Fix template parameter default args missed if redecled

[erichkeane]

Summary

This bug was found via self-build on lld, and worked around
here: https://reviews.llvm.org/rL316180

The issue is that the 'using' causes the lookup to pick up the
first decl. However, when setting inherited default parameters,
we only update 'forward', not 'backward'. SO, only the newest param
list has all the information about the default arguments.

This patch ensures that the list of parameters we look through checks
the newest decl's template parameter list so it doesn't miss a default.

[mstorsjo]

lib/Sema/SemaTemplate.cpp

4808 ↗

(On Diff #119665)

Typo in "recentdeclaration"

4811 ↗

(On Diff #119665)

How does this work if there's another forward declaration missing the parameter later? Is the logic with "most recent" ok, or should it be "most qualified/complete" instead?

[erichkeane]

lib/Sema/SemaTemplate.cpp

4811 ↗

(On Diff #119665)

I'm not sure the case you mean. Any future declarations with fewer parameters would be aspecialization and a different decl, right?

[mstorsjo]

lib/Sema/SemaTemplate.cpp

4811 ↗

(On Diff #119665)

Sorry, I meant a declaration with fewer defaults. E.g. the original forward declaration without defaults again after the actual definition.

E.g. by adding

namespace llvm {
  template<typename T > struct StringSet;
}

before the last namespace lld.

[erichkeane]

lib/Sema/SemaTemplate.cpp

4811 ↗

(On Diff #119665)

I think thats OK. The code propagates those forward. The problem here is that it does NOT propagate them backwards.

[mstorsjo]

lib/Sema/SemaTemplate.cpp

4811 ↗

(On Diff #119665)

Ok then, then I guess it sounds fine to me, although I'm very much not familiar with this code. So I'm not comfortable with giving it a formal approval.

[rnk]

lgtm

[rnk]

I think you forgot to svn add the test case

[erichkeane]

In D39127#904685, @rnk wrote:

I think you forgot to svn add the test case

Ugg... thanks for the heads up! Added in r316437

cfe/trunk/lib/Sema/SemaTemplate.cpp

bool Sema::CheckTemplateArgumentList(
     TemplateArgumentListInfo &TemplateArgs, bool PartialTemplateArgs,
     SmallVectorImpl<TemplateArgument> &Converted,
     bool UpdateArgsWithConversions) {
   // Make a copy of the template arguments for processing.  Only make the
   // changes at the end when successful in matching the arguments to the
   // template.
   TemplateArgumentListInfo NewArgs = TemplateArgs;
 
-  TemplateParameterList *Params = Template->getTemplateParameters();
+  // Make sure we get the template parameter list from the most
+  // recentdeclaration, since that is the only one that has is guaranteed to
+  // have all the default template argument information.
+  TemplateParameterList *Params =
+      cast<TemplateDecl>(Template->getMostRecentDecl())
+          ->getTemplateParameters();
 
   SourceLocation RAngleLoc = NewArgs.getRAngleLoc();
 
   // C++ [temp.arg]p1:
   //   [...] The type and form of each template-argument specified in
   //   a template-id shall match the type and form specified for the
   //   corresponding parameter declared by the template in its
   //   template-parameter-list.