c++: non-dependent call to consteval operator [PR105912]

Here we're crashing when substituting a non-dependent call to a
consteval operator, whose CALL_EXPR_OPERATOR_SYNTAX flag we try to
propagate to the result, but the result isn't a CALL_EXPR since the
selected function is consteval.  This patch fixes this by checking the
result of extract_call_expr accordingly.  (Note that we can't check
DECL_IMMEDIATE_FUNCTION_P here because we don't know which function was
selected by overload resolution from here.)

	PR c++/105912

gcc/cp/ChangeLog:

	* pt.cc (tsubst_copy_and_build) <case CALL_EXPR>: Guard against
	NULL_TREE extract_call_expr result.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/consteval31.C: New test.

gcc/cp/pt.cc

@@ -21206,12 +21206,12 @@ tsubst_copy_and_build (tree t,
 	    bool ord = CALL_EXPR_ORDERED_ARGS (t);
 	    bool rev = CALL_EXPR_REVERSE_ARGS (t);
 	    if (op || ord || rev)
-	      {
-		function = extract_call_expr (ret);
-		CALL_EXPR_OPERATOR_SYNTAX (function) = op;
-		CALL_EXPR_ORDERED_ARGS (function) = ord;
-		CALL_EXPR_REVERSE_ARGS (function) = rev;
-	      }
+	      if (tree call = extract_call_expr (ret))
+		{
+		  CALL_EXPR_OPERATOR_SYNTAX (call) = op;
+		  CALL_EXPR_ORDERED_ARGS (call) = ord;
+		  CALL_EXPR_REVERSE_ARGS (call) = rev;
+		}
 	  }
 	RETURN (ret);

gcc/testsuite/g++.dg/cpp2a/consteval31.C

+// PR c++/105912
+// { dg-do compile { target c++20 } }
+
+struct A {
+  consteval A operator+() {
+    return {};
+  }
+};
+
+consteval A operator~(A) {
+  return {};
+}
+
+consteval A operator+(A, A) {
+  return {};
+}
+
+template<class>
+void f() {
+  A a;
+  ~a;
+  a + a;
+  +a;
+}
+
+template void f<int>();