c++: poor diag w/ non-constexpr dtor called from constexpr ctor

When diagnosing a non-constexpr constructor call during constexpr
evaluation, explain_invalid_constexpr_fn was passing the genericized
body to require_potential_constant_expression rather than the saved
non-genericized one.

This meant for the below testcase (reduced from PR libstdc++/119282)
in which B::B() is deemed non-constexpr due to the local variable having
a non-constexpr destructor we would then issue the cryptic diagnostic:

constexpr-nonlit19.C:17:16: error: non-constant condition for static assertion
   17 | static_assert(f());
      |               ~^~
constexpr-nonlit19.C:17:16:   in ‘constexpr’ expansion of ‘f()’
constexpr-nonlit19.C:13:5: error: ‘constexpr B::B()’ called in a constant expression
   13 |   B b;
      |     ^
constexpr-nonlit19.C:6:13: note: ‘constexpr B::B()’ is not usable as a ‘constexpr’ function because:
    6 |   constexpr B() {
      |             ^
constexpr-nonlit19.C:8:5: error: ‘goto’ is not a constant expression
    8 |     for (int i = 0; i < 10; i++) { }
      |     ^~~

This patch makes us pass the non-genericized body to
require_potential_constant_expression, and so we now emit:

...
constexpr-nonlit19.C:6:13: note: ‘constexpr B::B()’ is not usable as a ‘constexpr’ function because:
    6 |   constexpr B() {
      |             ^
constexpr-nonlit19.C:9:3: error: call to non-‘constexpr’ function ‘A::~A()’
    9 |   }
      |   ^
constexpr-nonlit19.C:3:12: note: ‘A::~A()’ declared here
    3 | struct A { ~A() { } };
      |            ^

gcc/cp/ChangeLog:

	* constexpr.cc (explain_invalid_constexpr_fn): In the
	DECL_CONSTRUCTOR_P branch pass the non-genericized body to
	require_potential_constant_expression.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/constexpr-nonlit19.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

2 files changed, 22 insertions, 8 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 5b7b70f..fa754b9 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -1110,17 +1110,14 @@ explain_invalid_constexpr_fn (tree fun)
 	    body = fd->body;
 	  else
 	    body = DECL_SAVED_TREE (fun);
-	  body = massage_constexpr_body (fun, body);
-	  require_potential_rvalue_constant_expression (body);
+	  tree massaged = massage_constexpr_body (fun, body);
+	  require_potential_rvalue_constant_expression (massaged);
 	  if (DECL_CONSTRUCTOR_P (fun))
 	    {
-	      cx_check_missing_mem_inits (DECL_CONTEXT (fun), body, true);
+	      cx_check_missing_mem_inits (DECL_CONTEXT (fun), massaged, 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);
-		}
+		/* Also check the body, not just the ctor-initializer.  */
+		require_potential_rvalue_constant_expression (body);
 	    }
 	}
     }
diff --git a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit19.C b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit19.C
new file mode 100644
index 0000000..1b73e2d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit19.C
@@ -0,0 +1,17 @@
+// { dg-do compile { target c++23 } }
+
+struct A { ~A() { } };
+
+struct B {
+  constexpr B() {
+    A a;
+    for (int i = 0; i < 10; i++) { }
+  } // { dg-error "call to non-'constexpr' function 'A::~A..'" }
+};
+
+constexpr bool f() {
+  B b; // { dg-error "B::B..' called in a constant expression" }
+  return true;
+}
+
+static_assert(f()); // { dg-error "non-constant" }