Fix <rdar://problem/23433271> Swift compiler segfault in failure diagnosis

A crash in CSDiag that happened when we were unconditionally looking at the
getter of a subscript.  This failed on UnsafeMutablePointer because it only
has addressors, not getter/setters.

Author: lattner

lib/Sema/CSDiag.cpp

@@ -3326,10 +3326,13 @@ bool FailureDiagnosis::visitSubscriptExpr(SubscriptExpr *SE) {
     if (!SD) return { CC_GeneralMismatch, {}};
 
     // Check to make sure the base expr type is convertible to the expected base
-    // type.
+    // type.  We check either the getter, or if it isn't present, the addressor.
     auto selfConstraint = CC_ExactMatch;
+    auto getter = SD->getGetter();
+    if (!getter) getter = SD->getAddressor();
+    
     auto instanceTy =
-      SD->getGetter()->getImplicitSelfDecl()->getType()->getInOutObjectType();
+     getter->getImplicitSelfDecl()->getType()->getInOutObjectType();
     if (!isUnresolvedOrTypeVarType(baseType) &&
         !CS->TC.isConvertibleTo(baseType, instanceTy, CS->DC)) {
       selfConstraint = CC_SelfMismatch;

test/Constraints/diagnostics.swift

@@ -635,4 +635,11 @@ _ = -UnaryOp() // expected-error {{unary operator '-' cannot be applied to an op
 // expected-note @-1 {{overloads for '-' exist with these partially matching parameter lists: (Float), (Double),}}
 
 
+// <rdar://problem/23433271> Swift compiler segfault in failure diagnosis
+func f23433271(x : UnsafePointer<Int>) {}
+func segfault23433271(a : UnsafeMutablePointer<Void>) {
+  f23433271(a[0])  // expected-error {{cannot subscript a value of type 'UnsafeMutablePointer<Void>' (aka 'UnsafeMutablePointer<()>')}}
+}
+
+