Handle uninferred type parameter in array accesses (Fixes #3020)

checker/tests/nullness/Issue3020.java

@@ -0,0 +1,17 @@
+enum Issue3020 {
+    INSTANCE;
+
+    void retrieveConstant() {
+        Class<?> theClass = Issue3020.class;
+        // :: error: (accessing.nullable)
+        Object unused = passThrough(theClass.getEnumConstants())[0];
+    }
+
+    void nonNullArray(String[] p) {
+        Object unused = passThrough(p)[0];
+    }
+
+    <T> T passThrough(T t) {
+        return t;
+    }
+}

framework/src/main/java/org/checkerframework/framework/type/TypeFromExpressionVisitor.java

@@ -36,6 +36,7 @@
 import org.checkerframework.framework.type.AnnotatedTypeMirror.AnnotatedWildcardType;
 import org.checkerframework.framework.type.visitor.AnnotatedTypeMerger;
 import org.checkerframework.framework.util.AnnotatedTypes;
+import org.checkerframework.javacutil.BugInCF;
 import org.checkerframework.javacutil.Pair;
 import org.checkerframework.javacutil.TreeUtils;
 
@@ -224,8 +225,17 @@ public AnnotatedTypeMirror visitArrayAccess(ArrayAccessTree node, AnnotatedTypeF
             f.visitorState.setAssignmentContext(null);
 
             AnnotatedTypeMirror type = f.getAnnotatedType(node.getExpression());
-            assert type instanceof AnnotatedArrayType;
-            return ((AnnotatedArrayType) type).getComponentType();
+            if (type instanceof AnnotatedArrayType) {
+                return ((AnnotatedArrayType) type).getComponentType();
+            } else if (type instanceof AnnotatedWildcardType) {
+                // Clean-up after Issue #979.
+                AnnotatedWildcardType wc = (AnnotatedWildcardType) type;
+                AnnotatedTypeMirror wcbound = wc.getExtendsBound();
+                if (wcbound instanceof AnnotatedArrayType) {
+                    return ((AnnotatedArrayType) wcbound).getComponentType();
+                }
+            }
+            throw new BugInCF("Unexpected type: " + type);
         } finally {
             f.visitorState.setAssignmentContext(preAssCtxt);
         }