[ConstraintSystem] NFC: Extract opening of individual requirements into a method

It's a convenient way to use existing logic for default argument inference
because suhc inference cannot open whole signature but only conformance
and layout constraints associated generic parameters used in a particular
parameter position.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -4176,6 +4176,14 @@ class ConstraintSystem {
                                ConstraintLocatorBuilder locator,
                                llvm::function_ref<Type(Type)> subst);
 
+  // Record the given requirement in the constraint system.
+  void openGenericRequirement(DeclContext *outerDC,
+                              unsigned index,
+                              const Requirement &requirement,
+                              bool skipProtocolSelfConstraint,
+                              ConstraintLocatorBuilder locator,
+                              llvm::function_ref<Type(Type)> subst);
+
   /// Record the set of opened types for the given locator.
   void recordOpenedTypes(
          ConstraintLocatorBuilder locator,

lib/Sema/ConstraintSystem.cpp

@@ -1615,38 +1615,46 @@ void ConstraintSystem::openGenericRequirements(
     llvm::function_ref<Type(Type)> substFn) {
   auto requirements = signature.getRequirements();
   for (unsigned pos = 0, n = requirements.size(); pos != n; ++pos) {
-    const auto &req = requirements[pos];
-
-    Optional<Requirement> openedReq;
-    auto openedFirst = substFn(req.getFirstType());
-
-    auto kind = req.getKind();
-    switch (kind) {
-    case RequirementKind::Conformance: {
-      auto protoDecl = req.getProtocolDecl();
-      // Determine whether this is the protocol 'Self' constraint we should
-      // skip.
-      if (skipProtocolSelfConstraint && protoDecl == outerDC &&
-          protoDecl->getSelfInterfaceType()->isEqual(req.getFirstType()))
-        continue;
-      openedReq = Requirement(kind, openedFirst, req.getSecondType());
-      break;
-    }
-    case RequirementKind::Superclass:
-    case RequirementKind::SameType:
-      openedReq = Requirement(kind, openedFirst, substFn(req.getSecondType()));
-      break;
-    case RequirementKind::Layout:
-      openedReq = Requirement(kind, openedFirst, req.getLayoutConstraint());
-      break;
-    }
-
     auto openedGenericLoc =
-        locator.withPathElement(LocatorPathElt::OpenedGeneric(signature));
-    addConstraint(*openedReq,
-                  openedGenericLoc.withPathElement(
-                      LocatorPathElt::TypeParameterRequirement(pos, kind)));
+      locator.withPathElement(LocatorPathElt::OpenedGeneric(signature));
+    openGenericRequirement(outerDC, pos, requirements[pos],
+                           skipProtocolSelfConstraint, openedGenericLoc,
+                           substFn);
+  }
+}
+
+void ConstraintSystem::openGenericRequirement(
+    DeclContext *outerDC, unsigned index, const Requirement &req,
+    bool skipProtocolSelfConstraint, ConstraintLocatorBuilder locator,
+    llvm::function_ref<Type(Type)> substFn) {
+  Optional<Requirement> openedReq;
+  auto openedFirst = substFn(req.getFirstType());
+
+  auto kind = req.getKind();
+  switch (kind) {
+  case RequirementKind::Conformance: {
+    auto protoDecl = req.getProtocolDecl();
+    // Determine whether this is the protocol 'Self' constraint we should
+    // skip.
+    if (skipProtocolSelfConstraint && protoDecl == outerDC &&
+        protoDecl->getSelfInterfaceType()->isEqual(req.getFirstType()))
+      return;
+
+    openedReq = Requirement(kind, openedFirst, req.getSecondType());
+    break;
   }
+  case RequirementKind::Superclass:
+  case RequirementKind::SameType:
+    openedReq = Requirement(kind, openedFirst, substFn(req.getSecondType()));
+    break;
+  case RequirementKind::Layout:
+    openedReq = Requirement(kind, openedFirst, req.getLayoutConstraint());
+    break;
+  }
+
+  addConstraint(*openedReq,
+                locator.withPathElement(
+                    LocatorPathElt::TypeParameterRequirement(index, kind)));
 }
 
 /// Add the constraint on the type used for the 'Self' type for a member