Index: cfe/trunk/lib/Sema/SemaChecking.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaChecking.cpp
+++ cfe/trunk/lib/Sema/SemaChecking.cpp
@@ -12383,12 +12383,6 @@
     return;
 
   const Type *BaseType = ArrayTy->getElementType().getTypePtr();
-  // It is possible that the type of the base expression after IgnoreParenCasts
-  // is incomplete, even though the type of the base expression before
-  // IgnoreParenCasts is complete (see PR39746 for an example). In this case we
-  // have no information about whether the array access is out-of-bounds.
-  if (BaseType->isIncompleteType())
-    return;
 
   Expr::EvalResult Result;
   if (!IndexExpr->EvaluateAsInt(Result, Context, Expr::SE_AllowSideEffects))
@@ -12405,6 +12399,15 @@
     ND = ME->getMemberDecl();
 
   if (index.isUnsigned() || !index.isNegative()) {
+    // It is possible that the type of the base expression after
+    // IgnoreParenCasts is incomplete, even though the type of the base
+    // expression before IgnoreParenCasts is complete (see PR39746 for an
+    // example). In this case we have no information about whether the array
+    // access exceeds the array bounds. However we can still diagnose an array
+    // access which precedes the array bounds.
+    if (BaseType->isIncompleteType())
+      return;
+
     llvm::APInt size = ArrayTy->getSize();
     if (!size.isStrictlyPositive())
       return;
Index: cfe/trunk/test/SemaCXX/array-bounds.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/array-bounds.cpp
+++ cfe/trunk/test/SemaCXX/array-bounds.cpp
@@ -287,9 +287,12 @@
 
 namespace PR39746 {
   struct S;
-  extern S xxx[2];
+  extern S xxx[2]; // expected-note {{array 'xxx' declared here}}
   class C {};
 
   C &f() { return reinterpret_cast<C *>(xxx)[1]; } // no-warning
+  // We have no info on whether this is out-of-bounds.
   C &g() { return reinterpret_cast<C *>(xxx)[2]; } // no-warning
+  // We can still diagnose this.
+  C &h() { return reinterpret_cast<C *>(xxx)[-1]; } // expected-warning {{array index -1 is before the beginning of the array}}
 }