[clang] Improve error reporting for SFINAE

[paul Fultz]

This is to help improve error reporting with SFINAE as part of the bug report here:

http://llvm.org/bugs/show_bug.cgi?id=13309

So now when this program compiles:

template<class T> auto h(T x)->decltype(x.smurf()){return x.smurf();}
void h() {}
template<class T> auto g(T x)->decltype(h(x)){return h(x);}
template<class T> auto f(T x)->decltype(g(x)){return g(x);}
int main(){
  f(3);
}

Clang will report back this error with the true diagnostics:

smurf.cpp:5:3: error: no matching function for call to 'f'
  f(3);
  ^
smurf.cpp:1:42: note: candidate template ignored: substitution failure [with T = int]: member reference base type 'int' is not a structure or union
template<class T> auto h(T x)->decltype(x.smurf()){return x.smurf();}
                                         ^

Now this only does this when there is single overloads involved. If we change
the example to add an extra overload for `h`, like this:

template<class T> auto h(T x)->decltype(x.smurf()){return x.smurf();}
void h() {}
template<class T> auto g(T x)->decltype(h(x)){return h(x);}
template<class T> auto f(T x)->decltype(g(x)){return g(x);}
int main(){
  f(3);
}

Then we get an error like this instead:

smurf.cpp:6:3: error: no matching function for call to 'f'
  f(3);
  ^
smurf.cpp:3:41: note: candidate template ignored: substitution failure [with T = int]: no matching function for call to 'h'
template<class T> auto g(T x)->decltype(h(x)){return h(x);}
                                        ^

So clang basically stops when there is multiple overloads. If the user needs
to see more detail for this, we could look at adding an option for a full
backtrace in the future.

Either way, I think this is very useful to have, and improves the error
reporting in clang immensely. I would like to get this patch merged into
clang.

Index: include/clang/Sema/TemplateDeduction.h
===================================================================
--- include/clang/Sema/TemplateDeduction.h  (revision 230275)
+++ include/clang/Sema/TemplateDeduction.h  (working copy)
@@ -68,8 +68,10 @@
   /// \brief Take ownership of the SFINAE diagnostic.
   void takeSFINAEDiagnostic(PartialDiagnosticAt &PD) {
     assert(HasSFINAEDiagnostic);
-    PD.first = SuppressedDiagnostics.front().first;
-    PD.second.swap(SuppressedDiagnostics.front().second);
+    unsigned NumberOfDiagnostics = SuppressedDiagnostics.size();
+    PartialDiagnosticAt &PDiag = SuppressedDiagnostics[((NumberOfDiagnostics == 2) ? 1 : 0)];
+    PD.first = PDiag.first;
+    PD.second.swap(PDiag.second);
     SuppressedDiagnostics.clear();
     HasSFINAEDiagnostic = false;
   }
@@ -98,8 +100,6 @@
   /// \brief Add a new diagnostic to the set of diagnostics
   void addSuppressedDiagnostic(SourceLocation Loc,
                                PartialDiagnostic PD) {
-    if (HasSFINAEDiagnostic)
-      return;
     SuppressedDiagnostics.emplace_back(Loc, std::move(PD));
   }
 
Index: lib/Sema/SemaOverload.cpp
===================================================================
--- lib/Sema/SemaOverload.cpp   (revision 230275)
+++ lib/Sema/SemaOverload.cpp   (working copy)
@@ -9105,6 +9105,16 @@
       return;
     }
 
+    // If the diagnsotics is already for overload candidate substitution
+    // failure, then just emit the diagnostics as is.
+    if (PDiag && (PDiag->second.getDiagID() ==
+                      diag::note_ovl_candidate_substitution_failure ||
+                  PDiag->second.getDiagID() ==
+                      diag::note_ovl_candidate_disabled_by_enable_if)) {
+      S.Diag(PDiag->first, PDiag->second);
+      return;
+    }
+
     // Format the SFINAE diagnostic into the argument string.
     // FIXME: Add a general mechanism to include a PartialDiagnostic *'s
     //        formatted message in another diagnostic.
@@ -9116,7 +9126,7 @@
       PDiag->second.EmitToString(S.getDiagnostics(), SFINAEArgString);
     }
 
-    S.Diag(Templated->getLocation(),
+    S.Diag(PDiag->first,
            diag::note_ovl_candidate_substitution_failure)
         << TemplateArgString << SFINAEArgString << R;
     MaybeEmitInheritedConstructorNote(S, Templated);