Apply LitStyle-preserving logic to augmented assignment (+=)

Reviewed By: kinto0

Differential Revision: D103092401

fbshipit-source-id: 645f34cbdb213d3fb4eebfe7d342eb22a61fe13e

Author: rchen152

pyrefly/lib/alt/operators.rs

@@ -616,10 +616,13 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
                 } else if let Type::Any(style) = &rhs {
                     style.propagate()
                 } else if x.op == Operator::Add
-                    && base.is_literal_string()
-                    && rhs.is_literal_string()
+                    && let Some(lhs_style) = lhs.lit_string_style()
+                    && let Some(rhs_style) = rhs.lit_string_style()
                 {
-                    self.heap.mk_literal_string(LitStyle::Implicit)
+                    self.heap.mk_literal_string(match (lhs_style, rhs_style) {
+                        (LitStyle::Explicit, LitStyle::Explicit) => LitStyle::Explicit,
+                        _ => LitStyle::Implicit,
+                    })
                 } else if x.op == Operator::Add
                     && let Type::Tuple(ref l) = base
                     && let Type::Tuple(r) = rhs

pyrefly/lib/test/attributes.rs

@@ -2430,6 +2430,22 @@ def f(c: C):
     "#,
 );
 
+testcase!(
+    test_do_not_promote_augmented_addition_of_explicit_literal_strings,
+    r#"
+from typing import LiteralString, assert_type
+
+class C:
+    def __init__(self, x: LiteralString, y: LiteralString) -> None:
+        z = x
+        z += y
+        self.attr = z
+
+def f(c: C):
+    assert_type(c.attr, LiteralString)
+    "#,
+);
+
 testcase!(
     test_top_level_anonymous_typeddict,
     r#"