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.

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);

gcc/cp/decl.c

@@ -7299,17 +7299,7 @@ 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)

gcc/testsuite/g++.dg/cpp1z/constexpr-if34.C

+// 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()) ;
+  }
+};

gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated10.C

+// { 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;
+}