[gcc r11-7279] c++: Fix -std=c++20 ICE on virtual method call [PR99132]

[Jakub Jelinek]

https://gcc.gnu.org/g:3bfa96895b3219afd93a7038850baef4b63c1f82

commit r11-7279-g3bfa96895b3219afd93a7038850baef4b63c1f82
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Feb 18 16:21:52 2021 +0100

    c++: Fix -std=c++20 ICE on virtual method call [PR99132]
    
    On the following testcase we ICE in C++20 mode during cp_get_callee_fndecl
    -> constexpr evaluation.
    It is only in C++20 mode on this testcase because virtual methods can't
    be constexpr in C++17 and earlier and so potential_constant_expression_1
    rejects it earlier.
    And the ICE is caused by genericization changing the h PARM_DECL from having
    B<C *> type to B<C *> & DECL_BY_REFERENCE and the constexpr evaluation
    not being able to deal with that.
    I think this just shows that we shouldn't do the constexpr evaluation during
    genericization and later, and other spots e.g. during gimplification
    also don't call cp_get_callee_fndecl but cp_get_callee_fndecl_nofold.
    After all, cp_fold has already been run and it did the folding if there
    was any opportunity to do so.  And furthermore, what that cp_genericize_r
    spot does is check for any left-over immediate function calls (which can be
    ATM just std::source_location::current() call) and immediate functions
    outside of immediate functions can't have addresses leaked into the IL,
    so it will be always a direct call anyway.  And immediate functions
    themselves don't make it into genericization/gimplification.
    
    2021-02-18  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/99132
            * cp-gimplify.c (cp_genericize_r) <case CALL_EXPR>: Use
            cp_get_callee_fndecl_nofold instead of cp_get_callee_fndecl to check
            for immediate function calls.
    
            * g++.dg/cpp2a/constexpr-virtual18.C: New test.

Diff:
---
 gcc/cp/cp-gimplify.c                             |  2 +-
 gcc/testsuite/g++.dg/cpp2a/constexpr-virtual18.C | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index 1c5e15b957f..abb8a6ef078 100644
--- a/gcc/cp/cp-gimplify.c
+++ b/gcc/cp/cp-gimplify.c
@@ -1386,7 +1386,7 @@ cp_genericize_r (tree *stmt_p, int *walk_subtrees, void *data)
 	  break;
 	}
 
-      if (tree fndecl = cp_get_callee_fndecl (stmt))
+      if (tree fndecl = cp_get_callee_fndecl_nofold (stmt))
 	if (DECL_IMMEDIATE_FUNCTION_P (fndecl))
 	  {
 	    gcc_assert (source_location_current_p (fndecl));
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual18.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual18.C
new file mode 100644
index 00000000000..2fbf0b22da3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual18.C
@@ -0,0 +1,13 @@
+// PR c++/99132
+// { dg-do compile { target c++11 } }
+
+template <class T> struct A { T c; };
+template <class T> struct B {
+  A<T> d;
+  constexpr T operator-> () { return d.c; }
+  B (B &&);
+};
+struct C {
+  virtual void foo ();
+  void bar (B<C *> h) { h->foo (); }
+};