diff --git a/base-test/org.eclipse.jdt.groovy.core.tests.compiler/src/org/eclipse/jdt/groovy/core/tests/xform/StaticCompilationTests.java b/base-test/org.eclipse.jdt.groovy.core.tests.compiler/src/org/eclipse/jdt/groovy/core/tests/xform/StaticCompilationTests.java
index 09ed37ded8..77d7b3b69a 100644
--- a/base-test/org.eclipse.jdt.groovy.core.tests.compiler/src/org/eclipse/jdt/groovy/core/tests/xform/StaticCompilationTests.java
+++ b/base-test/org.eclipse.jdt.groovy.core.tests.compiler/src/org/eclipse/jdt/groovy/core/tests/xform/StaticCompilationTests.java
@@ -5166,4 +5166,37 @@ public void testCompileStatic9771() {
 
         runConformTest(sources, "[key:true]");
     }
+
+    @Test
+    public void testCompileStatic9786() {
+        //@formatter:off
+        String[] sources = {
+            "Main.groovy",
+            "interface I {\n" +
+            "  void m()\n" +
+            "}\n" +
+            "class A implements I {\n" +
+            "  void m() { print 'A' }\n" +
+            "}\n" +
+            "class B implements I {\n" +
+            "  void m() { print 'B' }\n" +
+            "}\n" +
+            "@groovy.transform.CompileStatic\n" +
+            "void test() {\n" +
+            "  I x\n" +
+            "  def y = false\n" +
+            "  def z = true \n" +
+            "  if (y) {\n" +
+            "    x = new A()\n" +
+            "  } else if (z) {\n" +
+            "    x = new B()\n" +
+            "  }\n" +
+            "  x.m()\n" +
+            "}\n" +
+            "test()\n",
+        };
+        //@formatter:on
+
+        runConformTest(sources, "B");
+    }
 }
diff --git a/base/org.codehaus.groovy25/src/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java b/base/org.codehaus.groovy25/src/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java
index 420464bf02..d380f4b56b 100644
--- a/base/org.codehaus.groovy25/src/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java
+++ b/base/org.codehaus.groovy25/src/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java
@@ -985,6 +985,7 @@ && isAssignment(enclosingBinaryExpression.getOperation().getType())) {
                         accessedVariable = new ParameterVariableExpression((Parameter) accessedVariable);
                     }
                     if (accessedVariable instanceof VariableExpression) {
+                        /* GRECLIPSE edit -- GROOVY-9786
                         VariableExpression var = (VariableExpression) accessedVariable;
                         List<ClassNode> types = typeCheckingContext.ifElseForWhileAssignmentTracker.get(var);
                         if (types == null) {
@@ -994,6 +995,9 @@ && isAssignment(enclosingBinaryExpression.getOperation().getType())) {
                             typeCheckingContext.ifElseForWhileAssignmentTracker.put(var, types);
                         }
                         types.add(resultType);
+                        */
+                        recordAssignment((VariableExpression) accessedVariable, resultType);
+                        // GRECLIPSE end
                     }
                 }
                 storeType(leftExpression, resultType);
@@ -4105,7 +4109,6 @@ protected void typeCheckClosureCall(final Expression callArguments, final ClassN
     @Override
     public void visitIfElse(final IfStatement ifElse) {
         Map<VariableExpression, List<ClassNode>> oldTracker = pushAssignmentTracking();
-
         try {
             // create a new temporary element in the if-then-else type info
             typeCheckingContext.pushTemporaryTypeInfo();
@@ -4126,20 +4129,26 @@ public void visitIfElse(final IfStatement ifElse) {
                 visitEmptyStatement((EmptyStatement) elseBlock);
             } else {
                 elseBlock.visit(this);
+                // GRECLIPSE add -- GROOVY-9786
+                Map<VariableExpression, ClassNode> updates =
+                    elseBlock.getNodeMetaData("assignments");
+                if (updates != null) {
+                    updates.forEach(this::recordAssignment);
+                }
+                // GRECLIPSE end
             }
         } finally {
-            popAssignmentTracking(oldTracker);
+            // GRECLIPSE add -- GROOVY-9786
+            ifElse.putNodeMetaData("assignments",
+            // GRECLIPSE end
+            popAssignmentTracking(oldTracker));
         }
-        BinaryExpression instanceOfExpression = findInstanceOfNotReturnExpression(ifElse);
-        if (instanceOfExpression == null) {
-        } else {
-            if (typeCheckingContext.enclosingBlocks.size() > 0) {
-                visitInstanceofNot(instanceOfExpression);
-            }
+
+        if (!typeCheckingContext.enclosingBlocks.isEmpty()) {
+            Optional.ofNullable(findInstanceOfNotReturnExpression(ifElse)).ifPresent(this::visitInstanceofNot);
         }
     }
 
-
     public void visitInstanceofNot(BinaryExpression be) {
         final BlockStatement currentBlock = typeCheckingContext.enclosingBlocks.getFirst();
         assert currentBlock != null;
@@ -4237,6 +4246,17 @@ public void visitCaseStatement(final CaseStatement statement) {
         restoreTypeBeforeConditional();
     }
 
