Allow method from interfaces to be inlined if it is private and is being called from within the interface

base/src/main/java/proguard/optimize/peephole/MethodInliner.java

@@ -654,11 +654,12 @@ implements            AttributeVisitor,
 
             DEBUG("Interface?")                                                                   &&
 
-            // Methods in interfaces should not be inlined since this can potentially
-            // lead to other methods in the interface needing broadened visibility,
-            // which can lead to either compilation errors during output writing
-            // or various issues at runtime.
-            (programClass.getAccessFlags() & AccessConstants.INTERFACE) == 0                   &&
+            // Methods in interfaces should only very rarely be inlined
+            // since this can potentially lead to other methods in the interface
+            // needing broadened visibility, which can lead to either compilation errors
+            // during output writing or various issues at runtime.
+            ((programClass.getAccessFlags() & AccessConstants.INTERFACE) == 0 ||
+              canInlineMethodFromInterface(programClass, programMethod))                       &&
 
             DEBUG("Synchronized?")                                                                &&
 
@@ -897,6 +898,17 @@ implements            AttributeVisitor,
                returnChar == TypeConstants.SHORT;
     }
 
+    /**
+     * We only inline methods from interfaces if both of these conditions hold:
+     * - The class that references the method is the interface itself.
+     * - The method is private.
+     */
+    private boolean canInlineMethodFromInterface(ProgramClass sourceClass, ProgramMethod sourceMethod)
+    {
+        return
+            sourceClass.equals(targetClass) &&
+            (sourceMethod.getAccessFlags() & AccessConstants.PRIVATE) != 0;
+    }
 
     /**
      * Indicates whether this method should be inlined. Subclasses can overwrite

base/src/test/kotlin/proguard/optimize/peephole/MethodInlinerJava9Test.kt

@@ -0,0 +1,78 @@
+package proguard.optimize.peephole
+
+import io.kotest.core.spec.IsolationMode
+import io.kotest.core.spec.style.FreeSpec
+import io.kotest.matchers.ints.shouldBeGreaterThan
+import proguard.classfile.Clazz
+import proguard.classfile.Method
+import proguard.classfile.ProgramClass
+import proguard.classfile.ProgramMethod
+import proguard.classfile.attribute.CodeAttribute
+import proguard.classfile.attribute.visitor.AllAttributeVisitor
+import proguard.classfile.visitor.AllMethodVisitor
+import proguard.classfile.visitor.ClassVisitor
+import proguard.classfile.visitor.MultiClassVisitor
+import proguard.optimize.info.ProgramClassOptimizationInfoSetter
+import proguard.optimize.info.ProgramMemberOptimizationInfoSetter
+import proguard.testutils.ClassPoolBuilder
+import proguard.testutils.JavaSource
+import testutils.RequiresJavaVersion
+
+@RequiresJavaVersion(9)
+class MethodInlinerJava9Test : FreeSpec({
+    isolationMode = IsolationMode.InstancePerTest
+
+    "Given a method calling a private method in the same interface" - {
+        val (programClassPool, _) = ClassPoolBuilder.fromSource(
+            JavaSource(
+                "Foo.java",
+                """interface Foo {
+                default void f1() {
+                    f2();
+                }
+        
+                private static void f2() {
+                    StringBuilder sb = new StringBuilder();
+                    sb.append(System.currentTimeMillis());
+                    System.out.println(sb.toString());
+                }
+            }""",
+            )
+        )
+
+        val clazz = programClassPool.getClass("Foo") as ProgramClass
+        val method = clazz.findMethod("f1", "()V") as ProgramMethod
+        val codeAttr = method.attributes.filterIsInstance<CodeAttribute>()[0]
+
+        val lengthBefore = codeAttr.u4codeLength
+
+        // Initialize optimization info (used when inlining).
+        val optimizationInfoInitializer: ClassVisitor = MultiClassVisitor(
+                ProgramClassOptimizationInfoSetter(),
+                AllMethodVisitor(
+                        ProgramMemberOptimizationInfoSetter()
+                )
+        )
+
+        programClassPool.classesAccept(optimizationInfoInitializer)
+
+        // Create a mock method inliner which always returns true.
+        val methodInliner = object : MethodInliner(false, true, true) {
+            override fun shouldInline(clazz: Clazz?, method: Method?, codeAttribute: CodeAttribute?): Boolean = true
+        }
+
+        "Then the interface method is inlined" {
+            programClassPool.classesAccept(
+                    AllMethodVisitor(
+                            AllAttributeVisitor(
+                                    methodInliner
+                            )
+                    )
+            )
+
+            val lengthAfter = codeAttr.u4codeLength
+
+            lengthAfter shouldBeGreaterThan lengthBefore
+        }
+    }
+})

base/src/test/kotlin/proguard/optimize/peephole/MethodInlinerTest.kt

@@ -283,7 +283,7 @@ class MethodInlinerTest : FreeSpec({
         }
     }
 
-    "Given a method calling another method in an interface" - {
+    "Given a method calling another non-private method in an interface" - {
         val (programClassPool, _) = ClassPoolBuilder.fromSource(
                 JavaSource(
                         "Foo.java",

docs/md/manual/releasenotes.md

@@ -13,7 +13,7 @@
 - Fix "NoClassDefFoundError: Failed resolution of: Lorg/apache/logging/log4j/LogManager" when using GSON optimization or `-addconfigurationdebugging`. (#326)
 - Don't drop Record attribute for records with no components. (proguard-core#118)
 - Fix potential duplication class when name obfuscating Kotlin multi-file facades.
-- Do not inline interface methods to avoid compilation errors during output writing due to an interface method being made package visible.
+- Do not inline interface methods during optimization to avoid compilation errors during output writing due to an interface method being made package visible.
 
 ## Version 7.3.2