c++: constexpr virtual and vbase thunk

C++20 allows virtual functions to be constexpr.  I don't think that calling
through a pointer to a vbase subobject is supposed to work in a constant
expression, since an object with virtual bases can't be constant, but the
call shouldn't ICE.

gcc/cp/ChangeLog:

	* constexpr.c (cxx_eval_thunk_call): Error instead of ICE
	on vbase thunk to constexpr function.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-virtual20.C: New test.

2 files changed, 32 insertions, 5 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 453007c..7c27131 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -2246,15 +2246,20 @@ cxx_eval_thunk_call (const constexpr_ctx *ctx, tree t, tree thunk_fndecl,
 {
   tree function = THUNK_TARGET (thunk_fndecl);
 
-  /* virtual_offset is only set in the presence of virtual bases, which make
-     the class non-literal, so we don't need to handle it here.  */
   if (THUNK_VIRTUAL_OFFSET (thunk_fndecl))
     {
-      gcc_assert (!DECL_DECLARED_CONSTEXPR_P (function));
       if (!ctx->quiet)
 	{
-	  error ("call to non-%<constexpr%> function %qD", function);
-	  explain_invalid_constexpr_fn (function);
+	  if (!DECL_DECLARED_CONSTEXPR_P (function))
+	    {
+	      error ("call to non-%<constexpr%> function %qD", function);
+	      explain_invalid_constexpr_fn (function);
+	    }
+	  else
+	    /* virtual_offset is only set for virtual bases, which make the
+	       class non-literal, so we don't need to handle it here.  */
+	    error ("calling constexpr member function %qD through virtual "
+		   "base subobject", function);
 	}
       *non_constant_p = true;
       return t;
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual20.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual20.C
new file mode 100644
index 0000000..3c411fa
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual20.C
@@ -0,0 +1,22 @@
+// Test for constexpr call through vbase thunk.
+// { dg-do compile { target c++20 } }
+
+class Rep {
+public:
+  constexpr virtual int foo() { return 1; }
+};
+
+class VBase {
+public:
+  constexpr virtual int foo() { return 2; }
+};
+
+class Main : public Rep, virtual public VBase {
+public:
+  constexpr virtual int foo() { return 5; }
+};
+
+int main() {
+  Main m;
+  static_assert(static_cast<VBase*>(&m)->foo() == 5); // { dg-error "" }
+}