fix <rdar://problem/20911927> False positive in the "variable was never mutated" warning with IUO

Swift SVN r28583

Author: lattner

lib/Sema/MiscDiagnostics.cpp

@@ -1068,6 +1068,8 @@ markBaseOfAbstractStorageDeclStore(Expr *base, ConcreteDeclRef decl) {
    if (auto *TEE = dyn_cast<TupleElementExpr>(E))
      return markStoredOrInOutExpr(TEE->getBase(), Flags);
 
+   if (auto *FVE = dyn_cast<ForceValueExpr>(E))
+     return markStoredOrInOutExpr(FVE->getSubExpr(), Flags);
 
    // If we don't know what kind of expression this is, assume it's a reference
    // and mark it as a read.

test/decl/var/usage.swift

@@ -133,3 +133,12 @@ func testTuple() {
   var tup : (x:Int, y:Int)  // expected-warning {{variable 'tup' was written to, but never read}}
   tup.x = 1
 }
+
+
+/// <rdar://problem/20911927> False positive in the "variable was never mutated" warning with IUO
+func testForceValueExpr() {
+  var a: X! = nil  // no warning, mutated through the !
+  a!.g()
+}
+
+