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universal type variables: initial prototype #521

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@@ -61,6 +61,7 @@

public enum Feature {
SWITCH_PATTERN_MATCHING,
UNIVERSAL_TVARS,
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@mcimadamore mcimadamore Aug 4, 2021

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As Dan noted yesterday, for other Valhalla features we do not generally use preview mechanisms (this code needs to be pushed in the valhalla repo). We can postpone the preview dance for later. In fact, to ease experiments in JDK code it would be better to just make universal type variable syntax available by default.

There are some other places where this UNIVERSAL_TVARS constant is used which should be cleaned up for now (e.g. Preview.java).

/**
* A key for testing.
*/
@@ -324,7 +324,12 @@ public enum LintCategory {
/**
* Warn about use of preview features.
*/
PREVIEW("preview");
PREVIEW("preview"),

/**
* Warn about use of universal type variables.
*/
UNIVERSAL("universal");

LintCategory(String option) {
this(option, false);
@@ -187,6 +187,7 @@ public boolean isPreview(Feature feature) {
return switch (feature) {
case CASE_NULL -> true;
case PATTERN_SWITCH -> true;
case UNIVERSAL_TVARS -> true;

//Note: this is a backdoor which allows to optionally treat all features as 'preview' (for testing).
//When real preview features will be added, this method can be implemented to return 'true'
@@ -234,6 +234,7 @@ public enum Feature {
CASE_NULL(JDK17, Fragments.FeatureCaseNull, DiagKind.NORMAL),
PATTERN_SWITCH(JDK17, Fragments.FeaturePatternSwitch, DiagKind.PLURAL),
REDUNDANT_STRICTFP(JDK17),
UNIVERSAL_TVARS(JDK18, Fragments.FeatureUniversalTvars, DiagKind.PLURAL),
;

enum DiagKind {
@@ -1943,12 +1943,34 @@ public static class TypeVar extends Type implements TypeVariable {
*/
public Type lower;

public boolean universal = false;
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@mcimadamore mcimadamore Aug 4, 2021

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Can we use a flag on the tsym for this? After all, being "universal" is a declaration (symbol) not a use (type) property?

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@vicente-romero-oracle vicente-romero-oracle Aug 4, 2021

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well yes and no, if we have:

class C<__universal T> {
    T.ref t;
}

T and T.ref share the same symbol but they have different types that's why the universal field has been set to the type

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yes, but universal is a property of the declaration, while ref-ness is a property of the use-site (hence type). Note that the underlying Valhalla compiler already has machinery to talk about reference projections, so perhaps we should use that to speak about T vs. T.ref, rather than inventing a new one. But we still need a flag somewhere to say whether the variable was declared as universal or not (as that affects what's compatible with that var).


public boolean createdFromUniversalTypeVar = false;

/** if this type variable is universal then it will also have a link to a pure reference
* type variable, it is important to know that a universal type variable and its
* corresponding referenceTypeVar share the same tsym. So if it is needed to double check if
* a type variable is universal or not, we need to check its type not the type of its tsym
*/
public TypeVar referenceProjection = null;

/** link back to universal type var when applicable, this field will have a value if this current
* type variable was derived form a type variable declaration using the .ref suffix, once the code
* is more mature we can fold fields referenceTypeVar and universalTypeVar
*/
public TypeVar universalTypeVar = null;

public TypeVar(Name name, Symbol owner, Type lower) {
this(name, owner, lower, false);
}

public TypeVar(Name name, Symbol owner, Type lower, boolean universal) {
super(null, TypeMetadata.EMPTY);
Assert.checkNonNull(lower);
tsym = new TypeVariableSymbol(0, name, this, owner);
this.setUpperBound(null);
this.lower = lower;
this.universal = universal;
}

public TypeVar(TypeSymbol tsym, Type bound, Type lower) {
@@ -1957,10 +1979,16 @@ public TypeVar(TypeSymbol tsym, Type bound, Type lower) {

public TypeVar(TypeSymbol tsym, Type bound, Type lower,
TypeMetadata metadata) {
this(tsym, bound, lower, metadata, false);
}

public TypeVar(TypeSymbol tsym, Type bound, Type lower,
TypeMetadata metadata, boolean universal) {
super(tsym, metadata);
Assert.checkNonNull(lower);
this.setUpperBound(bound);
this.lower = lower;
this.universal = universal;
}

@Override
@@ -2021,6 +2049,26 @@ public boolean isNullOrReference() {
public <R, P> R accept(TypeVisitor<R, P> v, P p) {
return v.visitTypeVariable(this, p);
}

public boolean isUniversal() {
return universal;
}

@Override
public Type withTypeVar(Type t) {
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Uhm - isn't this method used on wildcard types, and it all other types is supposed to just return this ?

if (t.hasTag(TYPEVAR) &&
((TypeVar)t).createdFromUniversalTypeVar &&
referenceProjection != null) {
return referenceProjection;
}
return this;
}

public void createReferenceProjection() {
referenceProjection = new TypeVar(tsym, _bound, lower, metadata, false);
referenceProjection.createdFromUniversalTypeVar = true;
referenceProjection.universalTypeVar = this;
}
}

