Don't merge starting If blocks without overloads

mypy/fastparse.py

@@ -486,7 +486,11 @@ def fix_function_overloads(self, stmts: List[Statement]) -> List[Statement]:
             if (
                 isinstance(stmt, IfStmt)
                 and len(stmt.body[0].body) == 1
-                and isinstance(stmt.body[0].body[0], (Decorator, FuncDef, OverloadedFuncDef))
+                and (
+                    isinstance(stmt.body[0].body[0], (Decorator, OverloadedFuncDef))
+                    or current_overload_name is not None
+                    and isinstance(stmt.body[0].body[0], FuncDef)
+                )
             ):
                 # Check IfStmt block to determine if function overloads can be merged
                 if_overload_name = self._check_ifstmt_for_overloads(stmt)

test-data/unit/check-functions.test

@@ -1400,9 +1400,8 @@ def top() -> None:
 from typing import Any
 x = None # type: Any
 if x:
-    pass  # some other node
     def f(): pass
-def f(): pass # E: Name "f" already defined on line 5
+def f(): pass # E: Name "f" already defined on line 4
 
 [case testIncompatibleConditionalFunctionDefinition]
 from typing import Any
@@ -1647,9 +1646,8 @@ from typing import Any
 x = None # type: Any
 class A:
     if x:
-        pass  # Some other node
         def f(self): pass
-    def f(self): pass # E: Name "f" already defined on line 6
+    def f(self): pass # E: Name "f" already defined on line 5
 
 [case testIncompatibleConditionalMethodDefinition]
 from typing import Any

test-data/unit/check-overloading.test

@@ -6153,11 +6153,11 @@ reveal_type(f5(A()))  # N: Revealed type is "__main__.A"
 reveal_type(f5(B()))  # N: Revealed type is "__main__.B"
 
 # Test from check-functions - testUnconditionalRedefinitionOfConditionalFunction
-# If IfStmt only contains FuncDef, block is ignore if uncertain about execution
-# Necessary to be able to ignore always-false cases
+# Don't merge If blocks if they appear before any overloads
+# and don't contain any overloads themselves.
 if maybe_true:  # E: Name "maybe_true" is not defined
     def f6(x): ...
-def f6(x): ...
+def f6(x): ...  # E: Name "f6" already defined on line 61
 
 if maybe_true:  # E: Name "maybe_true" is not defined
     pass  # Some other node