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 --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index 1c5e15b957fd9907f871780e11aca4a305b0d6e7..abb8a6ef0789ea37ad37157e25bd8dd9b96a8a44 100644 (file)
--- 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 (file)
index 0000000..2fbf0b2
--- /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 (); }
+};