[Sema] Fix uninitialized OverloadCandidate::FoundDecl member

[jkorous]

Summary

It's rather error-prone to leave that member variable uninitialized. The DeclAccessPair seems to be intentionally POD and it seems the make() method is supposed to be called as a constructor.

I take DeclAccessPair interface as given so the only options are to either change OverloadCandidate interface or actually initialize the member variable it to some sane default. I suggest the easiest option.

Sema/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS
   Support
   )
 
 add_clang_unittest(SemaTests
   ExternalSemaSourceTest.cpp
   CodeCompleteTest.cpp
+  OverloadTest.cpp
   )
 
 target_link_libraries(SemaTests
   PRIVATE
   clangAST
   clangBasic
   clangFrontend
   clangParse
   clangSema
   clangTooling
   )

Sema/OverloadTest.cpp

+//=== unittests/Sema/CodeCompleteTest.cpp - Code Complete tests ==============//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/Sema/Overload.h"
+#include "gtest/gtest.h"
+
+namespace {
+
+TEST(SemaOverloadTest, FoundDeclIsInitializedInOverloadCandidate) {
+  clang::OverloadCandidate foo;
+  EXPECT_EQ(foo.FoundDecl.getDecl(), nullptr);
+  EXPECT_EQ(foo.FoundDecl.getAccess(), clang::AS_none);
+}
+
+} // namespace

clang/Sema/Overload.h

class Sema;
 
   /// A list of implicit conversion sequences for the arguments of an
   /// OverloadCandidate.
   using ConversionSequenceList =
       llvm::MutableArrayRef<ImplicitConversionSequence>;
 
   /// OverloadCandidate - A single candidate in an overload set (C++ 13.3).
   struct OverloadCandidate {
+
+    OverloadCandidate()
+    : FoundDecl( DeclAccessPair::make(nullptr, AS_none))
+    { }
+
     /// Function - The actual function that this candidate
     /// represents. When NULL, this is a built-in candidate
     /// (C++ [over.oper]) or a surrogate for a conversion to a
     /// function pointer or reference (C++ [over.call.object]).
     FunctionDecl *Function;
 
     /// FoundDecl - The original declaration that was looked up /
     /// invented / otherwise found, together with its access.