[TypeChecker] Resolve a crash about ambiguous assignment closure

Having an invalid assignment in a closure could make the type checker
fail without providing a diagnostic, eventually crashing the compiler.
Now a diagnostic is provided.

This resolves SR-3053/SR-3073

Author: ahoppen

lib/Sema/CSApply.cpp

@@ -3294,8 +3294,11 @@ namespace {
       //
       // FIXME: This is also computed when the constraint system is set up.
       auto destTy = cs.computeAssignDestType(expr->getDest(), expr->getLoc());
-      if (!destTy)
+      if (!destTy) {
+        cs.TC.diagnose(expr->getLoc(), diag::type_of_expression_is_ambiguous)
+          .highlight(expr->getSourceRange());
         return nullptr;
+      }
       expr->getDest()->propagateLValueAccessKind(AccessKind::Write);
 
       // Convert the source to the simplified destination type.

test/expr/closure/inference.swift

@@ -45,3 +45,23 @@ func sr1976<T>(_ closure: (inout T) -> Void) {
 }
 
 sr1976({ $0 += 2 })
+
+// SR-3073: UnresolvedDotExpr in single expression closure
+
+func sr3073<S, T>(_ closure:(inout S, T) -> ()) {}
+
+sr3073({ $0.number1 = $1 }) //expected-error {{type of expression is ambiguous without more context}}
+
+struct SR3073Lense<Whole, Part> {
+  let set: (inout Whole, Part) -> ()
+}
+
+struct SR3073 {
+  var number1: Int
+  
+  func lenses() {
+    let _: SR3073Lense<SR3073, Int> = SR3073Lense(
+      set: { $0.number1 = $1 }
+    )
+  }
+}