DWARF doesn't describe templates itself but only actual template instantiations.
Because of that LLDB has to infer the parameters of the class template declarations
from the actual instantiations when creating the internal Clang AST from debug info
Because there is no dedicated DIE for the class template, LLDB also creates the ClassTemplateDecl
implicitly when parsing a template instantiation. To avoid creating one ClassTemplateDecls for every
instantiation, TypeSystemClang::CreateClassTemplateDecl will check if there is already a
ClassTemplateDecl in the requested DeclContext and will reuse a found fitting declaration.
The logic that checks if a found class template fits to an instantiation is currently just comparing
the name of the template. So right now we map template<typename T> struct S; to
an instantiation with the values S<1, 2, 3> even though they clearly don't belong together.
This causes crashes later on when for example the Itanium mangler's
TemplateArgManglingInfo::needExactType method tries to find fitting the class template parameter
that fits to an instantiation value. In the example above it will try to find the parameter for
the value 2 but will just trigger a boundary check when retrieving the parameter with index 1
from the class template.
There are two ways we can end up with an instantiation that doesn't fit to a class template with
the same name:
- We have two TUs with two templates that have the same name and internal linkage.
- A forward declared template instantiation is emitted by GCC and Clang without an empty list of parameter values.
This patch makes the check for whether a class template declaration can be reused more sophisticated by
also comparing whether the parameter values can fit to the found class template. If we can't find a fitting
class template we justcreate a second class template with the fitting parameters.