c++: fix truncated diagnostic in C++23 [PR111272]

In C++23, since P2448, a constexpr function F that calls a non-constexpr
function N is OK as long as we don't actually call F in a constexpr
context.  So instead of giving an error in maybe_save_constexpr_fundef,
we only give an error when evaluating the call.  Unfortunately, as shown
in this PR, the diagnostic can be truncated:

z.C:10:13: note: 'constexpr Jam::Jam()' is not usable as a 'constexpr' function because:
   10 |   constexpr Jam() { ft(); }
      |             ^~~

...because what?  With this patch, we say:

z.C:10:13: note: 'constexpr Jam::Jam()' is not usable as a 'constexpr' function because:
   10 |   constexpr Jam() { ft(); }
      |             ^~~
z.C:10:23: error: call to non-'constexpr' function 'int Jam::ft()'
   10 |   constexpr Jam() { ft(); }
      |                     ~~^~
z.C:8:7: note: 'int Jam::ft()' declared here
    8 |   int ft() { return 42; }
      |       ^~

Like maybe_save_constexpr_fundef, explain_invalid_constexpr_fn should
also check the body of a constructor, not just the mem-initializer.

	PR c++/111272

gcc/cp/ChangeLog:

	* constexpr.cc (explain_invalid_constexpr_fn): Also check the body of
	a constructor in C++14 and up.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-diag1.C: New test.

2 files changed, 30 insertions, 1 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 0f948db..dde4fec 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -1098,7 +1098,15 @@ explain_invalid_constexpr_fn (tree fun)
 	  body = massage_constexpr_body (fun, body);
 	  require_potential_rvalue_constant_expression (body);
 	  if (DECL_CONSTRUCTOR_P (fun))
-	    cx_check_missing_mem_inits (DECL_CONTEXT (fun), body, true);
+	    {
+	      cx_check_missing_mem_inits (DECL_CONTEXT (fun), body, true);
+	      if (cxx_dialect > cxx11)
+		{
+		  /* Also check the body, not just the ctor-initializer.  */
+		  body = DECL_SAVED_TREE (fun);
+		  require_potential_rvalue_constant_expression (body);
+		}
+	    }
 	}
     }
 }
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-diag1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-diag1.C
new file mode 100644
index 0000000..0e2909e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-diag1.C
@@ -0,0 +1,21 @@
+// PR c++/111272
+// { dg-do compile { target c++14 } }
+// { dg-options "-Werror=invalid-constexpr" }
+// { dg-prune-output "some warnings being treated as errors" }
+
+struct Jam
+{
+  // constexpr  // n.b.
+  int ft() { return 42; } // { dg-message "declared here" }
+
+  constexpr Jam() { ft(); } // { dg-error "call to non-.constexpr. function" }
+// { dg-message "declared here" "" { target c++20_down } .-1 }
+};
+
+constexpr bool test()
+{
+  Jam j; // { dg-error "called in a constant expression" }
+  return true;
+}
+
+static_assert(test(), ""); // { dg-error "non-constant condition" }