/** A captured type variable comes from wildcards which can have
@@ -2040,6 +2088,8 @@ public CapturedType(Name name,
this.lower = Assert.checkNonNull(lower);
this.setUpperBound(upper);
this.wildcard = wildcard;
this.universal = upper.hasTag(TYPEVAR) && ((TypeVar)upper).universal ||
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This probably requires tweaking - I see two issues:

  • right now it detects cases where upper/lower are type variables themselves - but doesn't detect cases originating from e.g. List<? extends Point> - where you just have a captured variable whose bound is Point.
  • the logic does not recurse - but maybe that's ok if universal is always set on creation

I think another approach could be to look at the wildcard type associated with the captured type. The wildcard type keeps track of the declared type variable it comes from, which means you can then look at the type symbol in there and decide if it's an universal variable or not. E.g.

captured.isUniversal() == captured.wildcard.bound.isUniversal()

lower.hasTag(TYPEVAR) && ((TypeVar)lower).universal;
}

public CapturedType(TypeSymbol tsym,
@@ -2050,6 +2100,8 @@ public CapturedType(TypeSymbol tsym,
TypeMetadata metadata) {
super(tsym, bound, lower, metadata);
this.wildcard = wildcard;
this.universal = upper.hasTag(TYPEVAR) && ((TypeVar)upper).universal ||
lower.hasTag(TYPEVAR) && ((TypeVar)lower).universal;
}

@Override
@@ -95,6 +95,7 @@ public class Types {
final boolean allowDefaultMethods;
final boolean mapCapturesToBounds;
final boolean allowValueBasedClasses;
final boolean allowUniversalTVars;
final Check chk;
final Enter enter;
JCDiagnostic.Factory diags;
@@ -125,7 +126,10 @@ protected Types(Context context) {
diags = JCDiagnostic.Factory.instance(context);
noWarnings = new Warner(null);
Options options = Options.instance(context);
Preview preview = Preview.instance(context);
allowValueBasedClasses = options.isSet("allowValueBasedClasses");
allowUniversalTVars = (!preview.isPreview(Feature.UNIVERSAL_TVARS) || preview.isEnabled()) &&
Feature.UNIVERSAL_TVARS.allowedInSource(source);
}
// </editor-fold>

@@ -608,6 +612,11 @@ public boolean isConvertible(Type t, Type s, Warner warn) {

boolean tValue = t.isPrimitiveClass();
boolean sValue = s.isPrimitiveClass();
if (allowUniversalTVars && (s.hasTag(TYPEVAR)) && ((TypeVar)s).isUniversal() &&
(t.hasTag(BOT) || t.hasTag(TYPEVAR) && !((TypeVar)t).isUniversal())) {
warn.warn(LintCategory.UNIVERSAL);
return true;
}
if (tValue != sValue) {
return tValue ?
isSubtype(t.referenceProjection(), s) :
@@ -1014,6 +1023,32 @@ public boolean isPrimitiveClass(Type t) {
return t != null && t.isPrimitiveClass();
}

@FunctionalInterface
public interface SubtypeTestFlavor {
boolean subtypeTest(Type t, Type s, Warner warn);
}

public boolean isBoundedBy(Type t, Type s, SubtypeTestFlavor subtypeTestFlavor) {
return isBoundedBy(t, s, noWarnings, subtypeTestFlavor);
}

/**
* Is type t bounded by s?
*/
public boolean isBoundedBy(Type t, Type s, Warner warn, SubtypeTestFlavor subtypeTestFlavor) {
boolean result = subtypeTestFlavor.subtypeTest(t, s, warn);
if (allowUniversalTVars && !result) {
if (isPrimitiveClass(t)) {
return isBoundedBy(t.referenceProjection(), s, warn, subtypeTestFlavor);
} else if (t.hasTag(TYPEVAR) && ((TypeVar)t).universal) {
return isBoundedBy(t.getUpperBound(), s, warn, subtypeTestFlavor);
} else if (s.hasTag(TYPEVAR) && ((TypeVar)s).universal) {
return isBoundedBy(t, s.getLowerBound(), warn, subtypeTestFlavor);
}
}
return result;
}

// <editor-fold defaultstate="collapsed" desc="isSubtype">
/**
* Is t an unchecked subtype of s?
@@ -1051,6 +1086,10 @@ private boolean isSubtypeUncheckedInternal(Type t, Type s, boolean capture, Warn
}
} else if (isSubtype(t, s, capture)) {
return true;
} else if (allowUniversalTVars && t.hasTag(TYPEVAR) && s.hasTag(TYPEVAR) && t.tsym == s.tsym) {
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@mcimadamore mcimadamore Aug 4, 2021

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Are you sure the check does what the comment says?

