[gcc r14-228] c++: value dependence of by-ref lambda capture [PR108975]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:3d674e29d7f89bf93fcfcc963ff0248c6347586d

commit r14-228-g3d674e29d7f89bf93fcfcc963ff0248c6347586d
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Apr 25 15:59:22 2023 -0400

    c++: value dependence of by-ref lambda capture [PR108975]
    
    We are still ICEing on the generic lambda version of the testcase from
    this PR, even after r13-6743-g6f90de97634d6f, due to the by-ref capture
    of the constant local variable 'dim' being considered value-dependent
    when regenerating the lambda (at which point processing_template_decl is
    set since the lambda is generic), which prevents us from constant folding
    its uses.  Later during prune_lambda_captures we end up not thoroughly
    walking the body of the lambda and overlook the (non-folded) uses of
    'dim' within the array bound and using-decls.
    
    We could fix this by making prune_lambda_captures walk the body of the
    lambda more thoroughly so that it finds these uses of 'dim', but ideally
    we should be able to constant fold all uses of 'dim' ahead of time and
    prune the implicit capture after all.
    
    To that end this patch makes value_dependent_expression_p return false
    for such by-ref captures of constant local variables, allowing their
    uses to get constant folded ahead of time.  It seems we just need to
    disable the predicate's conservative early exit for reference variables
    (added by r5-5022-g51d72abe5ea04e) when DECL_HAS_VALUE_EXPR_P.  This
    effectively makes us treat by-value and by-ref captures more consistently
    when it comes to value dependence.
    
            PR c++/108975
    
    gcc/cp/ChangeLog:
    
            * pt.cc (value_dependent_expression_p) <case VAR_DECL>:
            Suppress conservative early exit for reference variables
            when DECL_HAS_VALUE_EXPR_P.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp0x/lambda/lambda-const11a.C: New test.

Diff:
---
 gcc/cp/pt.cc                                        |  7 ++++---
 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const11a.C | 21 +++++++++++++++++++++
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 3e5f0104056..a668825b5f9 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -27959,9 +27959,7 @@ value_dependent_expression_p (tree expression)
     case VAR_DECL:
        /* A constant with literal type and is initialized
 	  with an expression that is value-dependent.  */
-      if (DECL_DEPENDENT_INIT_P (expression)
-	  /* FIXME cp_finish_decl doesn't fold reference initializers.  */
-	  || TYPE_REF_P (TREE_TYPE (expression)))
+      if (DECL_DEPENDENT_INIT_P (expression))
 	return true;
       if (DECL_HAS_VALUE_EXPR_P (expression))
 	{
@@ -27976,6 +27974,9 @@ value_dependent_expression_p (tree expression)
 		  && value_expr == error_mark_node))
 	    return true;
 	}
+      else if (TYPE_REF_P (TREE_TYPE (expression)))
+	/* FIXME cp_finish_decl doesn't fold reference initializers.  */
+	return true;
       return false;
 
     case DYNAMIC_CAST_EXPR:
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const11a.C b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const11a.C
new file mode 100644
index 00000000000..7fc3d48a8dd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const11a.C
@@ -0,0 +1,21 @@
+// PR c++/108975
+// A version of lambda-const11.C using a generic lambda.
+// { dg-do compile { target c++14 } }
+
+template<int> void g();
+template<int> struct A { };
+
+template<class T>
+void f() {
+  constexpr int dim = 1;
+  auto l = [&](auto) {
+    int n[dim * 1];
+    using ty1 = decltype(g<dim * 2>());
+    using ty2 = A<dim * 3>;
+  };
+  l(0);
+  // In f<int>, we shouldn't actually capture dim.
+  static_assert (sizeof(l) == 1, "");
+}
+
+template void f<int>();