Sema: Fix crash with UnresolvedType in coerceExistential()

Fixes <rdar://problem/34357545>.

Author: slavapestov

lib/Sema/CSApply.cpp

@@ -5543,9 +5543,11 @@ Expr *ExprRewriter::coerceExistential(Expr *expr, Type toType,
   Type toInstanceType = toType;
 
   // Look through metatypes
-  while (fromInstanceType->is<AnyMetatypeType>() &&
+  while ((fromInstanceType->is<UnresolvedType>() ||
+          fromInstanceType->is<AnyMetatypeType>()) &&
          toInstanceType->is<ExistentialMetatypeType>()) {
-    fromInstanceType = fromInstanceType->castTo<AnyMetatypeType>()->getInstanceType();
+    if (!fromInstanceType->is<UnresolvedType>())
+      fromInstanceType = fromInstanceType->castTo<AnyMetatypeType>()->getInstanceType();
     toInstanceType = toInstanceType->castTo<ExistentialMetatypeType>()->getInstanceType();
   }
 

test/Constraints/diagnostics.swift

@@ -1187,3 +1187,9 @@ func badTypes() {
   // expected-error@-1 {{type of expression is ambiguous without more context}}
   // FIXME: terrible diagnostic
 }
+
+// rdar://34357545
+func unresolvedTypeExistential() -> Bool {
+  return (Int.self==_{})
+  // expected-error@-1 {{ambiguous reference to member '=='}}
+}

validation-test/compiler_crashers/28739-unreachable-executed-at-swift-lib-ast-type-cpp-229.swift renamed to validation-test/compiler_crashers_fixed/28739-unreachable-executed-at-swift-lib-ast-type-cpp-229.swift

@@ -5,5 +5,5 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+// RUN: not %target-swift-frontend %s -emit-ir
 (Int==_{