Track type variable dependencies to guide instantiation decisions

compiler/src/dotty/tools/dotc/config/Config.scala

@@ -22,6 +22,11 @@ object Config {
    */
   inline val checkConstraintsNonCyclic = false
 
+  /** Check that reverse dependencies in constraints are correct and complete.
+   *  Can also be enabled using -Ycheck-constraint-deps.
+   */
+  inline val checkConstraintDeps = false
+
   /** Check that each constraint resulting from a subtype test
    *  is satisfiable. Also check that a type variable instantiation
    *  satisfies its constraints.
@@ -184,6 +189,9 @@ object Config {
   /** If set, prints a trace of all symbol completions */
   inline val showCompletions = false
 
+  /** If set, show variable/variable reverse dependencies when printing constraints. */
+  inline val showConstraintDeps = true
+
   /** If set, method results that are context functions are flattened by adding
    *  the parameters of the context function results to the methods themselves.
    *  This is an optimization that reduces closure allocations.

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -309,6 +309,7 @@ private sealed trait YSettings:
   val YforceSbtPhases: Setting[Boolean] = BooleanSetting("-Yforce-sbt-phases", "Run the phases used by sbt for incremental compilation (ExtractDependencies and ExtractAPI) even if the compiler is ran outside of sbt, for debugging.")
   val YdumpSbtInc: Setting[Boolean] = BooleanSetting("-Ydump-sbt-inc", "For every compiled foo.scala, output the API representation and dependencies used for sbt incremental compilation in foo.inc, implies -Yforce-sbt-phases.")
   val YcheckAllPatmat: Setting[Boolean] = BooleanSetting("-Ycheck-all-patmat", "Check exhaustivity and redundancy of all pattern matching (used for testing the algorithm).")
+  val YcheckConstraintDeps: Setting[Boolean] = BooleanSetting("-Ycheck-constraint-deps", "Check dependency tracking in constraints (used for testing the algorithm).")
   val YretainTrees: Setting[Boolean] = BooleanSetting("-Yretain-trees", "Retain trees for top-level classes, accessible from ClassSymbol#tree")
   val YshowTreeIds: Setting[Boolean] = BooleanSetting("-Yshow-tree-ids", "Uniquely tag all tree nodes in debugging output.")
   val YfromTastyIgnoreList: Setting[List[String]] = MultiStringSetting("-Yfrom-tasty-ignore-list", "file", "List of `tasty` files in jar files that will not be loaded when using -from-tasty")

compiler/src/dotty/tools/dotc/core/Constraint.scala

@@ -4,6 +4,7 @@ package core
 
 import Types._, Contexts._
 import printing.Showable
+import util.{SimpleIdentitySet, SimpleIdentityMap}
 
 /** Constraint over undetermined type parameters. Constraints are built
  *  over values of the following types:
@@ -128,7 +129,7 @@ abstract class Constraint extends Showable {
 
   /** Is `tv` marked as hard in the constraint? */
   def isHard(tv: TypeVar): Boolean
-  
+
   /** The same as this constraint, but with `tv` marked as hard. */
   def withHard(tv: TypeVar)(using Context): This
 
@@ -165,15 +166,32 @@ abstract class Constraint extends Showable {
    */
   def hasConflictingTypeVarsFor(tl: TypeLambda, that: Constraint): Boolean
 
-  /** Check that no constrained parameter contains itself as a bound */
-  def checkNonCyclic()(using Context): this.type
-
   /** Does `param` occur at the toplevel in `tp` ?
    *  Toplevel means: the type itself or a factor in some
    *  combination of `&` or `|` types.
    */
   def occursAtToplevel(param: TypeParamRef, tp: Type)(using Context): Boolean
 
+  /** A string that shows the reverse dependencies maintained by this constraint
+   *  (coDeps and contraDeps for OrderingConstraints).
+   */
+  def depsToString(using Context): String
+
+  /** Does the constraint restricted to variables outside `except` depend on `tv`
+   *  in the given direction `co`?
+   *  @param `co`  If true, test whether the constraint would change if the variable is made larger
+   *               otherwise, test whether the constraint would change if the variable is made smaller.
+   */
+  def dependsOn(tv: TypeVar, except: TypeVars, co: Boolean)(using Context): Boolean
+
+  /** Depending on Config settngs:
+   *   - Under `checkConstraintsNonCyclic`, check that no constrained
+   *     parameter contains itself as a bound.
+   *   - Under `checkConstraintDeps`, check hat reverse dependencies in
+   *     constraints are correct and complete.
+   */
+  def checkWellFormed()(using Context): this.type
+
   /** Check that constraint only refers to TypeParamRefs bound by itself */
   def checkClosed()(using Context): Unit