[gcc r17-2718] c++: parse trivial DMI immediately [PR96645]

https://gcc.gnu.org/g:e3e1c72649c3a0b48af70f88bcd942c46b0d00ab

commit r17-2718-ge3e1c72649c3a0b48af70f88bcd942c46b0d00ab
Author: Jason Merrill <[email protected]>
Date:   Fri Jul 24 15:17:32 2026 -0400

    c++: parse trivial DMI immediately [PR96645]
    
    The current direction of CWG2335 is to parse complete-class contexts as
    needed rather than always wait until the end of the outermost class.
    
    This doesn't fully implement that (nor fully fix PR96645), but takes a step
    in that direction by allowing simple cases that do no name lookup; then
    being in complete class scope makes no difference.
    
            CWG 2335
            PR c++/96645
    
    gcc/cp/ChangeLog:
    
            * parser.cc (cp_parser_early_parsing_nsdmi): New.
            (cp_parser_member_declaration): Call it.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp0x/nsdmi10.C: Distinguish simple case.
            * g++.dg/ext/is_constructible7.C: New.

Diff:
---
 gcc/cp/parser.cc                             | 42 +++++++++++++++++++++++++++-
 gcc/testsuite/g++.dg/cpp0x/nsdmi10.C         |  4 +--
 gcc/testsuite/g++.dg/ext/is_constructible7.C | 27 ++++++++++++++++++
 3 files changed, 70 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 13ebd64c4836..a4cfcfa480ef 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -3108,6 +3108,8 @@ static tree cp_parser_late_parse_one_default_arg
   (cp_parser *, tree, tree, tree);
 static void cp_parser_late_parsing_nsdmi
   (cp_parser *, tree);
+static bool cp_parser_early_parsing_nsdmi
+  (cp_parser *, tree);
 static void cp_parser_late_parsing_default_args
   (cp_parser *, tree);
 static tree cp_parser_sizeof_operand
@@ -31748,7 +31750,8 @@ cp_parser_member_declaration (cp_parser* parser)
 	      if (DECL_DECLARES_FUNCTION_P (decl))
 		cp_parser_save_default_args (parser, STRIP_TEMPLATE (decl));
 	      else if (TREE_CODE (decl) == FIELD_DECL
-		       && DECL_INITIAL (decl))
+		       && DECL_INITIAL (decl)
+		       && !cp_parser_early_parsing_nsdmi (parser, decl))
 		/* Add DECL to the queue of NSDMI to be parsed later.  */
 		vec_safe_push (unparsed_nsdmis, decl);
 	    }
@@ -37282,6 +37285,43 @@ cp_parser_late_parsing_nsdmi (cp_parser *parser, tree field)
   DECL_INITIAL (field) = def;
 }
 
+/* If the DEFERRED_PARSE for FIELD is safe to parse immediately, do so.
+   Returns true if deferred parsing is no longer needed.
+
+   CWG direction on issue 2335 is to parse DMI as needed rather than only at
+   the end of the class, which we do not yet implement; this function allows a
+   subset of trivial cases that are intended to be well-formed.
+
+   A further subset could be trying to parse immediately but giving up if name
+   lookup fails, since "A name N used in a class S shall refer to the same
+   declaration in its context and when re-evaluated in the completed scope of
+   S." ([basic.scope.class].
+
+   WIP for c++/96645 tries to parse as a pseudo-template and then
+   pseudo-instantiate when needed, like requires-expressions; this would
+   completely implement the CWG direction (and also rely on the above
+   rule).  */
+
+static bool
+cp_parser_early_parsing_nsdmi (cp_parser *parser, tree field)
+{
+  tree init = DECL_INITIAL (field);
+  if (TREE_CODE (init) != DEFERRED_PARSE)
+    return true;
+
+  cp_token_cache *tokens = DEFPARSE_TOKENS (init);
+  for (cp_token *p = tokens->first; p != tokens->last; ++p)
+    if (p->type == CPP_NAME
+	|| p->keyword == RID_THIS
+	|| p->keyword == RID_OPERATOR)
+      /* There's a name to look up or 'this', give up.  */
+      return false;
+
+  /* It's trivial, parse now.  */
+  cp_parser_late_parsing_nsdmi (parser, field);
+  return true;
+}
+
 /* FN is a FUNCTION_DECL which may contains a parameter with an
    unparsed DEFERRED_PARSE.  Parse the default args now.  This function
    assumes that the current scope is the scope in which the default
diff --git a/gcc/testsuite/g++.dg/cpp0x/nsdmi10.C b/gcc/testsuite/g++.dg/cpp0x/nsdmi10.C
index d8588b7f29ea..ee0d79570b4b 100644
--- a/gcc/testsuite/g++.dg/cpp0x/nsdmi10.C
+++ b/gcc/testsuite/g++.dg/cpp0x/nsdmi10.C
@@ -6,12 +6,12 @@ struct A1 {
     int y1 = 1;
   };
 
-  A1(const B1& opts = B1()) {}  // { dg-error "default member initializer" }
+  A1(const B1& opts = B1()) {}
 };
 
 struct A2 {
   struct B2 {
-    int x2, y2 = 1;
+    int x2, y2 = x2;
   };
 
   A2(const B2& opts = B2()) {}  // { dg-error "default member initializer" }
diff --git a/gcc/testsuite/g++.dg/ext/is_constructible7.C b/gcc/testsuite/g++.dg/ext/is_constructible7.C
new file mode 100644
index 000000000000..013a1df03c6a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_constructible7.C
@@ -0,0 +1,27 @@
+// PR c++/96645
+// { dg-do compile { target c++11 } }
+
+template<bool B>
+struct bool_constant
+{
+  static constexpr bool value = B;
+  using type = bool_constant;
+};
+
+using true_type = bool_constant<true>;
+
+template<typename T>
+struct is_default_constructible
+  : bool_constant<__is_constructible(T)>
+{ };
+
+void testVarStruct()
+{
+  struct DataWithStruct {
+    struct A {
+      int number = 5; // compiles, if remove initialization
+    };
+
+    is_default_constructible<A>::type t = true_type{};
+  };
+}