[CSBindings] Infer transitive protocols only for unresolved member base

Nothing besides static member refs on protocols feature is currently
using transitive protocols, so instead of trying to infer them on every
step let's do that only for base type of a dot-syntax reference when
there are no other bindings for it.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -461,7 +461,6 @@ void BindingSet::inferTransitiveBindings(
 
 void BindingSet::finalize(
     llvm::SmallDenseMap<TypeVariableType *, BindingSet> &inferredBindings) {
-  inferTransitiveProtocolRequirements(inferredBindings);
   inferTransitiveBindings(inferredBindings);
 
   determineLiteralCoverage();
@@ -478,11 +477,14 @@ void BindingSet::finalize(
       // func foo<T: P>(_: T) {}
       // foo(.bar) <- `.bar` should be a static member of `P`.
       // \endcode
-      if (!hasViableBindings() && TransitiveProtocols.hasValue()) {
-        for (auto *constraint : *TransitiveProtocols) {
-          auto protocolTy = constraint->getSecondType();
-          addBinding(
-              {protocolTy, AllowedBindingKind::Exact, constraint});
+      if (!hasViableBindings()) {
+        inferTransitiveProtocolRequirements(inferredBindings);
+
+        if (TransitiveProtocols.hasValue()) {
+          for (auto *constraint : *TransitiveProtocols) {
+            auto protocolTy = constraint->getSecondType();
+            addBinding({protocolTy, AllowedBindingKind::Exact, constraint});
+          }
         }
       }
     }

unittests/Sema/BindingInferenceTests.cpp

@@ -330,16 +330,7 @@ TEST_F(SemaTest, TestTransitiveProtocolInferenceThroughEquivalenceChains) {
   cs.addConstraint(ConstraintKind::ConformsTo, typeVar2, protocolTy0, nilLocator);
   cs.addConstraint(ConstraintKind::ConformsTo, typeVar3, protocolTy1, nilLocator);
 
-  llvm::SmallDenseMap<TypeVariableType *, BindingSet> cache;
-  for (auto *typeVar : cs.getTypeVariables()) {
-    cache.insert({typeVar, cs.getBindingsFor(typeVar, /*finalize=*/false)});
-  }
-
-  auto bindingSet = cache.find(typeVar0);
-  assert(bindingSet != cache.end());
-
-  auto &bindings = bindingSet->getSecond();
-  bindings.inferTransitiveProtocolRequirements(cache);
+  auto bindings = inferBindings(cs, typeVar0);
 
   ASSERT_TRUE(bool(bindings.TransitiveProtocols));
   verifyProtocolInferenceResults(*bindings.TransitiveProtocols,

unittests/Sema/SemaFixture.cpp

@@ -136,6 +136,7 @@ BindingSet SemaTest::inferBindings(ConstraintSystem &cs,
       continue;
 
     auto &bindings = cachedBindings->getSecond();
+    bindings.inferTransitiveProtocolRequirements(cache);
     bindings.finalize(cache);
   }