[5.1][Sema] Treat static methods as always applying self

Currently `areConservativelyCompatibleArgumentLabels` treats a reference
to a declaration as not applying its curried self parameter if it
doesn't have a base type. However this isn't correct for static
operators, which have their Self.Type parameter applied but don't have
a base type.

This commit changes `areConservativelyCompatibleArgumentLabels` to
always treat static methods as having their Self.Type parameters
applied, as there's currently no way to form an unapplied curried
reference to them.

Resolves SR-10843.

Author: hamishknight

lib/Sema/CSSimplify.cpp

@@ -130,6 +130,9 @@ bool constraints::areConservativelyCompatibleArgumentLabels(
   bool hasCurriedSelf;
   if (isa<SubscriptDecl>(decl)) {
     hasCurriedSelf = false;
+  } else if (decl->isStatic() && isa<AbstractFunctionDecl>(decl)) {
+    // Static methods always have their Self.Type parameter applied.
+    hasCurriedSelf = true;
   } else if (!baseType || baseType->is<ModuleType>()) {
     hasCurriedSelf = false;
   } else if (baseType->is<AnyMetatypeType>() && decl->isInstanceMember()) {

test/Constraints/operator.swift

@@ -223,3 +223,17 @@ func rdar46459603() {
   _ = [arr.values] == [[e]]
   // expected-error@-1 {{protocol type 'Any' cannot conform to 'Equatable' because only concrete types can conform to protocols}}
 }
+
+// SR-10843
+infix operator ^^^
+func ^^^ (lhs: String, rhs: String) {}
+
+struct SR10843 {
+  static func ^^^ (lhs: SR10843, rhs: SR10843) {}
+}
+
+func sr10843() {
+  let s = SR10843()
+  (^^^)(s, s)
+  _ = (==)(0, 0)
+}