[Sema] Fix the source compatibility check for member operators with

existential arguments to not return before checking the second
argument type.

Author: hborla

lib/Sema/TypeCheckDecl.cpp

@@ -1903,7 +1903,8 @@ bool swift::isMemberOperator(FuncDecl *decl, Type type) {
       // accepts member operators on protocols with existential
       // type arguments. We should consider banning this in Swift 6.
       if (auto existential = paramType->getAs<ExistentialType>()) {
-        return selfNominal == existential->getConstraintType()->getAnyNominal();
+        if (selfNominal == existential->getConstraintType()->getAnyNominal())
+          return true;
       }
 
       // For a protocol, is it the 'Self' type parameter?

test/type/explicit_existential.swift

@@ -185,3 +185,11 @@ enum E2: RawRepresentable {
     return ConcreteComposition()
   }
 }
+
+public protocol MyError {}
+
+extension MyError {
+  static func ~=(lhs: any Error, rhs: Self) -> Bool {
+    return true
+  }
+}