calls Sema::BuildNestedRequirement calls CheckConstraintSatisfaction, which results in another template instantiation (with empty template arguments). This seem to be an implementation detail to handle constraint satisfaction and is not required by the standard.
- The recursive template instantiation tries to find the parameter inside RequiresExprBody and fails with the corresponding assertion.
Note that this only happens as both instantiations happen with the class
partial template specialization set as Sema.CurContext, which is
considered a dependent DeclContext.
To fix the assertion, avoid doing the recursive template instantiation
and instead evaluate resulting expressions in-place.