[CSStep] Don't skip bindings inferred from defaults in diagnostic mode

Helps to diagnose some situations i.e. for-in loops where holes would
be propagated to the sequence otherwise.

Author: xedin

lib/Sema/CSStep.h

@@ -632,6 +632,14 @@ class TypeVariableStep final : public BindingStep<TypeVarBindingProducer> {
   bool attempt(const TypeVariableBinding &choice) override;
 
   bool shouldSkip(const TypeVariableBinding &choice) const override {
+    // Let's always attempt types inferred from "defaultable" constraints
+    // in diagnostic mode. This allows the solver to attempt i.e. `Any`
+    // for collection literals and produce better diagnostics for for-in
+    // statements like `for (x, y, z) in [] { ... }` when pattern type
+    // could not be inferred.
+    if (CS.shouldAttemptFixes())
+      return false;
+
     // If this is a defaultable binding and we have found solutions,
     // don't explore the default binding.
     return AnySolved && choice.isDefaultable();