Default-initialized local static constexpr in constant evaluation [GH #399]

The interpretation of the initialization statement of a local variable with
static or thread storage duration (which, in a constant evaluation, must be
usable in constant expressions) no longer allocates and initializes
interpreter storage for it: its value is obtained when it is referenced.
Previously, a default-initialized local "static constexpr" variable of class
type caused the constant evaluation to fail.

Author: daveedvdv-nvidia

src/interpret.c

@@ -9500,6 +9500,13 @@ loop body push a work item for it and continue in a later phase.
       { a_dynamic_init_ptr  dip = stmt->variant.dynamic_init;
         a_variable_ptr      vp = dip->variable;
         a_byte              *var_storage;
+        if (var_has_static_or_thread_storage_duration(vp)) {
+          /* A local static variable (e.g., a default-initialized one of
+             class type) that is usable in constant expressions (see
+             stmk_decl below): its value is obtained when it is
+             referenced. */
+          break;
+        }  /* if */
         /* Allocate and bind storage for the variable. */
         var_storage = do_constexpr_alloc_variable(ips, vp, &result);
         if (!result) break;

src/pending_changes/gh-399-default-initialized-local-static-constexpr.txt

@@ -0,0 +1,12 @@
+[GH #399]
+Default-initialized local static constexpr variables in constant evaluation
+
+In C++23 mode, a default-initialized local "static constexpr" variable of
+class type caused the constant evaluation of the enclosing function to fail.
+For example, with --c++23:
+
+  struct Q { int q = 1; };
+  constexpr int f() { static constexpr Q q; return q.q; }
+  static_assert(f() == 1);  // Previously failed.
+
+This is now fixed.

tests/tests/changes/.gh-399.rto/default.1.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=1 --c++23 Test_name.c

tests/tests/changes/gh-399.sft.cpp

@@ -0,0 +1,24 @@
+//type:fp
+//options:--c++23
+//remark:[GH #399] Constant evaluation with a default-initialized local
+//remark:static constexpr variable of class type
+
+struct Q { int q = 1; };
+constexpr int default_init() { static constexpr Q q; return q.q; }
+static_assert(default_init() == 1);  // Previously failed.
+constexpr int value_init() { static constexpr Q q{}; return q.q + 1; }
+static_assert(value_init() == 2);
+
+struct R { int r = 3; constexpr R() {} };
+constexpr int user_ctor() {
+  static constexpr R r;
+  constexpr const R* p = &r;
+  return p->r;
+}
+static_assert(user_ctor() == 3);  // Previously failed.
+
+template<typename T> constexpr int templ() {
+  static constexpr T t;
+  return t.q * 10;
+}
+static_assert(templ<Q>() == 10);  // Previously failed.