diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index 4baa3368f7a..9079a5b90ca 100644
--- a/gcc/cp/cp-gimplify.c
+++ b/gcc/cp/cp-gimplify.c
@@ -660,6 +660,14 @@ cp_gimplify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p)
       ret = GS_UNHANDLED;
       break;
 
+    case PTRMEM_CST:
+      *expr_p = cplus_expand_constant (*expr_p);
+      if (TREE_CODE (*expr_p) == PTRMEM_CST)
+	ret = GS_ERROR;
+      else
+	ret = GS_OK;
+      break;
+
     case RETURN_EXPR:
       if (TREE_OPERAND (*expr_p, 0)
 	  && (TREE_CODE (TREE_OPERAND (*expr_p, 0)) == INIT_EXPR
diff --git a/gcc/testsuite/g++.dg/cpp1z/pr99790.C b/gcc/testsuite/g++.dg/cpp1z/pr99790.C
new file mode 100644
index 00000000000..99961f5a4ee
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/pr99790.C
@@ -0,0 +1,9 @@
+// PR c++/99790
+// { dg-do compile { target c++17 } }
+
+struct A;
+struct B { void (*fn) (A *); };
+template <typename T>
+int foo (const T &);
+struct A { int a; static constexpr B b{[] (A *n) { n->*&A::a = 2; }}; };
+int a = foo (A::b);