[used before def] correctly handle walrus operator (python#14646)

Fixes python#14626.

I believe changing the way that we analyze call expression makes sense
(first, we analyze the callee, then we analyze the arguments).

mypy/traverser.py

@@ -253,9 +253,9 @@ def visit_yield_expr(self, o: YieldExpr) -> None:
             o.expr.accept(self)
 
     def visit_call_expr(self, o: CallExpr) -> None:
+        o.callee.accept(self)
         for a in o.args:
             a.accept(self)
-        o.callee.accept(self)
         if o.analyzed:
             o.analyzed.accept(self)
 

test-data/unit/check-python38.test

@@ -573,6 +573,14 @@ def foo() -> None:
     [x := x + y for y in [1, 2, 3]]
 [builtins fixtures/dict.pyi]
 
+[case testWalrusUsedBeforeDef]
+# flags: --python-version 3.8
+class C:
+    def f(self, c: 'C') -> None: pass
+
+(x := C()).f(y)  # E: Cannot determine type of "y"  # E: Name "y" is used before definition
+(y := C()).f(y)
+
 [case testOverloadWithPositionalOnlySelf]
 # flags: --python-version 3.8
 from typing import overload, Optional