[PATCH] c++: Fix ICE in tsubst_default_argument [PR92010]

[Patrick Palka]

This patch relaxes an assertion in tsubst_default_argument that exposes a latent
bug in how we substitute an array type into a cv-qualified wildcard function
parameter type.  Concretely, the latent bug is that given the function template

  template<typename T> void foo(const T t);

one would expect the type of foo<int[]> to be void(const int*), but we
(seemingly prematurely) strip function parameter types of their top-level
cv-qualifiers when building the function's TYPE_ARG_TYPES, and instead end up
obtaining void(int*) as the type of foo<int[]> after substitution and decaying.

We still however correctly substitute into and decay the formal parameter type,
obtaining const int* as the type of t after substitution.  But this then leads
to us tripping over the assert in tsubst_default_argument that verifies the
formal parameter type and the function type are consistent.

Assuming it's too late at this stage to fix the substitution bug, we can still
relax the assertion like so.  Tested on x86_64-pc-linux-gnu, does this look OK?

gcc/cp/ChangeLog:

	PR c++/92010
	* pt.c (tsubst_default_argument): Relax assertion to permit
	disagreements between the function type and the parameter type
	about the cv-qualification of the pointed-to type.

gcc/testsuite/ChangeLog:

	PR c++/92010
	* g++.dg/template/defarg22.C: New test.
---
 gcc/cp/pt.c                              | 17 ++++++++++++++++-
 gcc/testsuite/g++.dg/template/defarg22.C | 11 +++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/template/defarg22.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 9e1eb9416c9..923166276b8 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -13337,7 +13337,22 @@ tsubst_default_argument (tree fn, int parmnum, tree type, tree arg,
   if (parmtype == error_mark_node)
     return error_mark_node;
 
-  gcc_assert (same_type_ignoring_top_level_qualifiers_p (type, parmtype));
+  if (same_type_ignoring_top_level_qualifiers_p (type, parmtype))
+    ;
+  else if (POINTER_TYPE_P (type) && POINTER_TYPE_P (parmtype))
+    {
+      /* The function type and the parameter type can disagree about the
+	 cv-qualification of the pointed-to type; see PR92010.  */
+      gcc_assert (same_type_p (TREE_TYPE (type),
+			       strip_top_quals (TREE_TYPE (parmtype))));
+      /* Verify that this happens only when the dependent parameter type is a
+	 cv-qualified wildcard type.  */
+      tree pattern_parm = get_pattern_parm (parm, DECL_TI_TEMPLATE (fn));
+      gcc_assert (WILDCARD_TYPE_P (TREE_TYPE (pattern_parm))
+		  && cv_qualified_p (TREE_TYPE (pattern_parm)));
+    }
+  else
+    gcc_unreachable ();
 
   tree *slot;
   if (defarg_inst && (slot = defarg_inst->get (parm)))
diff --git a/gcc/testsuite/g++.dg/template/defarg22.C b/gcc/testsuite/g++.dg/template/defarg22.C
new file mode 100644
index 00000000000..cf6261916d8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/defarg22.C
@@ -0,0 +1,11 @@
+// PR c++/92010
+// { dg-do compile }
+
+template <typename T>
+void foo(const T t = 0)
+{ }
+
+int main()
+{
+  foo<char[]>();
+}
-- 
2.26.0.rc1.11.g30e9940356