[gcc r12-6485] c++: dependent bases and 'this' availability [PR103831]

[Patrick Palka]

https://gcc.gnu.org/g:0378f563b0321c44c4a9c98cf46d2a22b9160f76

commit r12-6485-g0378f563b0321c44c4a9c98cf46d2a22b9160f76
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Jan 11 13:00:48 2022 -0500

    c++: dependent bases and 'this' availability [PR103831]
    
    Here during satisfaction of B's constraints we're failing to reject the
    object-less call to the non-static member function A::size ultimately
    because satisfaction is performed in the (access) context of the class
    template B, which has a dependent base, and so the any_dependent_bases_p
    check within build_new_method_call causes us to not reject the call.
    (Subsequent constexpr evaluation of the call succeeds since the function
    is effectively static.)
    
    This patch fixes this by refining the any_dependent_bases_p check within
    build_new_method_call: if we're in a context where 'this' is unavailable,
    then we cannot resolve the implicit object regardless of the presence of
    a dependent base.  So let's also check current_class_ptr alongside a_d_b_p.
    
            PR c++/103831
    
    gcc/cp/ChangeLog:
    
            * call.c (build_new_method_call): Consider dependent bases only
            if 'this' is available.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/concepts-class3.C: New test.
            * g++.dg/template/non-dependent18.C: New test.

Diff:
---
 gcc/cp/call.c                                   |  2 +-
 gcc/testsuite/g++.dg/cpp2a/concepts-class3.C    | 12 ++++++++++++
 gcc/testsuite/g++.dg/template/non-dependent18.C | 19 +++++++++++++++++++
 3 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 88ab23392d2..d4a07a7a9b3 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -11125,7 +11125,7 @@ build_new_method_call (tree instance, tree fns, vec<tree, va_gc> **args,
 		     we know we really need it.  */
 		  cand->first_arg = instance;
 		}
-	      else if (any_dependent_bases_p ())
+	      else if (current_class_ptr && any_dependent_bases_p ())
 		/* We can't tell until instantiation time whether we can use
 		   *this as the implicit object argument.  */;
 	      else
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-class3.C b/gcc/testsuite/g++.dg/cpp2a/concepts-class3.C
new file mode 100644
index 00000000000..68b50b71278
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-class3.C
@@ -0,0 +1,12 @@
+// PR c++/103831
+// { dg-do compile { target c++20 } }
+
+struct A {
+  constexpr int size() { return 42; } // non-static
+};
+
+template<class T>
+  requires (T::size() == 42) // { dg-error "without object" }
+struct B : T { };
+
+template struct B<A>; // { dg-error "constraint" }
diff --git a/gcc/testsuite/g++.dg/template/non-dependent18.C b/gcc/testsuite/g++.dg/template/non-dependent18.C
new file mode 100644
index 00000000000..3e1556821a6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/non-dependent18.C
@@ -0,0 +1,19 @@
+// PR c++/103831
+// { dg-do compile { target c++11 } }
+
+struct A {
+  constexpr int size() { return 42; } // non-static
+};
+
+template<class T>
+struct B : T {
+  static_assert(A::size() == 42, ""); // { dg-error "without object" }
+
+  static int f() {
+    static_assert(A::size() == 42, ""); // { dg-error "without object" }
+    return A::size(); // { dg-error "without object" }
+  }
+
+  int n = A::size();
+  static const int m = A::size(); // { dg-error "without object" }
+};