Do not activate power assertions when a single union member containing a type constraint fails (#1462)

Prior to this change, this code would activate powers assertions /
instrumentation permanently:
```pkl
foo: String(contains("a")) | String(contains("b")) = "boo"
```

This is because the `contains("a")` constraint would fail, triggering
power assertions, but the subsequent check of the union's
`contains("b")` branch would succeed.
As observed in #1419, once instrumentation is enabled, all subsequent
evaluation slows significantly.
As with #1419, the fix here is to disable power assertions via
`VmLocalContext` until we know that all union members failed type
checking; then, each member is re-executed with power assertions allowed
to provide the improved user-facing error.

pkl-core/src/main/java/org/pkl/core/EvaluatorImpl.java

@@ -336,6 +336,16 @@ public final class EvaluatorImpl implements Evaluator {
     }
   }
 
+  // for use in tests to determine whether an evaluator ever triggered instrumentation
+  boolean isInstrumentationEverUsed() {
+    polyglotContext.enter();
+    try {
+      return VmLanguage.get(null).localContext.get().isInstrumentationEverUsed();
+    } finally {
+      polyglotContext.leave();
+    }
+  }
+
   String evaluateOutputText(VmTyped fileOutput) {
     return doEvaluate(() -> VmUtils.readTextProperty(fileOutput));
   }

pkl-core/src/main/java/org/pkl/core/ast/type/TypeNode.java

@@ -985,6 +985,11 @@ public abstract class TypeNode extends PklNode {
       // escape analysis should remove this allocation in compiled code
       var typeMismatches = new VmTypeMismatchException[elementTypeNodes.length];
 
+      // disallow power assertions from triggering in case one union member checks successfully
+      var localContext = VmLanguage.get(this).localContext.get();
+      var wasInTypeTest = localContext.isInTypeTest();
+      localContext.setInTypeTest(true);
+
       // Do eager checks (shallow-force) if there are two listings or two mappings represented.
       // (we can't know that `new Listing { 0; "hi" }[0]` fails for `Listing<Int>|Listing<String>`
       // without checking both index 0 and index 1).
@@ -992,15 +997,36 @@ public abstract class TypeNode extends PklNode {
       for (var i = 0; i < elementTypeNodes.length; i++) {
         var elementTypeNode = elementTypeNodes[i];
         try {
-          if (shouldEagerCheck) {
-            return elementTypeNode.executeEagerly(frame, value);
-          } else {
-            return elementTypeNode.executeLazily(frame, value);
-          }
+          var result =
+              shouldEagerCheck
+                  ? elementTypeNode.executeEagerly(frame, value)
+                  : elementTypeNode.executeLazily(frame, value);
+          localContext.setInTypeTest(wasInTypeTest);
+          return result;
         } catch (VmTypeMismatchException e) {
           typeMismatches[i] = e;
         }
       }
+
+      // all members failed to type check
+      // if enabled, re-execute type checks to generate power assertions
+      localContext.setInTypeTest(wasInTypeTest);
+      if (VmContext.get(this).getPowerAssertionsEnabled()
+          && (!wasInTypeTest || localContext.hasActiveTracker())) {
+        for (var i = 0; i < elementTypeNodes.length; i++) {
+          var elementTypeNode = elementTypeNodes[i];
+          try {
+            if (shouldEagerCheck) {
+              elementTypeNode.executeEagerly(frame, value);
+            } else {
+              elementTypeNode.executeLazily(frame, value);
+            }
+          } catch (VmTypeMismatchException e) {
+            typeMismatches[i] = e;
+          }
+        }
+      }
+
       throw new VmTypeMismatchException.Union(sourceSection, value, this, typeMismatches);
     }
 
@@ -1011,14 +1037,36 @@ public abstract class TypeNode extends PklNode {
       // escape analysis should remove this allocation in compiled code
       var typeMismatches = new VmTypeMismatchException[elementTypeNodes.length];
 
