Sema: Fix crash when diagnosing invalid assignment expression

CSGen can see an AssignExpr where the destination has an
unresolved type, even in a case where no diagnostic has been
emitted yet.

It is then wrong to return an empty type from here, because
this stops us from attempting to solve the expression, which
results in no diagnostic being emitted in the end.

The test case I added produces a decent diagnostic now,
but the original one in the radar now just says 'ambiguous
without more context'. Still, better than crashing.

Fixes <rdar://problem/30685195>.

Author: slavapestov

lib/Sema/CSGen.cpp

@@ -2672,8 +2672,12 @@ namespace {
       // Compute the type to which the source must be converted to allow
       // assignment to the destination.
       auto destTy = CS.computeAssignDestType(expr->getDest(), expr->getLoc());
-      if (!destTy || destTy->getRValueType()->is<UnresolvedType>())
+      if (!destTy)
         return Type();
+      if (destTy->getRValueType()->is<UnresolvedType>()) {
+        return CS.createTypeVariable(CS.getConstraintLocator(expr),
+                                     TVO_CanBindToLValue);
+      }
 
       // The source must be convertible to the destination.
       CS.addConstraint(ConstraintKind::Conversion,

test/Constraints/lvalues.swift

@@ -232,3 +232,13 @@ func r23331567(_ fn: (_ x: inout Int) -> Void) {
 }
 r23331567 { $0 += 1 }
 
+// <rdar://problem/30685195> Compiler crash with invalid assignment
+struct G<T> {
+  subscript(x: Int) -> T { get { } nonmutating set { } }
+  // expected-note@-1 {{'subscript' declared here}}
+}
+
+func wump<T>(to: T, _ body: (G<T>) -> ()) {}
+
+wump(to: 0, { $0[] = 0 })
+// expected-error@-1 {{missing argument for parameter #1 in call}}