[CSGen] Use new UnresolvedMemberChainBase constraint

Author: xedin

lib/Sema/CSBindings.cpp

@@ -1004,8 +1004,8 @@ PotentialBindings::inferFromRelational(Constraint *constraint) {
     // Don't allow a protocol type to get propagated from the base to the result
     // type of a chain, Result should always be a concrete type which conforms
     // to the protocol inferred for the base.
-    if (locator->isLastElement<LocatorPathElt::UnresolvedMemberChainResult>() &&
-        kind == AllowedBindingKind::Supertypes && type->is<ProtocolType>())
+    if (constraint->getKind() == ConstraintKind::UnresolvedMemberChainBase &&
+        kind == AllowedBindingKind::Subtypes && type->is<ProtocolType>())
       return None;
   }
 
@@ -1103,7 +1103,8 @@ PotentialBindings::inferFromRelational(Constraint *constraint) {
 
     case ConstraintKind::Bind:
     case ConstraintKind::BindParam:
-    case ConstraintKind::Equal: {
+    case ConstraintKind::Equal:
+    case ConstraintKind::UnresolvedMemberChainBase: {
       EquivalentTo.insert({bindingTypeVar, constraint});
       break;
     }

lib/Sema/CSGen.cpp

@@ -1446,8 +1446,8 @@ namespace {
       // whole chain, and the type of the whole chain must be equal to the base.
       CS.addConstraint(ConstraintKind::Conversion, memberTy, chainResultTy,
                        locator);
-      CS.addConstraint(ConstraintKind::Equal, chainBaseTy, chainResultTy,
-                       locator);
+      CS.addConstraint(ConstraintKind::UnresolvedMemberChainBase, chainResultTy,
+                       chainBaseTy, locator);
 
       return chainResultTy;
     }

test/expr/delayed-ident/member_chains.swift

@@ -149,8 +149,8 @@ let _: ImplicitMembers = .implicit[optional: ()] // expected-error {{value of op
 let _: ImplicitMembers = .implicit[funcOptional: ()]() // expected-error {{value of optional type 'ImplicitMembers?' must be unwrapped to a value of type 'ImplicitMembers'}} expected-note {{coalesce using '??' to provide a default when the optional value contains 'nil'}} {{55-55= ?? <#default value#>}} expected-note {{force-unwrap using '!' to abort execution if the optional value contains 'nil'}} {{55-55=!}}
 
 // FIXME: Improve these diagnostics (should probably offer unwrapping, as above)
-let _: ImplicitMembers = .implicit.anotherOptional?.another // expected-error{{type of expression is ambiguous without more context}}
-let _: ImplicitMembers = .implicit[optionalFunc: ()]?() // expected-error{{type of expression is ambiguous without more context}}
+let _: ImplicitMembers = .implicit.anotherOptional?.another // expected-error{{cannot convert value of type 'Optional<_>' to specified type 'ImplicitMembers'}}
+let _: ImplicitMembers = .implicit[optionalFunc: ()]?() // expected-error{{cannot convert value of type 'Optional<_>' to specified type 'ImplicitMembers'}}
 
 
 let _: ImplicitMembers = .other.implicit

unittests/Sema/BindingInferenceTests.cpp

@@ -270,7 +270,8 @@ TEST_F(SemaTest, TestComplexTransitiveProtocolInference) {
   // T2 <: T3, T2 <: T1, T3 == T1
   cs.addConstraint(ConstraintKind::Subtype, typeVar2, typeVar3, nilLocator);
   cs.addConstraint(ConstraintKind::Conversion, typeVar2, typeVar1, nilLocator);
-  cs.addConstraint(ConstraintKind::Equal, typeVar3, typeVar1, nilLocator);
+  cs.addConstraint(ConstraintKind::UnresolvedMemberChainBase, typeVar3,
+                   typeVar1, nilLocator);
   // T1 == T5, T <: T6
   cs.addConstraint(ConstraintKind::Equal, typeVar1, typeVar5, nilLocator);
   cs.addConstraint(ConstraintKind::Conversion, typeVar5, typeVar6, nilLocator);