Do pass 1 semanatic analysis on the FuncDef of decorated functions (p…

…ython#5654)

This populates fullname properly, fixing a crash where a decorated
function is used as a type annotation in an import cycle,
as well as fixing an obscure bug in sys.platform checks inside
decorated functions.

Fixes python#5652.

mypy/semanal_pass1.py

@@ -147,14 +147,20 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
             for lval in s.lvalues:
                 self.analyze_lvalue(lval, explicit_type=s.type is not None)
 
-    def visit_func_def(self, func: FuncDef) -> None:
+    def visit_func_def(self, func: FuncDef, decorated: bool = False) -> None:
+        """Process a func def.
+
+        decorated is true if we are processing a func def in a
+        Decorator that needs a _fullname and to have its body analyzed but
+        does not need to be added to the symbol table.
+        """
         sem = self.sem
         if sem.type is not None:
             # Don't process methods during pass 1.
             return
         func.is_conditional = sem.block_depth[-1] > 0
         func._fullname = sem.qualified_name(func.name())
-        at_module = sem.is_module_scope()
+        at_module = sem.is_module_scope() and not decorated
         if (at_module and func.name() == '__getattr__' and
                 self.sem.cur_mod_node.is_package_init_file() and self.sem.cur_mod_node.is_stub):
             if isinstance(func.type, CallableType):
@@ -311,6 +317,7 @@ def visit_decorator(self, d: Decorator) -> None:
             return
         d.var._fullname = self.sem.qualified_name(d.var.name())
         self.add_symbol(d.var.name(), SymbolTableNode(self.kind_by_scope(), d), d)
+        self.visit_func_def(d.func, decorated=True)
 
     def visit_if_stmt(self, s: IfStmt) -> None:
         infer_reachability_of_if_statement(s, self.sem.options)

test-data/unit/check-modules.test

@@ -733,6 +733,27 @@ import m.a
 [file m/a.py]
 [out]
 
+[case testCheckDecoratedFuncAsAnnotWithImportCycle]
+import a
+[file a.py]
+from typing import TypeVar
+import b
+
+T = TypeVar('T')
+def idf(x: T) -> T: return x
+
+@idf
+def Session() -> None: pass
+
+[file b.py]
+MYPY = False
+if MYPY:
+    from a import Session
+
+def f(self, session: Session) -> None:  # E: Invalid type "a.Session"
+    pass
+[builtins fixtures/bool.pyi]
+
 
 -- Checks dealing with submodules and different kinds of imports
 -- -------------------------------------------------------------

test-data/unit/check-unreachable-code.test

@@ -416,6 +416,25 @@ x = 1
 [builtins fixtures/ops.pyi]
 [out]
 
+[case testSysPlatformInFunctionImport3]
+from typing import Callable
+import sys
+
+def idf(x: Callable[[], None]) -> Callable[[], None]: return x
+
+@idf
+def foo() -> None:
+    if sys.platform == 'fictional':
+        import b as a
+    else:
+        import a
+    a.x
+[file a.py]
+x = 1
+[builtins fixtures/ops.pyi]
+[out]
+
+
 [case testSysPlatformInMethodImport2]
 import sys
 class A: