From a05304a534446298a99906b13eb376834efd5ef6 Mon Sep 17 00:00:00 2001
From: Paul King <paulk@asert.com.au>
Date: Tue, 4 Aug 2026 07:44:06 +1000
Subject: [PATCH] GROOVY-12228: STC: multiple assignment ignores the declared
 type of the target variables

---
 .../stc/StaticTypeCheckingVisitor.java        | 11 ++++-
 .../transform/stc/STCAssignmentTest.groovy    | 40 +++++++++++++++++++
 2 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/src/main/java/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java b/src/main/java/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java
index 8261cfafc5e..3f16865efc8 100644
--- a/src/main/java/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java
+++ b/src/main/java/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java
@@ -1521,13 +1521,20 @@ private boolean typeCheckMultipleAssignmentPositional(final Expression leftExpre
         }
 
         for (int i = 0, n = tupleExpressions.size(); i < n; i += 1) {
+            Expression tupleExpression = tupleExpressions.get(i);
             ClassNode valueType = getType(valueExpressions.get(i));
-            ClassNode targetType = getType(tupleExpressions.get(i));
+            ClassNode targetType = getType(tupleExpression);
             if (!isAssignableTo(valueType, targetType)) {
                 addStaticTypeError("Cannot assign value of type " + prettyPrintType(valueType) + " to variable of type " + prettyPrintType(targetType), rightExpression);
                 return false;
             }
-            storeType(tupleExpressions.get(i), valueType);
+            // GROOVY-12228: check for implicit conversion like "String a = 123" or "Long b = 1";
+            // as for single assignment, the target then holds its declared type, not the value type
+            ClassNode originType = getOriginalDeclarationType(tupleExpression);
+            if (!implementsInterfaceOrIsSubclassOf(wrapTypeIfNecessary(valueType), wrapTypeIfNecessary(originType))) {
+                valueType = originType;
+            }
+            storeType(tupleExpression, valueType);
         }
 
         return true;
diff --git a/src/test/groovy/groovy/transform/stc/STCAssignmentTest.groovy b/src/test/groovy/groovy/transform/stc/STCAssignmentTest.groovy
index f2708ed78e2..fd07f4c7e02 100644
--- a/src/test/groovy/groovy/transform/stc/STCAssignmentTest.groovy
+++ b/src/test/groovy/groovy/transform/stc/STCAssignmentTest.groovy
@@ -1467,6 +1467,46 @@ class STCAssignmentTest extends StaticTypeCheckingTestCase {
         '''
     }
 
+    // GROOVY-12228
+    @Test
+    void testMultiAssignRetainsDeclaredType1() {
+        assertScript '''
+            def m() {
+                String a, b
+                (a, b) = ["x${1}", "y${2}"]
+                assert a instanceof String
+                assert a.class === String && b.class === String
+                assert a == 'x1' && b == 'y2'
+            }
+            m()
+        '''
+    }
+
+    // GROOVY-12228
+    @Test
+    void testMultiAssignRetainsDeclaredType2() {
+        assertScript '''
+            def m() {
+                def (String a, String b) = ["x${1}", "y${2}"]
+                assert a.class === String && b.class === String
+            }
+            m()
+        '''
+    }
+
+    // GROOVY-12228
+    @Test
+    void testMultiAssignRetainsDeclaredType3() {
+        assertScript '''
+            def m() {
+                Long a, b
+                (a, b) = [1, 2]
+                assert a.class === Long && b.class === Long
+            }
+            m()
+        '''
+    }
+
     // GROOVY-10943
     @Test
     void testMultiAssignUnderscorePlaceholder1() {