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@vicente-romero-oracle vicente-romero-oracle Aug 5, 2021

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will remove the comment, probably a self comment I guess

// we are seeing a case of a universal type variable being assigned to a non-universal one
warn.warn(LintCategory.UNIVERSAL);
return true;
} else if (t.hasTag(TYPEVAR)) {
return isSubtypeUncheckedInternal(t.getUpperBound(), s, false, warn);
} else if (!s.isRaw()) {
@@ -1102,6 +1141,9 @@ public final boolean isSubtypeNoCapture(Type t, Type s) {
public boolean isSubtype(Type t, Type s, boolean capture) {
if (t.equalsIgnoreMetadata(s))
return true;
if (t.hasTag(TYPEVAR) && s.hasTag(TYPEVAR) && t.tsym == s.tsym) {
return true;
}
if (s.isPartial())
return isSuperType(s, t);

@@ -1143,6 +1185,9 @@ public Boolean visitType(Type t, Type s) {
case TYPEVAR:
return isSubtypeNoCapture(t.getUpperBound(), s);
case BOT:
if (allowUniversalTVars && s.hasTag(TYPEVAR) && ((TypeVar)s).universal) {
warnStack.head.warn(LintCategory.UNIVERSAL);
}
return
s.hasTag(BOT) || (s.hasTag(CLASS) && !isPrimitiveClass(s)) ||
s.hasTag(ARRAY) || s.hasTag(TYPEVAR);
@@ -1641,8 +1686,10 @@ public Boolean visitWildcardType(WildcardType t, Type s) {
// ---------------------------------------------------------------------------
return isSameWildcard(t, s)
|| isCaptureOf(s, t)
|| ((t.isExtendsBound() || isSubtypeNoCapture(wildLowerBound(t), wildLowerBound(s))) &&
(t.isSuperBound() || isSubtypeNoCapture(wildUpperBound(s), wildUpperBound(t))));
|| ((t.isExtendsBound() || isBoundedBy(wildLowerBound(t), wildLowerBound(s),
(t1, s1, w) -> isSubtypeNoCapture(t1, s1))) &&
(t.isSuperBound() || isBoundedBy(wildUpperBound(s), wildUpperBound(t),
(t1, s1, w) -> isSubtypeNoCapture(t1, s1))));
}
}

@@ -1655,6 +1702,16 @@ public Boolean visitUndetVar(UndetVar t, Type s) {
}
}

@Override
public Boolean visitTypeVar(TypeVar t, Type s) {
if (s.hasTag(TYPEVAR)) {
TypeVar other = (TypeVar)s;
if (allowUniversalTVars && t.universal != other.universal && t.tsym == other.tsym)
return true;
}
return isSameType(t, s);
}

@Override
public Boolean visitErrorType(ErrorType t, Type s) {
return true;
@@ -2075,7 +2132,7 @@ public boolean notSoftSubtype(Type t, Type s) {
if (!s.hasTag(WILDCARD))
s = cvarUpperBound(s);

return !isSubtype(t, relaxBound(s));
return !isBoundedBy(t, relaxBound(s), (t1, s1, w) -> isSubtype(t1, s1));
}

private Type relaxBound(Type t) {
@@ -3555,6 +3612,13 @@ public Type visitTypeVar(TypeVar t, Void ignored) {
if (t.equalsIgnoreMetadata(from.head)) {
return to.head.withTypeVar(t);
}
if (allowUniversalTVars &&
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Not sure about this - the main issue here seems that t.equalsIgnoreMetadata is too weak and would return false for ref vs.val mismatches. But again I'm uncertain about the need of overriding withTypeVar (see comment above).

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@vicente-romero-oracle vicente-romero-oracle Aug 5, 2021

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not sure I follow you here, t.equalsIgnoreMetadata is already being used in the method, not sure I see in what case it can give an incorrect answer

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@mcimadamore mcimadamore Aug 5, 2021

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What I mean is that the newly added code seems to workaround the fact that in some cases we do not call withTypeVar. I'm saying that because you added another test which, if satisfied, ends up calling to.head.withTypeVar(t); again, which seems to suggest that you found cases where this call was needed, but was not triggered - probably because of a mismatch between T and T.ref?

t.createdFromUniversalTypeVar &&
from.head.hasTag(TYPEVAR) &&
((TypeVar)from.head).referenceProjection != null &&
t.equalsIgnoreMetadata(((TypeVar)from.head).referenceProjection)) {
return to.head.withTypeVar(t);
}
}
return t;
}
@@ -3704,7 +3768,11 @@ public List<Type> newInstances(List<Type> tvars) {
private static final TypeMapping<Void> newInstanceFun = new TypeMapping<Void>() {
@Override
public TypeVar visitTypeVar(TypeVar t, Void _unused) {
return new TypeVar(t.tsym, t.getUpperBound(), t.getLowerBound(), t.getMetadata());
TypeVar newTV = new TypeVar(t.tsym, t.getUpperBound(), t.getLowerBound(), t.getMetadata(), t.universal);
if (t.referenceProjection != null) {
newTV.createReferenceProjection();
}
return newTV;
}
};
// </editor-fold>