gcc: do not warn about unreachable switches

The C code is generated naively and we do not try to remove dead parts.
Thus silent C compiler warnings about unreachable things.

Close nitlang#2780

Signed-off-by: Jean Privat <[email protected]>

src/compiler/abstract_compiler.nit

@@ -386,7 +386,7 @@ endif
 ifeq ($(origin CXX), default)
         CXX = ccache c++
 endif
-CFLAGS ?= -g {{{if not debug then "-O2" else ""}}} -Wno-unused-value -Wno-switch -Wno-attributes -Wno-trigraphs
+CFLAGS ?= -g {{{if not debug then "-O2" else ""}}} -Wno-unused-value -Wno-switch -Wno-attributes -Wno-trigraphs -Wno-switch-unreachable
 CINCL =
 LDFLAGS ?=
 LDLIBS  ?= -lm {{{linker_options.join(" ")}}}

tests/sav/test_generic_nullable.res

@@ -0,0 +1,4 @@
+0
+0
+1
+2

tests/test_generic_nullable.nit

@@ -0,0 +1,35 @@
+# This file is part of NIT ( http://www.nitlanguage.org ).
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import core::kernel
+
+class C[E]
+        var x: E
+        fun read_or_die
+        do
+                if x != null then
+                        x.as(not null).output
+		else
+			0.output
+                end
+        end
+end
+var c1 = new C[nullable Object](null)
+var c2 = new C[nullable Int](null)
+c1.read_or_die
+c2.read_or_die
+c1.x = 1
+c2.x = 2
+c1.read_or_die
+c2.read_or_die