Alternative solution for issue 3281

checker/tests/regex/Issue3281.java

@@ -0,0 +1,16 @@
+// Test case for Issue 3281:
+// https://github.com/typetools/checker-framework/issues/3281
+
+import java.util.regex.Pattern;
+import org.checkerframework.checker.regex.RegexUtil;
+
+public class Issue3281 {
+
+    void bar(String s) {
+        RegexUtil.isRegex(s);
+        if (true) {
+            // :: error: (argument.type.incompatible)
+            Pattern.compile(s);
+        }
+    }
+}

dataflow/src/main/java/org/checkerframework/dataflow/cfg/CFGBuilder.java

@@ -1613,6 +1613,9 @@ protected static class CFGTranslationPhaseOne extends TreePathScanner<Node, Void
          */
         protected final List<LambdaExpressionTree> declaredLambdas;
 
+        /** Statements in a method that are followed by if-statement. */
+        protected final HashSet<StatementTree> statementBeforeIf;
+
         /**
          * @param treeBuilder builder for new AST nodes
          * @param annotationProvider extracts annotations from AST nodes
@@ -1655,6 +1658,7 @@ public CFGTranslationPhaseOne(
             returnNodes = new ArrayList<>();
             declaredClasses = new ArrayList<>();
             declaredLambdas = new ArrayList<>();
+            statementBeforeIf = new HashSet<>();
         }
 
         /**
@@ -3360,6 +3364,14 @@ public Node visitBinary(BinaryTree tree, Void p) {
 
         @Override
         public Node visitBlock(BlockTree tree, Void p) {
+            List<? extends StatementTree> statements = tree.getStatements();
+            // Collect all expression satatments that are followed by a if statment
+            for (int i = 0; i < statements.size() - 1; i++) {
+                if (statements.get(i).getKind() == Kind.EXPRESSION_STATEMENT
+                        && statements.get(i + 1).getKind() == Kind.IF) {
+                    statementBeforeIf.add(statements.get(i));
+                }
+            }
             for (StatementTree n : tree.getStatements()) {
                 scan(n, null);
             }
@@ -3616,7 +3628,21 @@ public Node visitErroneous(ErroneousTree tree, Void p) {
 
         @Override
         public Node visitExpressionStatement(ExpressionStatementTree tree, Void p) {
-            return scan(tree.getExpression(), p);
+            if (statementBeforeIf.contains(tree)) {
+                // For an expression statment followed by an if-statment, create a local variable
+                // and assign the expression to that local variable, which would lead merging of the
+                // two store branches during dataflow analysis.
+                String name = uniqueName("mergeStoreBeforeIf");
+                Element owner = findOwner();
+                ExpressionTree initializer = tree.getExpression();
+                VariableTree auxillaryVariable =
+                        treeBuilder.buildVariableDecl(
+                                TreeUtils.typeOf(initializer), name, owner, initializer);
+                return scan(auxillaryVariable, p);
+
+            } else {
+                return scan(tree.getExpression(), p);
+            }
         }
 
         @Override