diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index 8b097c46158..b80010e0983 100644
--- a/gcc/cp/cp-gimplify.c
+++ b/gcc/cp/cp-gimplify.c
@@ -166,8 +166,15 @@ genericize_if_stmt (tree *stmt_p)
      can contain unfolded immediate function calls, we have to discard
      the then_ block regardless of whether else_ has side-effects or not.  */
   if (IF_STMT_CONSTEVAL_P (stmt))
-    stmt = build3 (COND_EXPR, void_type_node, boolean_false_node,
-		   void_node, else_);
+    {
+      if (block_may_fallthru (then_))
+	stmt = build3 (COND_EXPR, void_type_node, boolean_false_node,
+		       void_node, else_);
+      else
+	stmt = else_;
+    }
+  else if (IF_STMT_CONSTEXPR_P (stmt))
+    stmt = integer_nonzerop (cond) ? then_ : else_;
   else
     stmt = build3 (COND_EXPR, void_type_node, cond, then_, else_);
   protected_set_expr_location_if_unset (stmt, locus);
diff --git a/gcc/cp/cp-objcp-common.c b/gcc/cp/cp-objcp-common.c
index fd6fe17e7da..b31c9e3a636 100644
--- a/gcc/cp/cp-objcp-common.c
+++ b/gcc/cp/cp-objcp-common.c
@@ -313,6 +313,13 @@ cxx_block_may_fallthru (const_tree stmt)
       return false;
 
     case IF_STMT:
+      if (IF_STMT_CONSTEXPR_P (stmt))
+	{
+	  if (integer_nonzerop (IF_COND (stmt)))
+	    return block_may_fallthru (THEN_CLAUSE (stmt));
+	  if (integer_zerop (IF_COND (stmt)))
+	    return block_may_fallthru (ELSE_CLAUSE (stmt));
+	}
       if (block_may_fallthru (THEN_CLAUSE (stmt)))
 	return true;
       return block_may_fallthru (ELSE_CLAUSE (stmt));
diff --git a/gcc/testsuite/g++.dg/warn/Wreturn-type-13.C b/gcc/testsuite/g++.dg/warn/Wreturn-type-13.C
new file mode 100644
index 00000000000..fa2952b735c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wreturn-type-13.C
@@ -0,0 +1,35 @@
+// PR c++/103991
+// { dg-do compile { target c++17 } }
+
+struct S { ~S(); };
+int
+foo ()
+{
+  S s;
+  if constexpr (true)
+    return 0;
+  else
+    return 1;
+}			// { dg-bogus "control reaches end of non-void function" }
+
+#if __cpp_if_consteval >= 202106L
+constexpr int
+bar ()
+{
+  S s;
+  if consteval
+    {
+      return 0;
+    }
+  else
+    {
+      return 1;
+    }
+}			// { dg-bogus "control reaches end of non-void function" }
+
+int
+baz ()
+{
+  return bar ();
+}
+#endif