Use variable name to determined NamedTuple class name

This lets us avoid inserting line numbers into the name when the
variable name and argument to NamedTuple disagree. This is good,
because the line numbers are ugly and when combined with bug #6548
causes problems when a namedtuple is reexported.

Author: msullivan

mypy/semanal_namedtuple.py

@@ -178,14 +178,20 @@ def check_namedtuple(self,
             info = self.build_namedtuple_typeinfo(name, [], [], {}, node.line)
             self.store_namedtuple_info(info, name, call, is_typed)
             return True, info
-        name = cast(Union[StrExpr, BytesExpr, UnicodeExpr], call.args[0]).value
-        if name != var_name or is_func_scope:
-            # There are three special cases where need to give it a unique name derived
+
+        # We use the variable name as the class name if it exists. If
+        # it doesn't, we use the name passed as an argument. We prefer
+        # the variable name because it should be unique inside a
+        # module, and so we don't need to disambiguate it with a line
+        # number.
+        if var_name:
+            name = var_name
+        else:
+            name = cast(Union[StrExpr, BytesExpr, UnicodeExpr], call.args[0]).value
+
+        if var_name is None or is_func_scope:
+            # There are two special cases where need to give it a unique name derived
             # from the line number:
-            #   * There is a name mismatch with l.h.s., therefore we need to disambiguate
-            #     situations like:
-            #         A = NamedTuple('Same', [('x', int)])
-            #         B = NamedTuple('Same', [('y', str)])
             #   * This is a base class expression, since it often matches the class name:
             #         class NT(NamedTuple('NT', [...])):
             #             ...

test-data/unit/check-incremental.test

@@ -5036,7 +5036,7 @@ NT = NamedTuple('BadName', [('x', int)])
 [builtins fixtures/tuple.pyi]
 [out]
 [out2]
-tmp/a.py:3: note: Revealed type is 'Tuple[builtins.int, fallback=b.BadName@2]'
+tmp/a.py:3: note: Revealed type is 'Tuple[builtins.int, fallback=b.NT]'
 
 [case testNewAnalyzerIncrementalBrokenNamedTupleNested]
 

test-data/unit/fine-grained.test

@@ -9572,3 +9572,25 @@ c.py:2: note: Revealed type is 'a.A'
 ==
 c.py:2: note: Revealed type is 'a.<subclass of "A" and "B">'
 
+[case testReexportNamedTupleChange]
+from m import M
+
+def f(x: M) -> None: ...
+
+f(M(0))
+
+[file m.py]
+from n import M
+
+[file n.py]
+from typing import NamedTuple
+M = NamedTuple('_N', [('x', int)])
+
+[file n.py.2]
+# change the line numbers
+from typing import NamedTuple
+M = NamedTuple('_N', [('x', int)])
+
+[builtins fixtures/tuple.pyi]
+[out]
+==