feat[next]: Check fencil/program args in ITIR type inference (GridToo…

…ls#1565)

When the new ITIR type inference was introduced this broke the code generation in icon4py. This PR adds a small assert and improves the docstring.

src/gt4py/next/iterator/type_system/inference.py

@@ -331,6 +331,11 @@ def apply(
             allow_undeclared_symbols: Allow references to symbols that don't have a corresponding
               declaration. This is useful for testing or inference on partially inferred sub-nodes.
 
+        Preconditions:
+
+        All parameters in :class:`itir.Program` and :class:`itir.FencilDefinition` must have a type
+        defined, as they are the starting point for type propagation.
+
         Design decisions:
         - Lamba functions are monomorphic
         Builtin functions like ``plus`` are by design polymorphic and only their argument and return
@@ -378,6 +383,12 @@ def apply(
         if not allow_undeclared_symbols:
             node = RemoveTypes().visit(node)
 
+        if isinstance(node, (itir.FencilDefinition, itir.Program)):
+            assert all(isinstance(param.type, ts.DataType) for param in node.params), (
+                "All parameters in 'itir.Program' and 'itir.FencilDefinition' must have a type "
+                "defined, as they are the starting point for type propagation.",
+            )
+
         instance = cls(
             offset_provider=offset_provider,
             dimensions=(