[GSB] Infer same-type constraints among same-named associated types.

When we have an equivalence class that contains two unrelated
associated types with the same name, infer a same-type constraint
between those two associated types. This is a more principled way to
introduce these constraints that we had before, fixing a
recently-introduced regression.

Author: DougGregor

lib/AST/GenericSignatureBuilder.cpp

@@ -1662,6 +1662,7 @@ TypeDecl *EquivalenceClass::lookupNestedType(
 
   // Look for types with the given name in protocols that we know about.
   AssociatedTypeDecl *bestAssocType = nullptr;
+  llvm::SmallSetVector<AssociatedTypeDecl *, 4> assocTypeAnchors;
   SmallVector<TypeDecl *, 4> concreteDecls;
   for (const auto &conforms : conformsTo) {
     ProtocolDecl *proto = conforms.first;
@@ -1674,6 +1675,7 @@ TypeDecl *EquivalenceClass::lookupNestedType(
       if (auto assocType = dyn_cast<AssociatedTypeDecl>(member)) {
         // Retrieve the associated type anchor.
         assocType = assocType->getAssociatedTypeAnchor();
+        assocTypeAnchors.insert(assocType);
 
         if (!bestAssocType ||
              compareAssociatedTypes(assocType, bestAssocType) < 0)
@@ -1731,6 +1733,22 @@ TypeDecl *EquivalenceClass::lookupNestedType(
     }
   }
 
+  // Infer same-type constraints among same-named associated type anchors.
+  if (assocTypeAnchors.size() > 1) {
+    auto &builder = *members.front()->getBuilder();
+    auto anchorType = getAnchor({ });
+    auto inferredSource = FloatingRequirementSource::forInferred(nullptr);
+    for (auto assocType : assocTypeAnchors) {
+      if (assocType == bestAssocType) continue;
+
+      builder.addRequirement(
+        Requirement(RequirementKind::SameType,
+                    DependentMemberType::get(anchorType, bestAssocType),
+                    DependentMemberType::get(anchorType, assocType)),
+        inferredSource, nullptr);
+    }
+  }
+
   // Form the new cache entry.
   CachedNestedType entry;
   entry.numConformancesPresent = conformsTo.size();

test/Generics/requirement_inference.swift

@@ -154,8 +154,7 @@ protocol P10 {
 }
 
 // CHECK-LABEL: sameTypeConcrete1@
-// CHECK: Canonical generic signature: <τ_0_0 where τ_0_0 : P10, τ_0_0 : P9, τ_0_0.A == X3, τ_0_0.B == Int, τ_0_0.C == Int>
-// FIXME: Should have a second τ_0_0.A == X3
+// CHECK: Canonical generic signature: <τ_0_0 where τ_0_0 : P10, τ_0_0 : P9, τ_0_0.A == X3, τ_0_0.A == X3, τ_0_0.B == Int, τ_0_0.C == Int>
 func sameTypeConcrete1<T : P9 & P10>(_: T) where T.A == X3, T.C == T.B, T.C == Int { }
 
 // CHECK-LABEL: sameTypeConcrete2@