Fixes to isPureExpr

 - erasedValue[<ConstantType>] is now considered to be pure
 - calls of synthetic case class apply are considered pure if the case
   class is NoInits
 - Companions of Scala-2 classes Tuple and Some are assumed to be NoInits

Author: odersky

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -590,9 +590,13 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
     case New(_) | Closure(_, _, _) =>
       Pure
     case TypeApply(fn, _) =>
+      val sym = fn.symbol
       if tree.tpe.isInstanceOf[MethodOrPoly] then exprPurity(fn)
-      else if fn.symbol == defn.QuotedTypeModule_of || fn.symbol == defn.Predef_classOf then Pure
-      else if fn.symbol == defn.Compiletime_erasedValue && tree.tpe.dealias.isInstanceOf[ConstantType] then Pure
+      else if sym == defn.QuotedTypeModule_of
+          || sym == defn.Predef_classOf
+          || sym == defn.Compiletime_erasedValue && tree.tpe.dealias.isInstanceOf[ConstantType]
+          || defn.capsErasedValueMethods.contains(sym)
+      then Pure
       else Impure
     case Apply(fn, args) =>
       val factorPurity = minOf(exprPurity(fn), args.map(exprPurity))
@@ -636,6 +640,15 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
 
   def isPureBinding(tree: Tree)(using Context): Boolean = statPurity(tree) >= Pure
 
+  def isPureSyntheticCaseApply(sym: Symbol)(using Context): Boolean =
+    sym.isAllOf(SyntheticMethod)
+    && sym.name == nme.apply
+    && sym.owner.is(Module)
+    && {
+      val cls = sym.owner.companionClass
+      cls.is(Case) && cls.isNoInitsRealClass
+    }
+
   /** Is the application `tree` with function part `fn` known to be pure?
    *  Function value and arguments can still be impure.
    */
@@ -647,6 +660,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
 
     tree.tpe.isInstanceOf[ConstantType] && tree.symbol != NoSymbol && isKnownPureOp(tree.symbol) // A constant expression with pure arguments is pure.
     || fn.symbol.isStableMember && fn.symbol.isConstructor // constructors of no-inits classes are stable
+    || isPureSyntheticCaseApply(fn.symbol)
 
   /** The purity level of this reference.
    *  @return

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

@@ -2006,7 +2006,9 @@ class Definitions {
 
   /** A allowlist of Scala-2 classes that are known to be pure */
   def isAssuredNoInits(sym: Symbol): Boolean =
-    (sym `eq` SomeClass) || isTupleClass(sym)
+    (sym `eq` SomeClass)
+    || isTupleClass(sym)
+    || sym.is(Module) && isAssuredNoInits(sym.companionClass)
 
   /** If `cls` is Tuple1..Tuple22, add the corresponding *: type as last parent to `parents` */
   def adjustForTuple(cls: ClassSymbol, tparams: List[TypeSymbol], parents: List[Type]): List[Type] = {

compiler/src/dotty/tools/dotc/util/DiffUtil.scala

@@ -103,7 +103,6 @@ object DiffUtil {
         case Deleted(str) => deleted(str)
       }.mkString
 
-    (expectedDiff, actualDiff)
     val pad = " " * 0.max(expectedSize - expected.length)
 
     expectedDiff + pad + "  |  " + actualDiff

library/src/scala/CanThrow.scala

@@ -12,5 +12,5 @@ erased class CanThrow[-E <: Exception] extends caps.SharedCapability
 
 @experimental
 object unsafeExceptions:
-  given canThrowAny: CanThrow[Exception] = compiletime.erasedValue
+  inline given canThrowAny: CanThrow[Exception] = compiletime.erasedValue
 

tests/init/warn/inner30.scala

@@ -8,7 +8,7 @@ class Scanners {
 
   class Scanner {
     def foo() =
-      Conc(Run('a', 3), Run('b', 4))
+      Conc(Run('a', 3), Run('b', 4))  // warn
       new LookAheadScanner
 
     class LookAheadScanner() extends Scanner