[PATCH] c++: ICE with auto(0) in requires-expression [PR103408]

[Marek Polacek <polacek@redhat.com>]

Here we ICE on

  int f() requires (auto(0));

in do_auto_deduction when handling the auto: we're in a non-templated
requires-expression which are parsed under processing_template_decl == 1
and empty current_template_parms, so 'current_template_args ()' will
crash.  This code is invalid as per "C++20 CA378: Remove non-templated
constrained functions", but of course we shouldn't crash.

Since in the scenario above it's expected that current_template_parms is
null, I've just added a check, and let grokfndecl issue an error.
I guess another approch would be to fake up a template parameter list
before calling do_auto_deduction.

For good measure, I've added several well-formed cases with auto(x) in
a requires-expression.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

	PR c++/103408

gcc/cp/ChangeLog:

	* pt.c (do_auto_deduction): Check current_template_parms before
	current_template_args ().

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/auto-fncast9.C: New test.
---
 gcc/cp/pt.c                               |  2 +-
 gcc/testsuite/g++.dg/cpp23/auto-fncast9.C | 27 +++++++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp23/auto-fncast9.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index f4b9d9673fb..012ca5d06c0 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -30041,7 +30041,7 @@ do_auto_deduction (tree type, tree init, tree auto_node,
        (but we still may have used them for constraint checking above).  */;
   else if (context == adc_unify)
     targs = add_to_template_args (outer_targs, targs);
-  else if (processing_template_decl)
+  else if (processing_template_decl && current_template_parms)
     targs = add_to_template_args (current_template_args (), targs);
   return tsubst (type, targs, complain, NULL_TREE);
 }
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast9.C b/gcc/testsuite/g++.dg/cpp23/auto-fncast9.C
new file mode 100644
index 00000000000..45a0ff9b460
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast9.C
@@ -0,0 +1,27 @@
+// PR c++/103408
+// { dg-do compile { target c++23 } }
+
+int bad1() requires (auto(true)); // { dg-error "constraints on a non-templated function" }
+
+template<bool B>
+int f1() requires (auto(B));
+
+template<typename T>
+struct S { T t; constexpr operator bool() { return true; } };
+
+int bad2() requires (bool(S{1})); // { dg-error "constraints on a non-templated function" }
+int bad3() requires (bool(S(1))); // { dg-error "constraints on a non-templated function" }
+
+template<int N>
+int f2() requires (bool(S{N}));
+
+template<int N>
+int f3() requires (bool(S(N)));
+
+void
+g ()
+{
+  f1<true>();
+  f2<42>();
+  f3<42>();
+}

base-commit: 6b8ecbc6d6652d061d7c72c64352d51eca2df6ca
-- 
2.33.1