Count overloaded functions signatures correctly in error message (#1407)

Author: rwbarton

mypy/checker.py

@@ -498,7 +498,7 @@ def check_overlapping_overloads(self, defn: OverloadedFuncDef) -> None:
                 sig1 = self.function_type(item.func)
                 sig2 = self.function_type(item2.func)
                 if is_unsafe_overlapping_signatures(sig1, sig2):
-                    self.msg.overloaded_signatures_overlap(i + 1, j + 2,
+                    self.msg.overloaded_signatures_overlap(i + 1, i + j + 2,
                                                            item.func)
 
     def is_generator_return_type(self, typ: Type) -> bool:

mypy/test/data/check-overloading.test

@@ -630,3 +630,14 @@ def f(x: Sequence[int]) -> int: pass
 # These are considered overlapping despite the bound on T due to runtime type erasure.
 [out]
 main:4: error: Overloaded function signatures 1 and 2 overlap with incompatible return types
+
+[case testOverlappingOverloadCounting]
+from typing import overload
+class A: pass
+class B(A): pass
+@overload
+def f(x: int) -> None: pass
+@overload
+def f(x: B) -> str: pass # E: Overloaded function signatures 2 and 3 overlap with incompatible return types
+@overload
+def f(x: A) -> int: pass