[Sema] Add a comment on an identified bug on default arguments.

[timshen]

Summary

The mis-compile is triggered by internal code, but I haven't reduced it to a small piece of code. Add a FIXME here, since a decent fix doesn't seem to be trivial.

The decent fix can be changing Decl::Init to PointerUnion<Stmt *, EvaluatedStmt *, ParamVarDecl *>, and make setUninstantiatedDefaultArg take a ParamVarDecl *, which contains the Expr * as the default argument. This way, getTemplateInstantiationArgs can take that ParamVarDecl and do the right thing.

clang/lib/Sema/SemaExpr.cpp

bool Sema::CheckCXXDefaultArgExpr(SourceLocation CallLoc, FunctionDecl *FD,
   
   if (Param->hasUninstantiatedDefaultArg()) {
     Expr *UninstExpr = Param->getUninstantiatedDefaultArg();
 
     EnterExpressionEvaluationContext EvalContext(
         *this, ExpressionEvaluationContext::PotentiallyEvaluated, Param);
 
     // Instantiate the expression.
+    //
+    // FIXME: Pass in a correct Pattern argument, otherwise
+    // getTemplateInstantiationArgs uses the lexical context of FD, e.g.
+    //
+    // template<typename T>
+    // struct A {
+    //   static int FooImpl();
+    //
+    //   template<typename Tp>
+    //   // bug: default argument A<T>::FooImpl() is evaluated with 2-level
+    //   // template argument list [[T], [Tp]], should be [[Tp]].
+    //   friend A<Tp> Foo(int a);
+    // };
+    //
+    // template<typename T>
+    // A<T> Foo(int a = A<T>::FooImpl());
     MultiLevelTemplateArgumentList MutiLevelArgList
       = getTemplateInstantiationArgs(FD, nullptr, /*RelativeToPrimary=*/true);
 
     InstantiatingTemplate Inst(*this, CallLoc, Param,
                                MutiLevelArgList.getInnermost());
     if (Inst.isInvalid())
       return true;
     if (Inst.isAlreadyInstantiating()) {