[ConstraintSystem] Attempt conjunction before closure result or generic parameter holes

Closure result type or generic parameter associated with such a location
could bw inferred from a body of a multi-statement closure (when inference
is enabled), so we need to give closure a chance to run before attemtping
a hole for such positions in diagnostic mode.

Author: xedin

include/swift/Sema/CSBindings.h

@@ -338,6 +338,14 @@ class BindingSet {
 
   TypeVariableType *getTypeVariable() const { return Info.TypeVar; }
 
+  /// Check whether this binding set belongs to a type variable
+  /// that represents a result type of a closure.
+  bool forClosureResult() const;
+
+  /// Check whether this binding set belongs to a type variable
+  /// that represents a generic parameter.
+  bool forGenericParameter() const;
+
   bool canBeNil() const;
 
   /// If this type variable doesn't have any viable bindings, or

lib/Sema/CSBindings.cpp

@@ -25,6 +25,14 @@ using namespace swift;
 using namespace constraints;
 using namespace inference;
 
+bool BindingSet::forClosureResult() const {
+  return Info.TypeVar->getImpl().isClosureResultType();
+}
+
+bool BindingSet::forGenericParameter() const {
+  return bool(Info.TypeVar->getImpl().getGenericParameter());
+}
+
 bool BindingSet::canBeNil() const {
   auto &ctx = CS.getASTContext();
   return Literals.count(

lib/Sema/CSStep.cpp

@@ -345,6 +345,19 @@ StepResult ComponentStep::take(bool prevFailed) {
   auto *disjunction = CS.selectDisjunction();
   auto bestBindings = CS.determineBestBindings();
 
+  if (CS.shouldAttemptFixes()) {
+    if ((bestBindings &&
+         (bestBindings->forClosureResult() ||
+          bestBindings->forGenericParameter()) &&
+         bestBindings->isHole()) &&
+        !disjunction) {
+      if (auto *conjunction = CS.selectConjunction()) {
+        return suspend(
+            std::make_unique<ConjunctionStep>(CS, conjunction, Solutions));
+      }
+    }
+  }
+
   if (bestBindings &&
       (!disjunction || bestBindings->favoredOverDisjunction(disjunction))) {
     // Produce a type variable step.