Report a non-bool condition of the conditional operator as a type mismatch.

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

@@ -652,10 +652,11 @@ private OverloadResolution resolveOverload(
       }
     }
     if (resultType == null) {
-      // Non-bool operands of the logical operators are reported individually, as this is more
-      // precise than reporting that no overload matches.
-      boolean reportedLogicalOperandsErrors = reportLogicalOperandsErrors(function.name(), args);
-      if (!reportedLogicalOperandsErrors) {
+      // Non-bool operands of the logical operators and non-bool conditions of the conditional
+      // operator are reported individually, as this is more precise than reporting that no
+      // overload matches.
+      boolean reportedBoolOperandsErrors = reportBoolOperandsErrors(function.name(), args);
+      if (!reportedBoolOperandsErrors) {
         env.reportError(
             callExprId,
             position,
@@ -672,19 +673,25 @@ private OverloadResolution resolveOverload(
   }
 
   /**
-   * Reports a type mismatch for each operand of a logical operator ({@code _&&_}, {@code _||_})
-   * which is not a {@code bool}.
+   * Reports a type mismatch for each operand of a logical operator ({@code _&&_}, {@code _||_}),
+   * and for the condition of the conditional operator ({@code _?_:_}), which is not a {@code bool}.
    *
    * @return whether any error was reported.
    */
-  private boolean reportLogicalOperandsErrors(String functionName, List<CelMutableExpr> args) {
-    if (!functionName.equals(Operator.LOGICAL_AND.getFunction())
-        && !functionName.equals(Operator.LOGICAL_OR.getFunction())) {
+  private boolean reportBoolOperandsErrors(String functionName, List<CelMutableExpr> args) {
+    List<CelMutableExpr> boolArgs;
+    if (functionName.equals(Operator.LOGICAL_AND.getFunction())
+        || functionName.equals(Operator.LOGICAL_OR.getFunction())) {
+      boolArgs = args;
+    } else if (functionName.equals(Operator.CONDITIONAL.getFunction())) {
+      // Only the condition must be a bool: the branches can have any type.
+      boolArgs = args.subList(0, 1);
+    } else {
       return false;
     }
 
     boolean reported = false;
-    for (CelMutableExpr arg : args) {
+    for (CelMutableExpr arg : boolArgs) {
       CelType argType = env.getType(arg);
       if (!inferenceContext.isAssignable(argType, SimpleType.BOOL)) {
         env.reportError(

checker/src/test/java/dev/cel/checker/ExprCheckerTest.java

@@ -183,6 +183,17 @@ public void operatorsBoolErrors() throws Exception {
     source = "'a' || 1";
     runTest();
 
+    source = "1 ? 'a' : 'b'";
+    runTest();
+
+    // The condition is a bool, but the branches don't agree on a type.
+    source = "true ? 'a' : 1";
+    runTest();
+
+    // The filter macro expands its predicate into the condition of a conditional.
+    source = "[1, 2].filter(x, x + 1)";
+    runTest();
+
     // Bool operands still match no overload if _&&_ has no bool overload.
     declareFunction(
         "_&&_",

checker/src/test/resources/operatorsBoolErrors.baseline

@@ -13,6 +13,24 @@ ERROR: test_location:1:8: expected type 'bool' but found 'int'
  | 'a' || 1
  | .......^
 
+Source: 1 ? 'a' : 'b'
+=====>
+ERROR: test_location:1:1: expected type 'bool' but found 'int'
+ | 1 ? 'a' : 'b'
+ | ^
+
+Source: true ? 'a' : 1
+=====>
+ERROR: test_location:1:6: found no matching overload for '_?_:_' applied to '(bool, string, int)' (candidates: (bool, %A0, %A0))
+ | true ? 'a' : 1
+ | .....^
+
+Source: [1, 2].filter(x, x + 1)
+=====>
+ERROR: test_location:1:20: expected type 'bool' but found 'int'
+ | [1, 2].filter(x, x + 1)
+ | ...................^
+
 Source: true && false
 declare _&&_ {
   function logical_and_int (int, int) -> int

extensions/src/test/java/dev/cel/extensions/CelComprehensionsExtensionsTest.java

@@ -307,7 +307,8 @@ public void unparseAST_twoVarComprehension(
   @TestParameters(
       "{expr: '{}.transformMapEntry(i.j, k, {k: i.j})', err: 'argument must be a simple name'}")
   @TestParameters(
-      "{expr: '{}.transformMapEntry(j, k, \"bad filter\", {k: j})', err: 'no matching overload'}")
+      "{expr: '{}.transformMapEntry(j, k, \"bad filter\", {k: j})', err: \"expected type 'bool'"
+          + " but found 'string'\"}")
   @TestParameters(
       "{expr: '[1, 2].transformList(i, v, v % 2 == 0 ? [v] : v)', err: 'no matching overload'}")
   @TestParameters(