c++: ICE with -Wall and constexpr if [PR94937]

An ICE arises here because we call cp_get_callee_fndecl_nofold in a
template, and we've got a CALL_EXPR whose CALL_EXPR_FN is a BASELINK.
This tickles the INDIRECT_TYPE_P assert in cp_get_fndecl_from_callee.

Fixed by turning the assert into a condition and returning NULL_TREE
in that case.

	PR c++/94937
	* cvt.c (cp_get_fndecl_from_callee): Return NULL_TREE if the function
	type is not INDIRECT_TYPE_P.
	* decl.c (omp_declare_variant_finalize_one): Call
	cp_get_callee_fndecl_nofold instead of looking for the function decl
	manually.

	* g++.dg/cpp1z/constexpr-if34.C: New test.
	* g++.dg/cpp2a/is-constant-evaluated10.C: New test.

6 files changed, 62 insertions, 13 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index d8ad1d6..605cdf3 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,5 +1,14 @@
 2020-05-18  Marek Polacek  <polacek@redhat.com>
 
+	PR c++/94937
+	* cvt.c (cp_get_fndecl_from_callee): Return NULL_TREE if the function
+	type is not INDIRECT_TYPE_P.
+	* decl.c (omp_declare_variant_finalize_one): Call
+	cp_get_callee_fndecl_nofold instead of looking for the function decl
+	manually.
+
+2020-05-18  Marek Polacek  <polacek@redhat.com>
+
 	PR c++/90915
 	* parser.c (cp_parser_has_attribute_expression): Sorry on a
 	type-dependent argument.
diff --git a/gcc/cp/cvt.c b/gcc/cp/cvt.c
index 656e7fd..3710028 100644
--- a/gcc/cp/cvt.c
+++ b/gcc/cp/cvt.c
@@ -995,9 +995,8 @@ cp_get_fndecl_from_callee (tree fn, bool fold /* = true */)
   if (TREE_CODE (fn) == FUNCTION_DECL)
     return fn;
   tree type = TREE_TYPE (fn);
-  if (type == unknown_type_node)
+  if (type == NULL_TREE || !INDIRECT_TYPE_P (type))
     return NULL_TREE;
-  gcc_assert (INDIRECT_TYPE_P (type));
   if (fold)
     fn = maybe_constant_init (fn);
   STRIP_NOPS (fn);
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 7c7ca1f..6469850 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -7295,17 +7295,7 @@ omp_declare_variant_finalize_one (tree decl, tree attr)
   if (variant == error_mark_node && !processing_template_decl)
     return true;
 
-  variant = cp_get_callee (variant);
-  if (variant)
-    {
-      if (TREE_CODE (variant) == FUNCTION_DECL)
-	;
-      else if (TREE_TYPE (variant) && INDIRECT_TYPE_P (TREE_TYPE (variant)))
-	variant = cp_get_fndecl_from_callee (variant, false);
-      else
-	variant = NULL_TREE;
-    }
-
+  variant = cp_get_callee_fndecl_nofold (variant);
   input_location = save_loc;
 
   if (variant)
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 40e9d66..67f2c55 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2020-05-18  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94937
+	* g++.dg/cpp1z/constexpr-if34.C: New test.
+	* g++.dg/cpp2a/is-constant-evaluated10.C: New test.
+
 2020-05-18  Martin Sebor  <msebor@redhat.com>
 
 	PR middle-end/92815
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-if34.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-if34.C
new file mode 100644
index 0000000..6e0b259
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-if34.C
@@ -0,0 +1,15 @@
+// PR c++/94937 - ICE with -Wall and constexpr if.
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wall" }
+
+struct B {
+  static constexpr bool foo() { return false; }
+};
+
+template<typename T>
+struct C {
+  static void bar ()
+  {
+    if constexpr (B::foo()) ;
+  }
+};
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated10.C b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated10.C
new file mode 100644
index 0000000..7b2e345
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated10.C
@@ -0,0 +1,30 @@
+// { dg-do compile { target c++2a } }
+// { dg-options "-Wtautological-compare" }
+
+namespace std {
+  constexpr inline bool
+  is_constant_evaluated () noexcept
+  {
+    return __builtin_is_constant_evaluated ();
+  }
+}
+
+template<typename>
+constexpr int
+foo(int i)
+{
+  if constexpr (std::is_constant_evaluated ()) // { dg-warning ".std::is_constant_evaluated. always evaluates to true in .if constexpr." }
+    return 42;
+  else
+    return i;
+}
+
+template<typename>
+constexpr int
+foo2(int i)
+{
+  if constexpr (__builtin_is_constant_evaluated ()) // { dg-warning ".std::is_constant_evaluated. always evaluates to true in .if constexpr." }
+    return 42;
+  else
+    return i;
+}