Align inference resolution with javac by applying hints from Maurizio

See https://mail.openjdk.org/archives/list/compiler-dev@openjdk.org/message/GN6RTCGMME6I5JVLSFZRIR32XY6QKOI2/

Author: stephan-herrmann

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/lookup/BoundSet.java

@@ -1063,6 +1063,23 @@ public boolean hasOnlyTrivialExceptionBounds(InferenceVariable variable, TypeBin
 		return true;
 	}
 
+	public int rankIVar(InferenceVariable ivar) {
+		// implements the ranking of ivars according to their bounds as explained in
+		// https://mail.openjdk.org/archives/list/compiler-dev@openjdk.org/message/GN6RTCGMME6I5JVLSFZRIR32XY6QKOI2/
+		ThreeSets three = this.boundsPerVariable.get(ivar.prototype());
+		if (three != null) {
+			if (three.sameBounds != null)
+				for (TypeBound bound :three.sameBounds)
+					if (!(bound.right instanceof InferenceVariable))
+						return 1;
+			if (three.superBounds != null)
+				for (TypeBound bound :three.superBounds)
+					if (!(bound.right instanceof InferenceVariable))
+						return 2;
+		}
+		return 3;
+	}
+
 	/**
 	 * JLS 18.1.3:
 	 * Answer all upper bounds for the given inference variable as defined by any bounds in this set.

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/lookup/InferenceContext18.java

@@ -16,6 +16,7 @@
 
 import java.util.*;
 import java.util.Map.Entry;
+import java.util.stream.Collectors;
 import org.eclipse.jdt.core.compiler.CharOperation;
 import org.eclipse.jdt.internal.compiler.ast.*;
 import org.eclipse.jdt.internal.compiler.lookup.TypeConstants.BoundCheckStatus;
@@ -1169,19 +1170,14 @@ public boolean reduceAndIncorporate(ConstraintFormula constraint) throws Inferen
 			// find a minimal set of dependent variables:
 			Set<InferenceVariable> variableSet;
 			boolean[] hasSkippedSuperBound = { false };
-			while ((variableSet = getSmallestVariableSet(tmpBoundSet, toResolveSet, maySkipSuperBound, hasSkippedSuperBound)) != null) {
+			while ((variableSet = getSmallestVariableSet(tmpBoundSet, toResolveSet, false, hasSkippedSuperBound)) != null) {
 				int oldNumUninstantiated = tmpBoundSet.numUninstantiatedVariables(this.inferenceVariables);
-				final int numVars = variableSet.size();
-				if (numVars > 0) {
-					final InferenceVariable[] variables = variableSet.toArray(new InferenceVariable[numVars]);
-					// NON-JLS: prioritize ivars in 'inThrows' as those may pull in new information by extra rule below (=RuntimeException)
-					BoundSet tSet = tmpBoundSet;
-					Arrays.sort(variables, (v1, v2) -> {
-						int r1 = tSet.inThrows.contains(v1) ? -1 : 0;
-						int r2 = tSet.inThrows.contains(v2) ? -1 : 0;
-						return r1 - r2;
-					});
-					//
+				List<InferenceVariable> ofRank;
+				while ((ofRank = pickIvarsByRank(variableSet, tmpBoundSet)) != null) {
+					final int numVars = ofRank.size();
+					if (numVars == 0)
+						continue;
+					final InferenceVariable[] variables = ofRank.toArray(new InferenceVariable[numVars]);
 					variables: if (!isRecordPatternTypeInference && !tmpBoundSet.hasCaptureBound(variableSet)) {
 						// try to instantiate this set of variables in a fresh copy of the bound set:
 						BoundSet prevBoundSet = tmpBoundSet;
@@ -1190,6 +1186,7 @@ public boolean reduceAndIncorporate(ConstraintFormula constraint) throws Inferen
 							InferenceVariable variable = variables[j];
 							if (tmpBoundSet.isInstantiated(variable)) { // NON-JLS: may happen when exception bound has been incorporated
 								toResolveSet.remove(variable);
+								variableSet.remove(variable);
 								continue;
 							}
 							// try lower bounds:
@@ -1236,11 +1233,13 @@ public boolean reduceAndIncorporate(ConstraintFormula constraint) throws Inferen
 								}
 							}
 							toResolveSet.remove(variable);
+							variableSet.remove(variable);
 						}
 						if (tmpBoundSet.incorporate(this))
 							continue;
 						tmpBoundSet = prevBoundSet;// clean-up for second attempt
 					}
+
 					if (maySkipSuperBound && hasSkippedSuperBound[0])
 						return resolve(toResolve, isRecordPatternTypeInference, false);
 					// Otherwise, a second attempt is made...
@@ -1348,6 +1347,7 @@ public TypeBinding substitute(TypeVariableBinding typeVariable) {
 						if (!typeboundCreated)
 							tmpBoundSet.addBound(new TypeBound(variable, zsj, ReductionResult.SAME), this.environment);
 						toResolveSet.remove(variable);
+						variableSet.remove(variable);
 					}
 					if (tmpBoundSet.incorporate(this)) {
 						if (tmpBoundSet.numUninstantiatedVariables(this.inferenceVariables) == oldNumUninstantiated)
@@ -1458,7 +1458,9 @@ public Set<InferenceVariable> getSmallestVariableSet(BoundSet bounds, Set<Infere
 			//  "... and ii) there exists no non-empty proper subset of { α1, ..., αn } with this property."
 			// -> find a smallest among candidate sets:
 			if (set == null) {
-				return Collections.singleton(currentVariable);
+				set = new HashSet<>();
+				set.add(currentVariable);
+				return set;
 			}
 			for (Iterator<InferenceVariable> iter = set.iterator(); iter.hasNext();) {
 				InferenceVariable iv = iter.next();
@@ -1476,6 +1478,22 @@ public Set<InferenceVariable> getSmallestVariableSet(BoundSet bounds, Set<Infere
 		return result;
 	}
 
+	static List<InferenceVariable> pickIvarsByRank(Set<InferenceVariable> variableSet, BoundSet tmpBoundSet) {
+		// apply the ranking of ivars according to their bounds as explained in
+		// https://mail.openjdk.org/archives/list/compiler-dev@openjdk.org/message/GN6RTCGMME6I5JVLSFZRIR32XY6QKOI2/
+		Map<Integer, List<InferenceVariable>> byRank = variableSet.stream().collect(Collectors.groupingBy(tmpBoundSet::rankIVar));
+		for (int rank = 0; rank < 4; rank++) {
+			List<InferenceVariable> ofRank = byRank.get(rank);
+			if (ofRank == null)
+				continue;
+			final int numVars = ofRank.size();
+			if (numVars == 0)
+				continue;
+			return ofRank;
+		}
+		return null;
+	}
+
 	/**
 	 * Collect dependencies of all our ivars based on TypeBounds of 'bounds'
 	 * @param bounds consider all its TypeBounds