[cfe-dev] Smart pointers for canonical types

[Douglas Gregor]

Hello all,

Last week a few of us discussed potential improvements to Clang's use  
of canonical types. At issue is our continuing concern that it is too  
easy to accidentally compare non-canonical QualTypes, and each place  
we've made this mistake will only be exposed when some poor user has a  
typedef where we haven't yet seen one.

The attached patch introduces a new class template CanQual<T>, which  
is a smart pointer that refers to a canonical type whose dynamic type  
is T or some subclass thereof. T will often be Type, but we also  
expect that, e.g., CanQual<PointerType> will be used rather than const  
PointerType*.

ASTContext's getCanonicalType() now returns a CanQual<Type> (also  
named CanQualType). CanQual<T> provides the appropriate conversions to  
QualType and const T* so that existing code continues to work. We can  
gradually move type-checking code over toward CanQual<T> to improve  
safety. The design is such that, once complete, accessing members of  
PointerType through CanQual<PointerType> will always return  
*canonical* types (e.g., a CanQual<Type>) so that it's easy to stay  
within the static canonical-types system.

Comments and questions greatly appreciated!

	- Doug

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