diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -7863,8 +7863,13 @@
             return Result::deleted();
           }
         } else {
+          QualType T = Best->BuiltinParamTypes[0];
+          assert(T == Best->BuiltinParamTypes[1] &&
+                 "builtin comparison for different types?");
+          assert(Best->BuiltinParamTypes[2].isNull() &&
+                 "invalid builtin comparison");
           Optional<ComparisonCategoryType> Cat =
-              getComparisonCategoryForBuiltinCmp(Args[0]->getType());
+              getComparisonCategoryForBuiltinCmp(T);
           assert(Cat && "no category for builtin comparison?");
           R.Category = *Cat;
         }
diff --git a/clang/test/CXX/class/class.compare/class.spaceship/p2.cpp b/clang/test/CXX/class/class.compare/class.spaceship/p2.cpp
--- a/clang/test/CXX/class/class.compare/class.spaceship/p2.cpp
+++ b/clang/test/CXX/class/class.compare/class.spaceship/p2.cpp
@@ -172,3 +172,23 @@
     int C::*x;                                   // expected-note {{because there is no viable three-way comparison function for member 'x'}}
   };
 }
+
+namespace PR50591 {
+  struct a1 {
+    operator int() const;
+  };
+  struct b1 {
+    auto operator<=>(b1 const &) const = default;
+    a1 f;
+  };
+  std::strong_ordering cmp_b1 = b1() <=> b1();
+
+  struct a2 {
+    operator float() const;
+  };
+  struct b2 {
+    auto operator<=>(b2 const &) const = default;
+    a2 f;
+  };
+  std::partial_ordering cmp_b2 = b2() <=> b2();
+}