+      // disallow power assertions from triggering in case one union member checks successfully
+      var localContext = VmLanguage.get(this).localContext.get();
+      var wasInTypeTest = localContext.isInTypeTest();
+      localContext.setInTypeTest(true);
+
       for (var i = 0; i < elementTypeNodes.length; i++) {
         // eager checks
         try {
-          return elementTypeNodes[i].executeEagerly(frame, value);
+          var result = elementTypeNodes[i].executeEagerly(frame, value);
+          localContext.setInTypeTest(wasInTypeTest);
+          return result;
         } catch (VmTypeMismatchException e) {
           typeMismatches[i] = e;
         }
       }
+
+      // all members failed to type check
+      // if enabled, re-execute type checks to generate power assertions
+      localContext.setInTypeTest(wasInTypeTest);
+      if (VmContext.get(this).getPowerAssertionsEnabled()
+          && (!wasInTypeTest || localContext.hasActiveTracker())) {
+        for (var i = 0; i < elementTypeNodes.length; i++) {
+          try {
+            elementTypeNodes[i].executeEagerly(frame, value);
+          } catch (VmTypeMismatchException e) {
+            typeMismatches[i] = e;
+          }
+        }
+      }
+
       throw new VmTypeMismatchException.Union(sourceSection, value, this, typeMismatches);
     }
   }

pkl-core/src/main/java/org/pkl/core/runtime/VmLocalContext.java

@@ -28,6 +28,8 @@ public class VmLocalContext {
    */
   private int activeTrackerDepth = 0;
 
+  private boolean instrumentationEverUsed = false;
+
   public VmLocalContext() {}
 
   public void shouldEagerTypecheck(boolean shouldEagerTypecheck) {
@@ -48,6 +50,7 @@ public class VmLocalContext {
 
   public void enterTracker() {
     activeTrackerDepth++;
+    instrumentationEverUsed = true;
   }
 
   public void exitTracker() {
@@ -57,4 +60,8 @@ public class VmLocalContext {
   public boolean hasActiveTracker() {
     return activeTrackerDepth > 0;
   }
+
+  public boolean isInstrumentationEverUsed() {
+    return instrumentationEverUsed;
+  }
 }

pkl-core/src/test/kotlin/org/pkl/core/EvaluatorTest.kt

@@ -566,6 +566,69 @@ class EvaluatorTest {
       )
   }
 
+  @Test
+  fun `constraint failures activate instrumentation`() {
+    val evaluator =
+      with(EvaluatorBuilder.preconfigured()) {
+        powerAssertionsEnabled = true
+        build()
+      }
+
+    val exc =
+      assertThrows<PklException> {
+        evaluator.evaluate(
+          text(
+            """
+    foo: String(chars.first == "a") = "boo"
+    """
+              .trimIndent()
+          )
+        )
+      }
+
+    assertThat((evaluator as EvaluatorImpl).isInstrumentationEverUsed()).isTrue
+  }
+
+  @Test
+  fun `union single-member constraint failures do not activate instrumentation`() {
+    val evaluator =
+      with(EvaluatorBuilder.preconfigured()) {
+        powerAssertionsEnabled = true
+        build()
+      }
+
+    evaluator.evaluate(
+      text(
+        """
+    foo: String(startsWith("a")) | String(startsWith("b")) | String(startsWith("c")) = "cool"
+    """
+          .trimIndent()
+      )
+    )
+
+    assertThat((evaluator as EvaluatorImpl).isInstrumentationEverUsed()).isFalse
+  }
+
+  @Test
+  fun `type test failures do not activate instrumentation`() {
+    val evaluator =
+      with(EvaluatorBuilder.preconfigured()) {
+        powerAssertionsEnabled = true
+        build()
+      }
+
+    evaluator.evaluate(
+      text(
+        """
+    foo = "bar" is Int(this > 0)
+    """
+          .trimIndent()
+      )
+    )
+
+    assertThat((evaluator as EvaluatorImpl).isInstrumentationEverUsed()).isFalse
+  }
+
   private fun checkModule(module: PModule) {
     assertThat(module.properties.size).isEqualTo(2)
     assertThat(module.getProperty("name")).isEqualTo("pigeon")