Allow type parameters reusing the name missing from current module (#20081)

Fixes #18507. Fixes #19526. Fixes #19946 (already closed without
reproducer, but star imports may cause such issues,
`testPEP695TypeVarNameClashStarImport` fails on current master).

Even if a type parameter is reusing the name from outer scope, it still
cannot be a redefinition, so we can safely store it even in presence of
unresolved star imports or variables that do not refer to a type.

Author: sterliakov

mypy/semanal.py

@@ -1794,7 +1794,9 @@ def push_type_args(
             if self.is_defined_type_param(p.name):
                 self.fail(f'"{p.name}" already defined as a type parameter', context)
             else:
-                self.add_symbol(p.name, tv, context, no_progress=True, type_param=True)
+                assert self.add_symbol(
+                    p.name, tv, context, no_progress=True, type_param=True
+                ), "Type parameter should not be discarded"
 
         return tvs
 
@@ -6830,6 +6832,7 @@ def add_symbol_table_node(
             else:
                 # see note in docstring describing None contexts
                 self.defer()
+
         if (
             existing is not None
             and context is not None
@@ -6849,7 +6852,9 @@ def add_symbol_table_node(
                     self.add_redefinition(names, name, symbol)
                 if not (isinstance(new, (FuncDef, Decorator)) and self.set_original_def(old, new)):
                     self.name_already_defined(name, context, existing)
-        elif name not in self.missing_names[-1] and "*" not in self.missing_names[-1]:
+        elif type_param or (
+            name not in self.missing_names[-1] and "*" not in self.missing_names[-1]
+        ):
             names[name] = symbol
             if not no_progress:
                 self.progress = True

test-data/unit/check-python312.test

@@ -1228,6 +1228,43 @@ class C[T]:
 def f[S, S](x: S) -> S:  # E: "S" already defined as a type parameter
     return x
 
+[case testPEP695TypeVarNameClashNoCrashForwardReference]
+# https://github.com/python/mypy/issues/18507
+from typing import TypeVar
+T = TypeVar("T", bound=Foo)  # E: Name "Foo" is used before definition
+
+class Foo: ...
+class Bar[T]: ...
+
+[case testPEP695TypeVarNameClashNoCrashDeferredSymbol]
+# https://github.com/python/mypy/issues/19526
+T = Unknown  # E: Name "Unknown" is not defined
+
+class Foo[T]: ...
+class Bar[*T]: ...
+class Baz[**T]: ...
+[builtins fixtures/tuple.pyi]
+
+[case testPEP695TypeVarNameClashTypeAlias]
+type Tb = object
+type Ta[Tb] = 'B[Tb]'
+class A[Ta]: ...
+class B[Tb](A[Ta]): ...
+
+[case testPEP695TypeVarNameClashStarImport]
+# Similar to
+# https://github.com/python/mypy/issues/19946
+import a
+
+[file a.py]
+from b import *
+class Foo[T]: ...
+
+[file b.py]
+from a import *
+class Bar[T]: ...
+[builtins fixtures/tuple.pyi]
+
 [case testPEP695ClassDecorator]
 from typing import Any