[FOLD] add test

Author: sdkrystian

clang/test/SemaCXX/decltype.cpp

@@ -147,6 +147,31 @@ namespace GH97646 {
   }
 }
 
+namespace GH99873 {
+struct B {
+  int x;
+};
+
+template<typename T>
+struct A {
+  template<typename U>
+  constexpr int f() const {
+    return 1;
+  }
+
+  template<>
+  constexpr int f<int>() const {
+    return decltype(B::x)();
+  }
+};
+
+// This shouldn't crash.
+static_assert(A<int>().f<int>() == 0, "");
+// The result should not be dependent.
+static_assert(A<int>().f<int>() != 0, ""); // expected-error {{static assertion failed due to requirement 'GH99873::A<int>().f<int>() != 0'}}
+                                           // expected-note@-1 {{expression evaluates to '0 != 0'}}
+}
+
 template<typename>
 class conditional {
 };