most specific check: accept lambdas when target type is type parameter (IDEA-124725)

java/java-psi-impl/src/com/intellij/psi/scope/conflictResolvers/JavaMethodsConflictResolver.java

@@ -153,6 +153,10 @@ public class JavaMethodsConflictResolver implements PsiConflictResolver{
         if (methodParameters.length == 0) continue;
         final PsiParameter param = i < methodParameters.length ? methodParameters[i] : methodParameters[methodParameters.length - 1];
         final PsiType paramType = param.getType();
+        // http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.12.2.1
+        // A lambda expression or a method reference expression is potentially compatible with a type variable if the type variable is a type parameter of the candidate method.
+        final PsiClass paramClass = PsiUtil.resolveClassInType(paramType);
+        if (paramClass instanceof PsiTypeParameter && ((PsiTypeParameter)paramClass).getOwner() == method) continue;
         if (!lambdaExpression.isAcceptable(((MethodCandidateInfo)conflict).getSubstitutor(false).substitute(paramType), lambdaExpression.hasFormalParameterTypes())) {
           iterator.remove();
         }

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/lambda/mostSpecific/TargetTypeParameter.java

@@ -0,0 +1,11 @@
+import java.util.function.Supplier;
+
+class Test {
+
+  public static void main(String... args) {
+    c<error descr="Cannot resolve method 'c(<lambda expression>, <lambda expression>)'">(() -> 3, () -> 10)</error>;
+  }
+
+  public static <T> void c(Supplier<T> s1, Supplier<T> s2) {}
+  public static <T> void c(Supplier<T> s1, T value) {}
+}

java/java-tests/testSrc/com/intellij/codeInsight/daemon/lambda/MostSpecificResolutionTest.java

@@ -99,6 +99,10 @@ public class MostSpecificResolutionTest extends LightDaemonAnalyzerTestCase {
     doTest();
   }
 
+  public void testTargetTypeParameter() throws Exception {
+    doTest(false);
+  }
+
   private void doTest() {
     doTest(true);
   }