[used-before-def] fix false positive inside loop (#14307)

A similar case was addressed in #14176 but I missed the part where it
doesn't need to be defined in a different branch. This makes the fix
more complete.

Author: ilinum

mypy/partially_defined.py

@@ -322,7 +322,10 @@ def process_definition(self, name: str) -> None:
         # Was this name previously used? If yes, it's a used-before-definition error.
         refs = self.tracker.pop_undefined_ref(name)
         for ref in refs:
-            self.var_used_before_def(name, ref)
+            if self.loops:
+                self.variable_may_be_undefined(name, ref)
+            else:
+                self.var_used_before_def(name, ref)
         self.tracker.record_definition(name)
 
     def visit_global_decl(self, o: GlobalDecl) -> None:

test-data/unit/check-possibly-undefined.test

@@ -382,7 +382,7 @@ def f2() -> None:
             x = 2
         w = x  # No error.
 
-[case testDefinedDifferentBranchPossiblyUndefined]
+[case testPossiblyUndefinedLoop]
 # flags: --enable-error-code possibly-undefined --enable-error-code used-before-def
 
 def f0() -> None:
@@ -423,6 +423,11 @@ def f3() -> None:
     else:
         y = x  # E: Name "x" may be undefined
 
+def f4() -> None:
+    while int():
+        y = x  # E: Name "x" may be undefined
+        x: int = 1
+
 [case testAssert]
 # flags: --enable-error-code possibly-undefined
 def f1() -> int: