[CSStep/TypeVar] Check if loop should be stopped even if current binding has failed

Eagerly check if optimal solution has already been found even if
current binding attempt has failed, because otherwise it might
check too many bindings which would result in ambiguity.

Author: xedin

lib/Sema/CSStep.cpp

@@ -394,6 +394,11 @@ StepResult TypeVariableStep::take(bool prevFailed) {
         // let's try to see if it leads to any solutions.
         return suspend(SplitterStep::create(CS, Solutions));
       }
+
+      // If this binding didn't match, let's check
+      // if we've checked enough bindings to stop.
+      if (shouldStop())
+        break;
     }
   }
 
@@ -417,12 +422,10 @@ StepResult TypeVariableStep::resume(bool prevFailed) {
     log.indent(CS.solverState->depth * 2) << ")\n";
   }
 
-  // If there has been at least one solution so far
-  // at a current batch of bindings is done it's a
-  // success because each new batch would be less
-  // and less precise.
-  if (AnySolved && Producer.needsToComputeNext())
-    return done(/*isSuccess=*/true);
+  // Let's check if we should stop right before
+  // attempting any new bindings.
+  if (shouldStop())
+    return done(/*isSuccess=*/AnySolved);
 
   // Attempt next type variable binding.
   return take(prevFailed);

lib/Sema/CSStep.h

@@ -424,6 +424,17 @@ class TypeVariableStep final : public SolverStep {
                                   SmallVectorImpl<Solution> &solutions) {
     return new TypeVariableStep(cs, bindings, solutions);
   }
+
+private:
+  /// Check whether attempting type variable binding choices should
+  /// be stopped, because optimal solution has already been found.
+  bool shouldStop() const {
+    // If there has been at least one solution so far
+    // at a current batch of bindings is done it's a
+    // success because each new batch would be less
+    // and less precise.
+    return AnySolved && Producer.needsToComputeNext();
+  }
 };
 
 class DisjunctionStep final : public SolverStep {