completion: presume fresh variables have always low bounds during completion

IDEA-190041

java/java-impl/src/com/intellij/psi/impl/source/resolve/CompletionParameterTypeInferencePolicy.java

@@ -66,4 +66,9 @@ public class CompletionParameterTypeInferencePolicy extends ProcessCandidatePara
   public boolean isVarargsIgnored() {
     return true;
   }
+
+  @Override
+  public boolean inferLowerBoundForFreshVariables() {
+    return true;
+  }
 }

java/java-psi-api/src/com/intellij/psi/impl/source/resolve/ParameterTypeInferencePolicy.java

@@ -42,4 +42,13 @@ public abstract class ParameterTypeInferencePolicy {
   public boolean isVarargsIgnored() {
     return false;
   }
+
+  /**
+   * For infinite type declarations, like {@code Foo<T extends Foo<T>>}, inference introduces fake fresh "fixed" type parameters.
+   * These fresh parameters respect constraints, created during inference session. For completion, it makes sense to define lower bounds
+   * even if no appropriate constraints were detected, as probably the corresponding argument is currently completed. 
+   */
+  public boolean inferLowerBoundForFreshVariables() {
+    return false;
+  }
 }

java/java-tests/testData/codeInsight/completion/smartType/LowerBoundOfFreshVariable-out.java

@@ -0,0 +1,9 @@
+
+interface Foo<T extends Foo<T>> {
+
+    static <F extends Foo<F>> void foo(F f) {
+        bar(f);<caret>
+    }
+
+    static <B extends Foo<B>> void bar(B b) { }
+}

java/java-tests/testData/codeInsight/completion/smartType/LowerBoundOfFreshVariable.java

@@ -0,0 +1,9 @@
+
+interface Foo<T extends Foo<T>> {
+
+    static <F extends Foo<F>> void foo(F f) {
+        bar(<caret>);
+    }
+
+    static <B extends Foo<B>> void bar(B b) { }
+}

java/java-tests/testSrc/com/intellij/java/codeInsight/completion/SmartType18CompletionTest.java

@@ -224,6 +224,11 @@ public void testConvertToObjectStream() {
     myFixture.complete(CompletionType.SMART, 1);
     checkResultByFile("/" + getTestName(false) + "-out.java");
   }
+  public void testLowerBoundOfFreshVariable() { 
+    configureByTestName();
+    myFixture.complete(CompletionType.SMART, 1);
+    checkResultByFile("/" + getTestName(false) + "-out.java");
+  }
 
   private void doTest() {
     doTest(true);