build.dsl: check type of resource number.

Fixes #599.

amaranth/build/dsl.py

@@ -197,8 +197,11 @@ class Resource(Subsignal):
             return cls(name_or_number + name_suffix, number, *ios)
 
     def __init__(self, name, number, *args):
-        super().__init__(name, *args)
+        if not isinstance(number, int):
+            raise TypeError("Resource number must be an integer, not {!r}"
+                            .format(number))
 
+        super().__init__(name, *args)
         self.number = number
 
     def __repr__(self):

tests/test_build_dsl.py

@@ -242,6 +242,12 @@ class ResourceTestCase(FHDLTestCase):
                                   " (subsignal rx (pins i A1))"
                                   " (attrs IOSTANDARD='LVCMOS33'))")
 
+    def test_number_wrong(self):
+        with self.assertRaisesRegex(TypeError,
+                r"^Resource number must be an integer, not \(pins o 1\)$"):
+            # number omitted by accident
+            Resource("led", Pins("1", dir="o"))
+
     def test_family(self):
         ios = [Subsignal("clk", Pins("A0", dir="o"))]
         r1  = Resource.family(0, default_name="spi", ios=ios)