[mypyc] fix: Final load in unreachable branches (#20617)

- avoid crashing when loading Final values in unreachable branches by
falling back to the declared type
- add an irbuild-unreachable regression test for Final name loads

Fixes mypyc/mypyc#1180

Author: BobTheBuidler

mypyc/irbuild/expression.py

@@ -49,7 +49,15 @@
     UnaryExpr,
     Var,
 )
-from mypy.types import Instance, ProperType, TupleType, TypeType, get_proper_type
+from mypy.types import (
+    AnyType,
+    Instance,
+    ProperType,
+    TupleType,
+    TypeOfAny,
+    TypeType,
+    get_proper_type,
+)
 from mypyc.common import MAX_SHORT_INT
 from mypyc.ir.class_ir import ClassIR
 from mypyc.ir.func_ir import FUNC_CLASSMETHOD, FUNC_STATICMETHOD
@@ -147,13 +155,11 @@ def transform_name_expr(builder: IRBuilder, expr: NameExpr) -> Value:
         return math_literal
 
     if isinstance(expr.node, Var) and expr.node.is_final:
+        final_type = builder.types.get(expr) or expr.node.type
+        if final_type is None:
+            final_type = AnyType(TypeOfAny.special_form)
         value = builder.emit_load_final(
-            expr.node,
-            fullname,
-            expr.name,
-            builder.is_native_ref_expr(expr),
-            builder.types[expr],
-            expr.line,
+            expr.node, fullname, expr.name, builder.is_native_ref_expr(expr), final_type, expr.line
         )
         if value is not None:
             return value
@@ -208,8 +214,11 @@ def transform_member_expr(builder: IRBuilder, expr: MemberExpr) -> Value:
     final = builder.get_final_ref(expr)
     if final is not None:
         fullname, final_var, native = final
+        final_type = builder.types.get(expr) or final_var.type
+        if final_type is None:
+            final_type = AnyType(TypeOfAny.special_form)
         value = builder.emit_load_final(
-            final_var, fullname, final_var.name, native, builder.types[expr], expr.line
+            final_var, fullname, final_var.name, native, final_type, expr.line
         )
         if value is not None:
             return value

mypyc/test-data/irbuild-unreachable.test

@@ -68,6 +68,34 @@ L3:
     y = r6
     return 1
 
+[case testUnreachableFinalNameExpr]
+from typing import Final
+
+ZERO: Final = 0
+
+def f(x: int) -> int:
+    if x is None:
+        return ZERO
+    return ZERO
+[out]
+def f(x):
+    x :: int
+    r0, r1 :: object
+    r2 :: bit
+    r3 :: object
+    r4 :: int
+L0:
+    r0 = load_address _Py_NoneStruct
+    r1 = box(int, x)
+    r2 = r1 == r0
+    if r2 goto L1 else goto L2 :: bool
+L1:
+    r3 = object 0
+    r4 = unbox(int, r3)
+    return r4
+L2:
+    return 0
+
 [case testUnreachableStatementAfterReturn]
 def f(x: bool) -> int:
     if x: