Allow adjacent conditionally-defined overloads (#19042)

Fixes #19015. Fixes #17521.

Author: sterliakov

mypy/fastparse.py

@@ -631,7 +631,7 @@ def fix_function_overloads(self, stmts: list[Statement]) -> list[Statement]:
         ret: list[Statement] = []
         current_overload: list[OverloadPart] = []
         current_overload_name: str | None = None
-        seen_unconditional_func_def = False
+        last_unconditional_func_def: str | None = None
         last_if_stmt: IfStmt | None = None
         last_if_overload: Decorator | FuncDef | OverloadedFuncDef | None = None
         last_if_stmt_overload_name: str | None = None
@@ -641,7 +641,7 @@ def fix_function_overloads(self, stmts: list[Statement]) -> list[Statement]:
             if_overload_name: str | None = None
             if_block_with_overload: Block | None = None
             if_unknown_truth_value: IfStmt | None = None
-            if isinstance(stmt, IfStmt) and seen_unconditional_func_def is False:
+            if isinstance(stmt, IfStmt):
                 # Check IfStmt block to determine if function overloads can be merged
                 if_overload_name = self._check_ifstmt_for_overloads(stmt, current_overload_name)
                 if if_overload_name is not None:
@@ -669,11 +669,18 @@ def fix_function_overloads(self, stmts: list[Statement]) -> list[Statement]:
                     last_if_unknown_truth_value = None
                 current_overload.append(stmt)
                 if isinstance(stmt, FuncDef):
-                    seen_unconditional_func_def = True
+                    # This is, strictly speaking, wrong: there might be a decorated
+                    # implementation. However, it only affects the error message we show:
+                    # ideally it's "already defined", but "implementation must come last"
+                    # is also reasonable.
+                    # TODO: can we get rid of this completely and just always emit
+                    # "implementation must come last" instead?
+                    last_unconditional_func_def = stmt.name
             elif (
                 current_overload_name is not None
                 and isinstance(stmt, IfStmt)
                 and if_overload_name == current_overload_name
+                and last_unconditional_func_def != current_overload_name
             ):
                 # IfStmt only contains stmts relevant to current_overload.
                 # Check if stmts are reachable and add them to current_overload,
@@ -729,7 +736,7 @@ def fix_function_overloads(self, stmts: list[Statement]) -> list[Statement]:
                 # most of mypy/mypyc assumes that all the functions in an OverloadedFuncDef are
                 # related, but multiple underscore functions next to each other aren't necessarily
                 # related
-                seen_unconditional_func_def = False
+                last_unconditional_func_def = None
                 if isinstance(stmt, Decorator) and not unnamed_function(stmt.name):
                     current_overload = [stmt]
                     current_overload_name = stmt.name

test-data/unit/check-overloading.test

@@ -6310,6 +6310,40 @@ reveal_type(f12(A()))  # N: Revealed type is "__main__.A"
 
 [typing fixtures/typing-medium.pyi]
 
+[case testAdjacentConditionalOverloads]
+# flags: --always-true true_alias
+from typing import overload
+
+true_alias = True
+
+if true_alias:
+    @overload
+    def ham(v: str) -> list[str]: ...
+
+    @overload
+    def ham(v: int) -> list[int]: ...
+
+def ham(v: "int | str") -> "list[str] | list[int]":
+    return []
+
+if true_alias:
+    @overload
+    def spam(v: str) -> str: ...
+
+    @overload
+    def spam(v: int) -> int: ...
+
+def spam(v: "int | str") -> "str | int":
+    return ""
+
+reveal_type(ham)  # N: Revealed type is "Overload(def (v: builtins.str) -> builtins.list[builtins.str], def (v: builtins.int) -> builtins.list[builtins.int])"
+reveal_type(spam)  # N: Revealed type is "Overload(def (v: builtins.str) -> builtins.str, def (v: builtins.int) -> builtins.int)"
+
+reveal_type(ham(""))  # N: Revealed type is "builtins.list[builtins.str]"
+reveal_type(ham(0))  # N: Revealed type is "builtins.list[builtins.int]"
+reveal_type(spam(""))  # N: Revealed type is "builtins.str"
+reveal_type(spam(0))  # N: Revealed type is "builtins.int"
+
 [case testOverloadIfUnconditionalFuncDef]
 # flags: --always-true True --always-false False
 from typing import overload