Delay checking against expected type for eta-expanded closures

Author: odersky

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -606,8 +606,23 @@ class CheckCaptures extends Recheck, SymTransformer:
       // rechecking the body.
       openClosures = (mdef.symbol, pt) :: openClosures
       try
+        def isEtaExpansion(mdef: DefDef): Boolean = mdef.paramss match
+          case (param :: _) :: _ if param.asInstanceOf[Tree].span.isZeroExtent =>
+            mdef.rhs match
+              case _: Apply => true
+              case closureDef(mdef1) => isEtaExpansion(mdef1)
+              case _ => false
+          case _ => false
         val res = recheckClosure(expr, pt, forceDependent = true)
-        checkConformsExpr(res, pt, expr)
+        if !isEtaExpansion(mdef) then
+          // If closure is an eta expanded method reference it's better to not constrain
+          // its internals early since that would give error messages in generated code
+          // which are less intelligible.
+          // Example is the line `a = x` in neg-custom-args/captures/vars.scala.
+          // For all other closures, early constraints are preferred since they
+          // give more localized error messages.
+          checkConformsExpr(res, pt, expr)
+        //else report.warning(i"skip test $mdef", mdef.srcPos)
         recheckDef(mdef, mdef.symbol)
         //println(i"RECHECK CLOSURE ${mdef.symbol.info}")
         res