Add guard against too many literal values

Author: cdce8p

mypy/checkexpr.py

@@ -207,6 +207,10 @@
 # see https://github.com/python/mypy/pull/5255#discussion_r196896335 for discussion.
 MAX_UNIONS: Final = 5
 
+# Use fallback type if literal addition of unions results in too many literal
+# values. Explicitly set on the safe side to prevent accidental issues.
+MAX_LITERAL_ADDITION_VALUES: Final = 15
+
 
 # Types considered safe for comparisons with --strict-equality due to known behaviour of __eq__.
 # NOTE: All these types are subtypes of AbstractSet.
@@ -3566,6 +3570,8 @@ def literal_expression_addition(
         )
         if len(values) == 1:
             return LiteralType(values[0], self.named_type(lvalue[1]))
+        elif len(values) > MAX_LITERAL_ADDITION_VALUES:
+            return None
         return make_simplified_union(
             [LiteralType(val, self.named_type(lvalue[1])) for val in values]
         )

test-data/unit/check-literal.test

@@ -3097,3 +3097,12 @@ def func(d: LookupDict, pos: Literal["top_", "bottom_", ""]) -> str:
 
 [builtins fixtures/dict.pyi]
 [typing fixtures/typing-typeddict.pyi]
+
+[case testLiteralAdditionGuardMaxValues]
+from typing_extensions import Literal
+
+HexDigit = Literal["0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "A", "B", "C", "D", "E", "F"]
+
+def foo(a: HexDigit, b: HexDigit, c: HexDigit) -> None:
+    reveal_type(a + b + c)  # N: Revealed type is "builtins.str"
+[builtins fixtures/primitives.pyi]