Fix crash in BuildCXXDefaultInitExpr.

[teemperor]

Summary

Fix crash in BuildCXXDefaultInitExpr when member of template class
has same name as the class itself.

[rnk]

lgtm

test/SemaCXX/pr27047-default-init-expr-name-conflict.cpp

6

More reduced:

template <typename T> struct A { int A = 0; };
A<int> a;

lib/Sema/SemaDeclCXX.cpp

ExprResult Sema::BuildCXXDefaultInitExpr(SourceLocation Loc, FieldDecl *Field) {
   // Maybe we haven't instantiated the in-class initializer. Go check the
   // pattern FieldDecl to see if it has one.
   CXXRecordDecl *ParentRD = cast<CXXRecordDecl>(Field->getParent());
 
   if (isTemplateInstantiation(ParentRD->getTemplateSpecializationKind())) {
     CXXRecordDecl *ClassPattern = ParentRD->getTemplateInstantiationPattern();
     DeclContext::lookup_result Lookup =
         ClassPattern->lookup(Field->getDeclName());
-    assert(Lookup.size() == 1);
-    FieldDecl *Pattern = cast<FieldDecl>(Lookup[0]);
+
+    FieldDecl *Pattern = nullptr;
+
+    // It's possible that both the current CXXRecordDecl and
+    // the wanted FieldDecl appear in Lookup, so we iterate
+    // over it and check that there is exactly one FieldDecl
+    // and an optional unused CXXRecordDecl.
+    for (NamedDecl *PossibleDecl : Lookup) {
+      FieldDecl *PossibleFieldDecl = dyn_cast<FieldDecl>(PossibleDecl);
+
+      if (PossibleFieldDecl) {
+        // Will fail if there is more than one FieldDecl in Lookup.
+        assert(Pattern == nullptr);
+
+        Pattern = PossibleFieldDecl;
+      } else {
+        // Assert that the other declaration is the unused
+        // parent CXXRecordDecl.
+        assert(isa<CXXRecordDecl>(PossibleDecl));
+      }
+    }
+
+    // Will fail if there is no FieldDecl in Lookup.
+    assert(Pattern != nullptr);
+
     if (InstantiateInClassInitializer(Loc, Field, Pattern,
                                       getTemplateInstantiationArgs(Field)))
       return ExprError();
     return CXXDefaultInitExpr::Create(Context, Loc, Field);
   }
 
   // DR1351:
   //   If the brace-or-equal-initializer of a non-static data member

test/SemaCXX/pr27047-default-init-expr-name-conflict.cpp

+// RUN: %clang_cc1 -fsyntax-only -std=c++11 %s
+
+struct A {};
+
+template <typename T>
+struct B {
+
+  // Don't crash here
+  A B{};
+};
+
+int main() {
+  B<int> b;
+}