Index: cfe/trunk/lib/Sema/SemaChecking.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaChecking.cpp
+++ cfe/trunk/lib/Sema/SemaChecking.cpp
@@ -12379,10 +12379,19 @@
       BaseExpr->getType()->getPointeeOrArrayElementType();
   BaseExpr = BaseExpr->IgnoreParenCasts();
   const ConstantArrayType *ArrayTy =
-    Context.getAsConstantArrayType(BaseExpr->getType());
+      Context.getAsConstantArrayType(BaseExpr->getType());
+
   if (!ArrayTy)
     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))
     return;
@@ -12402,7 +12411,6 @@
     if (!size.isStrictlyPositive())
       return;
 
-    const Type *BaseType = BaseExpr->getType()->getPointeeOrArrayElementType();
     if (BaseType != EffectiveType) {
       // Make sure we're comparing apples to apples when comparing index to size
       uint64_t ptrarith_typesize = Context.getTypeSize(EffectiveType);
Index: cfe/trunk/test/SemaCXX/array-bounds.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/array-bounds.cpp
+++ cfe/trunk/test/SemaCXX/array-bounds.cpp
@@ -284,3 +284,12 @@
 int test_struct_multiarray() {
   return multi2[4].arr[0]; // expected-warning {{array index 4 is past the end of the array (which contains 4 elements)}}
 }
+
+namespace PR39746 {
+  struct S;
+  extern S xxx[2];
+  class C {};
+
+  C &f() { return reinterpret_cast<C *>(xxx)[1]; } // no-warning
+  C &g() { return reinterpret_cast<C *>(xxx)[2]; } // no-warning
+}