From: Jason Merrill <jason@redhat.com>
Date: Wed, 22 Apr 2026 18:53:14 +0000 (-0400)
Subject: c++: consteval, array, modules [PR124973]
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b6e40cf38c60d11c4e105f9179a6d5cfb502bd9b;p=thirdparty%2Fgcc.git

c++: consteval, array, modules [PR124973]

Here the consteval holder constructor calls the defaulted element_array
constructor, which uses a VEC_INIT_EXPR to call the defaulted element
constructor.

When we read in the holder constructor, we need to clone it, so we call
finish_function, which calls cp_fold_function_non_odr_use, which tries to
constant-evaluate the call to the element_array constructor.  This
eventually wants to evaluate the VEC_INIT_EXPR, which wants to call the
element constructor (complete object clone).  But we haven't cloned the
element constructor yet, so mark_used tries to synthesize it again, which
breaks because the constructor is already defined, just not cloned yet.

We should have cloned the element constructor first, but we didn't know that
the element_array constructor depends on it because VEC_INIT_EXPR doesn't
express that; build_vec_init_expr calls build_vec_init_elt and then throws
it away.  Perhaps we want to add the elt_init as an additional operand that
is used to express dependencies, but ignored in expansion?

It would also be nice not to repeat all the finish_function passes when
loading a function from a module; we already did
cp_fold_function_non_odr_use and such for this function before writing out
the module, doing it again is a waste of time.

But also, trying to constant-evaluate the element_array constructor is wrong
for _non_odr_use, it shouldn't be doing any optimization folding.

Furthermore, since the TARGET_EXPR is wrapped in an INIT_EXPR, we should
never have tried to fold it by itself, before cp_genericize_init_expr has a
chance to elide it.  So let's only do that folding when ff_genericize, like
the other TARGET_EXPR transformations.  This is a much simpler fix for this
testcase.

While we're at it, let's also suppress the other flag_no_inline-conditional
folding when ff_only_non_odr.

	PR c++/124973
	PR c++/120502
	PR c++/120005

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_fold_r) <case TARGET_EXPR>: Only
	do optimization folding when ff_genericize.
	(cp_fold) <case CALL_EXPR>: Don't do
	optimization folding when ff_only_non_odr.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/consteval-1_a.C: New test.
	* g++.dg/modules/consteval-1_b.C: New test.
---

diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index 5bcf5168970..8ae66405817 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -1717,7 +1717,8 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
       break;
 
     case TARGET_EXPR:
-      if (!flag_no_inline)
+      if (!flag_no_inline
+	  && (data->flags && ff_genericize))
 	if (tree &init = TARGET_EXPR_INITIAL (stmt))
 	  {
 	    tree folded = maybe_constant_init (init, TARGET_EXPR_SLOT (stmt),
@@ -3769,6 +3770,7 @@ cp_fold (tree x, fold_flags_t flags)
 	if ((OPTION_SET_P (flag_fold_simple_inlines)
 	     ? flag_fold_simple_inlines
 	     : !flag_no_inline)
+	    && !(flags & ff_only_non_odr)
 	    && call_expr_nargs (x) == 1
 	    && decl_in_std_namespace_p (callee)
 	    && DECL_NAME (callee) != NULL_TREE
@@ -3881,6 +3883,7 @@ cp_fold (tree x, fold_flags_t flags)
 	   Do constexpr expansion of expressions where the call itself is not
 	   constant, but the call followed by an INDIRECT_REF is.  */
 	if (callee && DECL_DECLARED_CONSTEXPR_P (callee)
+	    && !(flags & ff_only_non_odr)
 	    && (!flag_no_inline
 		|| lookup_attribute ("always_inline",
 				     DECL_ATTRIBUTES (callee))))
diff --git a/gcc/testsuite/g++.dg/modules/consteval-1_a.C b/gcc/testsuite/g++.dg/modules/consteval-1_a.C
new file mode 100644
index 00000000000..dac678c32b1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/consteval-1_a.C
@@ -0,0 +1,36 @@
+// PR c++/124973
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fmodules" }
+
+export module right;		// { dg-module-cmi right }
+
+struct constexpr_move {
+    constexpr_move() = default;
+    constexpr_move(const constexpr_move&) = default;
+    constexpr constexpr_move(constexpr_move&&) {}
+};
+
+struct element {
+    constexpr_move a;
+    int b;
+};
+
+template <unsigned N>
+struct element_array {
+    element data[ N ];
+};
+
+template <unsigned N>
+struct holder {
+    consteval holder(element_array<N> values) :
+        values_{static_cast<element_array<N>&&>(values)} {}
+    element_array<N> values_;
+};
+
+template <unsigned N>
+constexpr auto holder_of = holder<N>{element_array<N>{}};
+
+export class environment {
+    decltype(holder_of<1>) s1_{holder_of<1>};
+    decltype(holder_of<2>) s2_{holder_of<2>};
+};
diff --git a/gcc/testsuite/g++.dg/modules/consteval-1_b.C b/gcc/testsuite/g++.dg/modules/consteval-1_b.C
new file mode 100644
index 00000000000..14676e5dcea
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/consteval-1_b.C
@@ -0,0 +1,8 @@
+// PR c++/124973
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fmodules" }
+
+export module left;		// { dg-module-cmi left }
+import right;
+
+export auto xx(environment&) -> void;