[mypyc] Optimize TYPE_CHECKING to False at Runtime (#16263)

Fixes [mypyc/mypyc#902](mypyc/mypyc#902)

This PR finds references of `typing.TYPE_CHECKING` or
`typing_extensions.TYPE_CHECKING` and optimizes them to `False` in
mypyc.

mypyc/irbuild/expression.py

@@ -126,6 +126,8 @@ def transform_name_expr(builder: IRBuilder, expr: NameExpr) -> Value:
         return builder.true()
     if fullname == "builtins.False":
         return builder.false()
+    if fullname in ("typing.TYPE_CHECKING", "typing_extensions.TYPE_CHECKING"):
+        return builder.false()
 
     math_literal = transform_math_literal(builder, fullname)
     if math_literal is not None:
@@ -185,6 +187,10 @@ def transform_name_expr(builder: IRBuilder, expr: NameExpr) -> Value:
 
 
 def transform_member_expr(builder: IRBuilder, expr: MemberExpr) -> Value:
+    # Special Cases
+    if expr.fullname in ("typing.TYPE_CHECKING", "typing_extensions.TYPE_CHECKING"):
+        return builder.false()
+
     # First check if this is maybe a final attribute.
     final = builder.get_final_ref(expr)
     if final is not None:

mypyc/test-data/irbuild-basic.test

@@ -3688,3 +3688,33 @@ def f(arg):
     arg :: __main__.A
 L0:
     return arg
+
+[case testTypeCheckingFlag]
+from typing import TYPE_CHECKING, List
+
+def f(arg: List[int]) -> int:
+    if TYPE_CHECKING:
+        from collections.abc import Sized
+    s: Sized = arg
+    return len(s)
+
+[out]
+def f(arg):
+    arg :: list
+    r0 :: bool
+    r1 :: int
+    r2 :: bit
+    s :: object
+    r3 :: int
+L0:
+    r0 = 0 << 1
+    r1 = extend r0: builtins.bool to builtins.int
+    r2 = r1 != 0
+    if r2 goto L1 else goto L2 :: bool
+L1:
+    goto L3
+L2:
+L3:
+    s = arg
+    r3 = CPyObject_Size(s)
+    return r3