Fix crash upon bad let expr

If the expr type is None, the program crashed.
Also add more typing hints.

Test case included

Author: boriel

src/libzxbc/zxbparser.py

@@ -13,12 +13,14 @@
 # PI Constant
 # PI = 3.1415927 is ZX Spectrum PI representation
 # But a better one is 3.141592654, so take it from math
+
 import math
 from math import pi as PI
 import collections
 
 # typings
 from typing import NamedTuple
+from typing import Optional
 
 # Compiler API
 from src.api.debug import __DEBUG__  # analysis:ignore
@@ -181,15 +183,18 @@ def make_nop():
     return symbols.NOP()
 
 
-def make_number(value, lineno, type_=None):
+def make_number(value, lineno: int, type_=None):
     """ Wrapper: creates a constant number node.
     """
     return symbols.NUMBER(value, type_=type_, lineno=lineno)
 
 
-def make_typecast(type_, node, lineno):
+def make_typecast(type_: symbols.TYPE, node: Optional[symbols.SYMBOL], lineno: int):
     """ Wrapper: returns a Typecast node
     """
+    if node is None or node.type_ is None:
+        return  # syntax / semantic error
+
     assert isinstance(type_, symbols.TYPE)
     return symbols.TYPECAST.make_node(type_, node, lineno)
 

src/parsetab/tabs.dbm.bak

@@ -1,4 +1,4 @@
 'zxbpp', (0, 72571)
 'asmparse', (72704, 254311)
 'zxnext_asmparse', (327168, 285299)
-'zxbparser', (612864, 712099)
+'zxbparser', (612864, 715296)

src/parsetab/tabs.dbm.dat

@@ -1,4 +1,4 @@
 'zxbpp', (0, 72571)
 'asmparse', (72704, 254311)
 'zxnext_asmparse', (327168, 285299)
-'zxbparser', (612864, 712099)
+'zxbparser', (612864, 715296)

src/parsetab/tabs.dbm.dir

@@ -0,0 +1,9 @@
+DIM a as Ubyte
+
+FUNCTION editStringFN(ByREF s AS Byte) as s
+    s = s + 1
+    RETURN s
+END FUNCTION
+
+a = editStringFN(a, 1, "i")
+