Short-circuit if expression for always true/always false vars and M…

…YPY/TYPE_CHECKING

 - Always short-circuit in the if's expression, instead of only
   short-circuiting for "ALWAYS_TRUE/FALSE" constants (e.g. sys.platform
   checks) and not "MYPY_TRUE/FALSE" constants (e.g. `MYPY`)

 - Make --always-true and --always-false map to ALWAYS_TRUE and
   ALWAYS_FALSE, respectively, instead of MYPY_TRUE and MYPY_FALSE.
   The only difference between the two is a change in import priority,
   but semantically uses of "--always-true" are probably closer to
   "this condition should be true at compile time and runtime" than
   "this condition should be true at compile time but false at
   runtime".

 - Fixes #5963

mypy/reachability.py

@@ -84,8 +84,8 @@ def infer_condition_value(expr: Expression, options: Options) -> int:
         name = expr.name
     elif isinstance(expr, OpExpr) and expr.op in ('and', 'or'):
         left = infer_condition_value(expr.left, options)
-        if ((left == ALWAYS_TRUE and expr.op == 'and') or
-                (left == ALWAYS_FALSE and expr.op == 'or')):
+        if ((left in (ALWAYS_TRUE, MYPY_TRUE) and expr.op == 'and') or
+                (left in (ALWAYS_FALSE, MYPY_FALSE) and expr.op == 'or')):
             # Either `True and <other>` or `False or <other>`: the result will
             # always be the right-hand-side.
             return infer_condition_value(expr.right, options)
@@ -105,9 +105,9 @@ def infer_condition_value(expr: Expression, options: Options) -> int:
         elif name == 'MYPY' or name == 'TYPE_CHECKING':
             result = MYPY_TRUE
         elif name in options.always_true:
-            result = MYPY_TRUE
+            result = ALWAYS_TRUE
         elif name in options.always_false:
-            result = MYPY_FALSE
+            result = ALWAYS_FALSE
     if negated:
         result = inverted_truth_mapping[result]
     return result

mypy/semanal.py

@@ -90,7 +90,8 @@
 from mypy.semanal_enum import EnumCallAnalyzer
 from mypy.semanal_newtype import NewTypeAnalyzer
 from mypy.reachability import (
-    infer_reachability_of_if_statement, infer_condition_value, ALWAYS_FALSE, ALWAYS_TRUE
+    infer_reachability_of_if_statement, infer_condition_value, ALWAYS_FALSE, ALWAYS_TRUE,
+    MYPY_TRUE, MYPY_FALSE
 )
 from mypy.typestate import TypeState
 
@@ -3102,12 +3103,12 @@ def visit_op_expr(self, expr: OpExpr) -> None:
 
         if expr.op in ('and', 'or'):
             inferred = infer_condition_value(expr.left, self.options)
-            if ((inferred == ALWAYS_FALSE and expr.op == 'and') or
-                    (inferred == ALWAYS_TRUE and expr.op == 'or')):
+            if ((inferred in (ALWAYS_FALSE, MYPY_FALSE) and expr.op == 'and') or
+                    (inferred in (ALWAYS_TRUE, MYPY_TRUE) and expr.op == 'or')):
                 expr.right_unreachable = True
                 return
-            elif ((inferred == ALWAYS_TRUE and expr.op == 'and') or
-                    (inferred == ALWAYS_FALSE and expr.op == 'or')):
+            elif ((inferred in (ALWAYS_TRUE, MYPY_TRUE) and expr.op == 'and') or
+                    (inferred in (ALWAYS_FALSE, MYPY_FALSE) and expr.op == 'or')):
                 expr.right_always = True
 
         expr.right.accept(self)

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

@@ -567,6 +567,42 @@ else:
 reveal_type(y)  # E: Revealed type is 'builtins.str'
 [builtins fixtures/ops.pyi]
 
+[case testShortCircuitNoEvaluation]
+# flags: --platform linux --always-false COMPILE_TIME_FALSE
+import sys
+
+if sys.platform == 'darwin':
+    mac_only = 'junk'
+
+# `mac_only` should not be evaluated
+if sys.platform == 'darwin' and mac_only:
+    pass
+if sys.platform == 'linux' or mac_only:
+    pass
+
+COMPILE_TIME_FALSE = 'junk'
+
+if COMPILE_TIME_FALSE:
+    compile_time_false_only = 'junk'
+
+# `compile_time_false_only` should not be evaluated
+if COMPILE_TIME_FALSE and compile_time_false_only:
+    pass
+if not COMPILE_TIME_FALSE or compile_time_false_only:
+    pass
+
+MYPY = False
+
+if not MYPY:
+    mypy_only = 'junk'
+
+# `mypy_only` should not be evaluated
+if not MYPY and mypy_only:
+    pass
+if MYPY or mypy_only:
+    pass
+[builtins fixtures/ops.pyi]
+
 [case testConditionalAssertWithoutElse]
 import typing