+    // GRECLIPSE add -- GROOVY-9786
+    private void recordAssignment(final VariableExpression lhsExpr, final ClassNode rhsType) {
+        typeCheckingContext.ifElseForWhileAssignmentTracker.computeIfAbsent(lhsExpr, lhs -> {
+            ClassNode lhsType = lhs.getNodeMetaData(StaticTypesMarker.INFERRED_TYPE);
+            List<ClassNode> types = new ArrayList<>(2);
+            types.add(lhsType);
+            return types;
+        }).add(rhsType);
+    }
+    // GRECLIPSE end
+
     private void restoreTypeBeforeConditional() {
         Set<Map.Entry<VariableExpression, List<ClassNode>>> entries = typeCheckingContext.ifElseForWhileAssignmentTracker.entrySet();
         for (Map.Entry<VariableExpression, List<ClassNode>> entry : entries) {
diff --git a/base/org.codehaus.groovy30/src/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java b/base/org.codehaus.groovy30/src/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java
index db05205cf1..19614f80fd 100644
--- a/base/org.codehaus.groovy30/src/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java
+++ b/base/org.codehaus.groovy30/src/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java
@@ -869,13 +869,15 @@ && isAssignment(enclosingBinaryExpression.getOperation().getType())) {
                 }
 
                 // if we are in an if/else branch, keep track of assignment
-                if (typeCheckingContext.ifElseForWhileAssignmentTracker != null && leftExpression instanceof VariableExpression
-                        && !isNullConstant(rightExpression)) {
+                if (!isNullConstant(rightExpression)
+                        && leftExpression instanceof VariableExpression
+                        && typeCheckingContext.ifElseForWhileAssignmentTracker != null) {
                     Variable accessedVariable = ((VariableExpression) leftExpression).getAccessedVariable();
                     if (accessedVariable instanceof Parameter) {
                         accessedVariable = new ParameterVariableExpression((Parameter) accessedVariable);
                     }
                     if (accessedVariable instanceof VariableExpression) {
+                        /* GRECLIPSE edit -- GROOVY-9786
                         VariableExpression var = (VariableExpression) accessedVariable;
                         List<ClassNode> types = typeCheckingContext.ifElseForWhileAssignmentTracker.get(var);
                         if (types == null) {
@@ -885,6 +887,9 @@ && isAssignment(enclosingBinaryExpression.getOperation().getType())) {
                             typeCheckingContext.ifElseForWhileAssignmentTracker.put(var, types);
                         }
                         types.add(resultType);
+                        */
+                        recordAssignment((VariableExpression) accessedVariable, resultType);
+                        // GRECLIPSE end
                     }
                 }
                 storeType(leftExpression, resultType);
@@ -3889,7 +3894,6 @@ protected void typeCheckClosureCall(final Expression callArguments, final ClassN
     @Override
     public void visitIfElse(final IfStatement ifElse) {
         Map<VariableExpression, List<ClassNode>> oldTracker = pushAssignmentTracking();
-
         try {
             // create a new temporary element in the if-then-else type info
             typeCheckingContext.pushTemporaryTypeInfo();
@@ -3904,15 +3908,26 @@ public void visitIfElse(final IfStatement ifElse) {
             restoreTypeBeforeConditional();
 
             ifElse.getElseBlock().visit(this);
+            // GRECLIPSE add -- GROOVY-9786
+            Map<VariableExpression, ClassNode> updates =
+                ifElse.getElseBlock().getNodeMetaData("assignments");
+            if (updates != null) {
+                updates.forEach(this::recordAssignment);
+            }
+            // GRECLIPSE end
         } finally {
-            popAssignmentTracking(oldTracker);
-        }
-        BinaryExpression instanceOfExpression = findInstanceOfNotReturnExpression(ifElse);
-        if (instanceOfExpression == null) {
-            instanceOfExpression = findNotInstanceOfReturnExpression(ifElse);
+            // GRECLIPSE add
+            ifElse.putNodeMetaData("assignments",
+            // GRECLIPSE end
+            popAssignmentTracking(oldTracker));
         }
-        if (instanceOfExpression != null) {
-            if (!typeCheckingContext.enclosingBlocks.isEmpty()) {
+
+        if (!typeCheckingContext.enclosingBlocks.isEmpty()) {
+            BinaryExpression instanceOfExpression = findInstanceOfNotReturnExpression(ifElse);
+            if (instanceOfExpression == null) {
+                instanceOfExpression = findNotInstanceOfReturnExpression(ifElse);
+            }
+            if (instanceOfExpression != null) {
                 visitInstanceofNot(instanceOfExpression);
             }
         }
@@ -4062,6 +4077,17 @@ public void visitCaseStatement(final CaseStatement statement) {
         restoreTypeBeforeConditional();
     }
 
+    // GRECLIPSE add -- GROOVY-9786
+    private void recordAssignment(final VariableExpression lhsExpr, final ClassNode rhsType) {
+        typeCheckingContext.ifElseForWhileAssignmentTracker.computeIfAbsent(lhsExpr, lhs -> {
+            ClassNode lhsType = lhs.getNodeMetaData(INFERRED_TYPE);
+            List<ClassNode> types = new ArrayList<>(2);
+            types.add(lhsType);
+            return types;
+        }).add(rhsType);
+    }
+    // GRECLIPSE end
+
     private void restoreTypeBeforeConditional() {
         Set<Map.Entry<VariableExpression, List<ClassNode>>> entries = typeCheckingContext.ifElseForWhileAssignmentTracker.entrySet();
         for (Map.Entry<VariableExpression, List<ClassNode>> entry : entries) {