Add string to boolean conversion function in the standard definition

checker/src/main/java/dev/cel/checker/Standard.java

@@ -475,7 +475,9 @@ private static ImmutableList<CelFunctionDecl> coreFunctionDeclarations() {
         CelFunctionDecl.newFunctionDeclaration(
             Function.BOOL.getFunction(),
             CelOverloadDecl.newGlobalOverload(
-                "bool_to_bool", "type conversion (identity)", SimpleType.BOOL, SimpleType.BOOL)));
+                "bool_to_bool", "type conversion (identity)", SimpleType.BOOL, SimpleType.BOOL),
+            CelOverloadDecl.newGlobalOverload(
+                "string_to_bool", "type conversion", SimpleType.BOOL, SimpleType.STRING)));
 
     // String functions
     celFunctionDeclBuilder.add(

checker/src/test/resources/standardEnvDump.baseline

@@ -144,6 +144,7 @@ declare _||_ {
 declare bool {
   value type(bool)
   function bool_to_bool (bool) -> bool
+  function string_to_bool (string) -> bool
 }
 declare bytes {
   value type(bytes)

runtime/src/main/java/dev/cel/runtime/StandardFunctions.java

@@ -156,6 +156,33 @@ public static void addNonInlined(
   private static void addBoolFunctions(Registrar registrar) {
     // Identity
     registrar.add("bool_to_bool", Boolean.class, (Boolean x) -> x);
+    // Conversion function
+    registrar.add(
+        "string_to_bool",
+        String.class,
+        (String str) -> {
+          // Note: this is a bit less permissive than what cel-go allows (it accepts '1', 't').
+          switch (str) {
+            case "true":
+            case "TRUE":
+            case "True":
+            case "t":
+            case "1":
+              return true;
+            case "false":
+            case "FALSE":
+            case "False":
+            case "f":
+            case "0":
+              return false;
+            default:
+              throw new InterpreterException.Builder(
+                      "Type conversion error from 'string' to 'bool': [%s]", str)
+                  .setErrorCode(CelErrorCode.BAD_FORMAT)
+                  .build();
+          }
+        });
+
     // The conditional, logical_or, logical_and, and not_strictly_false functions are special-cased.
     registrar.add("logical_not", Boolean.class, (Boolean x) -> !x);
 

runtime/src/test/resources/boolConversions.baseline

@@ -1,4 +1,26 @@
 Source: bool(true)
 =====>
 bindings: {}
-result:   true
+result:   true
+
+Source: bool('true') && bool('TRUE') && bool('True') && bool('t') && bool('1')
+=====>
+bindings: {}
+result:   true
+
+Source: bool('false') || bool('FALSE') || bool('False') || bool('f') || bool('0')
+=====>
+bindings: {}
+result:   false
+
+Source: bool('TrUe')
+=====>
+bindings: {}
+error:    evaluation error: Type conversion error from 'string' to 'bool': [TrUe]
+error_code:    BAD_FORMAT
+
+Source: bool('FaLsE')
+=====>
+bindings: {}
+error:    evaluation error: Type conversion error from 'string' to 'bool': [FaLsE]
+error_code:    BAD_FORMAT

testing/src/main/java/dev/cel/testing/BaseInterpreterTest.java

@@ -1568,6 +1568,18 @@ public void bytes() throws Exception {
   public void boolConversions() throws Exception {
     source = "bool(true)";
     runTest(Activation.EMPTY); // Identity
+
+    source = "bool('true') && bool('TRUE') && bool('True') && bool('t') && bool('1')";
+    runTest(Activation.EMPTY); // result is true
+
+    source = "bool('false') || bool('FALSE') || bool('False') || bool('f') || bool('0')";
+    runTest(Activation.EMPTY); // result is false
+
+    source = "bool('TrUe')";
+    runTest(Activation.EMPTY); // exception
+
+    source = "bool('FaLsE')";
+    runTest(Activation.EMPTY); // exception
   }
 
   @Test