Merge pull request #15759 from dotty-staging/mb/isConst-lazy

Do not fold IsConst applied to dependent parameters

Author: Kordyjan

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

@@ -64,7 +64,12 @@ object TypeEval:
         // currently not a concrete known type
         case _: TypeParamRef => None
         // constant if the term is constant
-        case t: TermRef => isConst(t.underlying)
+        case t: TermRef =>
+          if t.denot.symbol.flagsUNSAFE.is(Flags.Param) then
+            // might be substituted later
+            None
+          else
+            isConst(t.underlying)
         // an operation type => recursively check all argument compositions
         case applied: AppliedType if defn.isCompiletimeAppliedType(applied.typeSymbol) =>
           val argsConst = applied.args.map(isConst)
@@ -239,4 +244,4 @@ object TypeEval:
 
     case _ => NoType
   end tryCompiletimeConstantFold
-end TypeEval
+end TypeEval

tests/neg/singleton-ops-any.scala

@@ -37,4 +37,9 @@ object Test {
   val t51 : true = isConst2[1, 1]
   val t52 : false = isConst2[1, one.type]
   val t53 : true = isConst2[1, two.type]
+
+  val t54: 2 = 2
+  summon[IsConst[t54.type] =:= true]
+  def isConst(arg: Int)(using IsConst[arg.type] =:= true): Unit = ???
+  isConst(t54)
 }