8043179: Lambda expression can mutate final field

Reviewed-by: vromero

Author: archiecobbs

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java

@@ -2856,6 +2856,11 @@ public void visitLambda(JCLambda tree) {
             int nextadrPrev = nextadr;
             ListBuffer<PendingExit> prevPending = pendingExits;
             try {
+                // JLS 16.1.10: No rule allows V to be definitely unassigned before a lambda
+                // body. This is by design: a variable that was definitely unassigned before the
+                // lambda body may end up being assigned to later on, so we cannot conclude that
+                // the variable will be unassigned when the body is executed.
+                uninits.excludeFrom(firstadr);
                 returnadr = nextadr;
                 pendingExits = new ListBuffer<>();
                 for (List<JCVariableDecl> l = tree.params; l.nonEmpty(); l = l.tail) {

test/langtools/tools/javac/lambda/8043179/LambdaMutateFinalField.java

@@ -0,0 +1,13 @@
+/*
+ * @test /nodynamiccopyright/
+ * @summary Verify lambda expression can't mutate a final field
+ * @bug 8043179
+ * @compile/fail/ref=LambdaMutateFinalField.out -XDrawDiagnostics LambdaMutateFinalField.java
+ */
+class LambdaMutateFinalField {
+    final String x;
+    LambdaMutateFinalField() {
+        Runnable r1 = () -> x = "not ok";
+        this.x = "ok";
+    }
+}

test/langtools/tools/javac/lambda/8043179/LambdaMutateFinalField.out

@@ -0,0 +1,2 @@
+LambdaMutateFinalField.java:10:29: compiler.err.var.might.already.be.assigned: x
+1 error

test/langtools/tools/javac/lambda/8043179/LambdaMutateFinalVar.java

@@ -0,0 +1,13 @@
+/*
+ * @test /nodynamiccopyright/
+ * @summary Verify lambda expression can't mutate a final variable
+ * @bug 8043179
+ * @compile/fail/ref=LambdaMutateFinalVar.out -XDrawDiagnostics LambdaMutateFinalVar.java
+ */
+class LambdaMutateFinalVar {
+    LambdaMutateFinalVar() {
+        final String x;
+        Runnable r1 = () -> x = "not ok";
+        x = "ok";
+    }
+}

test/langtools/tools/javac/lambda/8043179/LambdaMutateFinalVar.out

@@ -0,0 +1,2 @@
+LambdaMutateFinalVar.java:10:29: compiler.err.var.might.already.be.assigned: x
+1 error