IDEA-123289 Infer nullability of 'for each' variable from type annotations of the collection parameter

java/java-analysis-impl/src/com/intellij/codeInspection/dataFlow/value/DfaVariableValue.java

@@ -25,6 +25,7 @@
 package com.intellij.codeInspection.dataFlow.value;
 
 import com.intellij.codeInsight.NullableNotNullManager;
+import com.intellij.codeInsight.daemon.impl.analysis.JavaGenericsUtil;
 import com.intellij.codeInspection.dataFlow.DfaPsiUtil;
 import com.intellij.codeInspection.dataFlow.Nullness;
 import com.intellij.openapi.util.Comparing;
@@ -160,6 +161,16 @@ public class DfaVariableValue extends DfaValue {
       return nullability;
     }
 
+    if (var instanceof PsiParameter && var.getParent() instanceof PsiForeachStatement) {
+      PsiExpression iteratedValue = ((PsiForeachStatement)var.getParent()).getIteratedValue();
+      if (iteratedValue != null) {
+        PsiType itemType = JavaGenericsUtil.getCollectionItemType(iteratedValue);
+        if (itemType != null) {
+          return DfaPsiUtil.getElementNullability(itemType, var);
+        }
+      }
+    }
+
     if (var instanceof PsiField && DfaPsiUtil.isFinalField((PsiVariable)var) && myFactory.isHonorFieldInitializers()) {
       List<PsiExpression> initializers = DfaPsiUtil.findAllConstructorInitializers((PsiField)var);
       if (initializers.isEmpty()) {

java/java-tests/testData/inspection/dataFlow/fixture/NullableForeachVariable.java

@@ -14,6 +14,13 @@ class Foo {
       System.out.println(<warning descr="Method invocation 'i.intValue()' may produce 'java.lang.NullPointerException'">i.intValue()</warning>);
     }
   }
+  void foo3() {
+    List<@Nullable String> list = new ArrayList<>();
+    list.add(null);
+    for (String s : list) {
+      System.out.println(<warning descr="Method invocation 's.length()' may produce 'java.lang.NullPointerException'">s.length()</warning>);
+    }
+  }
 
 }