[cfe-dev] Type source info for function template instances.

[Enea Zaffanella]

John McCall wrote:
> Can you not "reattach" default argument expressions during
> TemplateDeclInstantiator::SubstFunctionType()?  Or even better,
> transform them appropriately in TreeTransform.  I would be much more
> comfortable doing either of these than suppressing diagnostics
> assumed to be redundant.
> 
> John.
> 

Hello John.

Please find attached a revised patch.

In this version, we no longer visit parameters twice,
so we no longer need to suppress diagnostics.

We (re-)discovered that we are NOT allowed to substitute the default 
argument expressions when instantiating the function decl, because this 
could eagerly generate diagnostics that should only be produced if and 
when the default argument is actually used.

We added a new helper named
    SubstFunctionTypeSourceInfo
which mimics the behavior of the existing
    SubstFunctionType
but produces a TypeSourceInfo instead of a QualType.
We factored the common code that checks parameters in
    CheckInstantiatedParams.


The patch passes all but a single test:

Failing Tests (1):
     Clang :: SemaTemplate/instantiate-expr-2.cpp

An assertion is failing:

clang: Sema.h:3416: clang::Decl* 
clang::Sema::LocalInstantiationScope::getInstantiationOf(const 
clang::Decl*): Assertion `Result && "declaration was not instantiated in 
this scope!"' failed.

The relevant code in the example is the following:
================
template<typename T, unsigned long N> struct IntegralConstant { };

template<typename T>
struct X0 {
   void f(T x, IntegralConstant<T, sizeof(x)>);
};

void test_X0(X0<int> x, IntegralConstant<int, sizeof(int)> ic) {
   x.f(5,ic);
}
================

The assertion crash disappears if you replace sizeof(x) with sizeof(T) 
in the declaration of method X0<T>::f.

Could this be an already existing, but hidden bug that was uncovered by 
our patch?

Cheers,
Enea Zaffanella.
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