Sema: Fix bug with protocol compositions in expression context

If the left hand side was not a TypeExpr, we would just drop
it on the floor instead of giving up on forming a protocol
composition.

Author: slavapestov

lib/Sema/TypeCheckConstraints.cpp

@@ -1324,7 +1324,8 @@ TypeExpr *PreCheckExpression::simplifyTypeExpr(Expr *E) {
             return nullptr;
         else
           return nullptr;
-      }
+      } else
+        return nullptr;
 
       // Add the rhs which is just a TypeExpr
       auto *rhs = dyn_cast<TypeExpr>(binaryExpr->getArg()->getElement(1));

test/type/protocol_composition.swift

@@ -163,3 +163,13 @@ struct S05<T> where T : P5? & P6 {} // expected-error {{inheritance from non-nam
 struct S3124<T: protocol<P1, P3>> {} // expected-warning {{'protocol<...>' composition syntax is deprecated; join the protocols using '&'}} {{17-34=P1 & P3>}}
 func f3124_1<U where U: protocol<P1, P3>>(x: U)  {} // expected-warning {{'protocol<...>' composition syntax is deprecated; join the protocols using '&'}} {{25-42=P1 & P3>}} // expected-error {{'where' clause}}
 func f3124_2<U : protocol<P1>>(x: U)  {} // expected-warning {{'protocol<...>' composition syntax is deprecated and not needed here}} {{18-31=P1>}}
+
+// Make sure we correctly form compositions in expression context
+func takesP1AndP2(_: [AnyObject & P1 & P2]) {}
+
+takesP1AndP2([AnyObject & P1 & P2]())
+takesP1AndP2([Swift.AnyObject & P1 & P2]())
+takesP1AndP2([AnyObject & protocol_composition.P1 & P2]())
+takesP1AndP2([AnyObject & P1 & protocol_composition.P2]())
+takesP1AndP2([DoesNotExist & P1 & P2]()) // expected-error {{use of unresolved identifier 'DoesNotExist'}}
+takesP1AndP2([Swift.DoesNotExist & P1 & P2]()) // expected-error {{module 'Swift' has no member named 'DoesNotExist'}}