Coverage: Do not lift applications of context functions (#18498)

closes lampepfl#16502

compiler/src/dotty/tools/dotc/transform/InstrumentCoverage.scala

@@ -452,8 +452,13 @@ class InstrumentCoverage extends MacroTransform with IdentityDenotTransformer:
          * they shouldn't be lifted.
          */
         val sym = fun.symbol
-        sym.exists && (isShortCircuitedOp(sym) || StringInterpolatorOpt.isCompilerIntrinsic(sym) || sym == defn.Object_synchronized)
-      end
+        sym.exists && (
+          isShortCircuitedOp(sym)
+          || StringInterpolatorOpt.isCompilerIntrinsic(sym)
+          || sym == defn.Object_synchronized
+          || isContextFunctionApply(fun)
+        )
+      end isUnliftableFun
 
       val fun = tree.fun
       val nestedApplyNeedsLift = fun match
@@ -463,6 +468,12 @@ class InstrumentCoverage extends MacroTransform with IdentityDenotTransformer:
       nestedApplyNeedsLift ||
       !isUnliftableFun(fun) && !tree.args.isEmpty && !tree.args.forall(LiftCoverage.noLift)
 
+    private def isContextFunctionApply(fun: Tree)(using Context): Boolean =
+      fun match
+        case Select(prefix, nme.apply) =>
+          defn.isContextFunctionType(prefix.tpe.widen)
+        case _ => false
+
     /** Check if an Apply can be instrumented. Prevents this phase from generating incorrect code. */
     private def canInstrumentApply(tree: Apply)(using Context): Boolean =
       def isSecondaryCtorDelegateCall: Boolean = tree.fun match

tests/coverage/pos/i16502.scala

@@ -0,0 +1,8 @@
+import scala.concurrent.*
+
+def asyncSum: ExecutionContext ?=> Future[Int] = Future(1)
+
+@main
+def Test(): Unit =
+  import scala.concurrent.ExecutionContext.Implicits.global
+  asyncSum

tests/coverage/pos/i16502.scoverage.check

@@ -0,0 +1,88 @@
+# Coverage data, format version: 3.0
+# Statement data:
+# - id
+# - source path
+# - package name
+# - class name
+# - class type (Class, Object or Trait)
+# - full class name
+# - method name
+# - start offset
+# - end offset
+# - line number
+# - symbol name
+# - tree name
+# - is branch
+# - invocations count
+# - is ignored
+# - description (can be multi-line)
+# '' sign
+# ------------------------------------------
+0
+i16502.scala
+<empty>
+i16502$package$
+Object
+<empty>.i16502$package$
+$anonfun
+76
+85
+2
+apply
+Apply
+false
+0
+false
+Future(1)
+
+1
+i16502.scala
+<empty>
+i16502$package$
+Object
+<empty>.i16502$package$
+asyncSum
+27
+39
+2
+asyncSum
+DefDef
+false
+0
+false
+def asyncSum
+
+2
+i16502.scala
+<empty>
+i16502$package$
+Object
+<empty>.i16502$package$
+Test
+174
+182
+7
+apply
+Apply
+false
+0
+false
+asyncSum
+
+3
+i16502.scala
+<empty>
+i16502$package$
+Object
+<empty>.i16502$package$
+Test
+87
+101
+5
+Test
+DefDef
+false
+0
+false
+@main\ndef Test
+