[Macros] Introduce an example where member operators aren't found

Operators introduced by member macros are not getting found by global
operator lookup. This is technically a bug, but due to the potential
cost of having to expand macros on/in types just to find operators,
for now we're just going to document here that it doesn't work and
reconsider the semantics.

Author: DougGregor

test/Macros/Inputs/syntax_macro_definitions.swift

@@ -1553,3 +1553,15 @@ public struct SelfAlwaysEqualOperator: DeclarationMacro {
     ]
   }
 }
+
+extension SelfAlwaysEqualOperator: MemberMacro {
+  public static func expansion(
+    of node: AttributeSyntax,
+    providingMembersOf decl: some DeclGroupSyntax,
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax] {
+    return [
+      "static func ==(lhs: Self, rhs: Bool) -> Bool { true }",
+    ]
+  }
+}

test/Macros/macro_expand.swift

@@ -446,6 +446,8 @@ func testHasAnExpandedStatic() {
 
 @freestanding(declaration, names: named(==)) public macro addSelfEqualsOperator() = #externalMacro(module: "MacroDefinition", type: "SelfAlwaysEqualOperator")
 @freestanding(declaration, names: arbitrary) public macro addSelfEqualsOperatorArbitrary() = #externalMacro(module: "MacroDefinition", type: "SelfAlwaysEqualOperator")
+@attached(member, names: named(==)) public macro AddSelfEqualsMemberOperator() = #externalMacro(module: "MacroDefinition", type: "SelfAlwaysEqualOperator")
+@attached(member, names: arbitrary) public macro AddSelfEqualsMemberOperatorArbitrary() = #externalMacro(module: "MacroDefinition", type: "SelfAlwaysEqualOperator")
 
 struct HasEqualsSelf {
   #addSelfEqualsOperator
@@ -455,7 +457,26 @@ struct HasEqualsSelf2 {
   #addSelfEqualsOperatorArbitrary
 }
 
-func testHasEqualsSelf(x: HasEqualsSelf, y: HasEqualsSelf2) {
+@AddSelfEqualsMemberOperator
+struct HasEqualsSelf3 {
+}
+
+@AddSelfEqualsMemberOperatorArbitrary
+struct HasEqualsSelf4 {
+}
+
+func testHasEqualsSelf(
+  x: HasEqualsSelf, y: HasEqualsSelf2, z: HasEqualsSelf3, w: HasEqualsSelf4
+) {
   _ = (x == true)
   _ = (y == true)
+#if TEST_DIAGNOSTICS
+  // FIXME: This is technically a bug, because we should be able to find the
+  // == operator introduced through a member operator. However, we might
+  // want to change the rule rather than implement this.
+  _ = (z == true) // expected-error{{binary operator '==' cannot be applied to operands}}
+  // expected-note@-1{{overloads for '==' exist with these partially matching parameter lists}}
+  _ = (w == true) // expected-error{{binary operator '==' cannot be applied to operands}}
+  // expected-note@-1{{overloads for '==' exist with these partially matching parameter lists}}
+  #endif
 }