From 0ae37634614383713d4c590afb8b772ea8b02e33 Mon Sep 17 00:00:00 2001
From: Andrew Reynolds <andrew.j.reynolds@gmail.com>
Date: Tue, 19 Jul 2022 10:21:54 -0500
Subject: [PATCH] Fix bug in strict symbolic length strip (#8973)

Leads to incorrect rewrites. Fixes #8970.
---
 src/theory/strings/strings_entail.cpp                     | 7 ++++++-
 test/regress/cli/CMakeLists.txt                           | 1 +
 .../cli/regress1/strings/issue8970-strip-sym-len.smt2     | 8 ++++++++
 3 files changed, 15 insertions(+), 1 deletion(-)
 create mode 100644 test/regress/cli/regress1/strings/issue8970-strip-sym-len.smt2

diff --git a/src/theory/strings/strings_entail.cpp b/src/theory/strings/strings_entail.cpp
index a9e77615e..e5111c94a 100644
--- a/src/theory/strings/strings_entail.cpp
+++ b/src/theory/strings/strings_entail.cpp
@@ -202,7 +202,12 @@ bool StringsEntail::stripSymbolicLength(std::vector<Node>& n1,
       }
     }
   }
-  if (sindex > 0 && (!strict || curr == zero))
+  if (strict && curr != zero)
+  {
+    // return false if we did not strip the entire length
+    ret = false;
+  }
+  else if (sindex > 0)
   {
     if (dir == 1)
     {
diff --git a/test/regress/cli/CMakeLists.txt b/test/regress/cli/CMakeLists.txt
index 51836ea02..41bf5cac9 100644
--- a/test/regress/cli/CMakeLists.txt
+++ b/test/regress/cli/CMakeLists.txt
@@ -2657,6 +2657,7 @@ set(regress_1_tests
   regress1/strings/issue8918-str-nth-crange-red.smt2
   regress1/strings/issue8932-cmi-unit.smt2
   regress1/strings/issue8944-sygus-inst.smt2
+  regress1/strings/issue8970-strip-sym-len.smt2
   regress1/strings/kaluza-fl.smt2
   regress1/strings/loop002.smt2
   regress1/strings/loop003.smt2
diff --git a/test/regress/cli/regress1/strings/issue8970-strip-sym-len.smt2 b/test/regress/cli/regress1/strings/issue8970-strip-sym-len.smt2
new file mode 100644
index 000000000..f44d5ae5a
--- /dev/null
+++ b/test/regress/cli/regress1/strings/issue8970-strip-sym-len.smt2
@@ -0,0 +1,8 @@
+; EXPECT: sat
+(set-logic QF_SLIA)
+(declare-fun t () String)
+(assert (str.prefixof "-" (str.substr t 0 1)))
+(assert (> (str.len (str.substr t 0 2)) 1))
+(assert (= "-0" (str.update "-0" 0 t)))
+(assert (str.suffixof (str.replace t "-0" "-") "-"))
+(check-sat)
-- 
2.30.2