Detect abstract inline method calls after inlining

compiler/src/dotty/tools/dotc/typer/Inliner.scala

@@ -1487,7 +1487,10 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
     override def hasInliningErrors(using Context) = ctx.reporter.errorCount > initialErrorCount
 
     private def inlineIfNeeded(tree: Tree)(using Context): Tree =
-      if Inliner.needsInlining(tree) then Inliner.inlineCall(tree)
+      val meth = tree.symbol
+      if meth.isAllOf(DeferredInline) then
+        errorTree(tree, i"Deferred inline ${meth.showLocated} cannot be invoked")
+      else if Inliner.needsInlining(tree) then Inliner.inlineCall(tree)
       else tree
 
     override def typedUnadapted(tree: untpd.Tree, pt: Type, locked: TypeVars)(using Context): Tree =

tests/neg/i12555.scala

@@ -0,0 +1,6 @@
+trait Noop {
+  inline def noop: String
+}
+
+inline def boom: String = (??? : Noop).noop
+def test: Unit = boom // error

tests/neg/i12555b.scala

@@ -0,0 +1,18 @@
+trait Noop[T]:
+  //note: making this "not inline" fixes the result
+  inline def noop(fa: T): T
+
+object Noop {
+  inline def noop[T](alg: T)(using n: Noop[T]): T = n.noop(alg)
+}
+
+import Noop.*
+
+final case class User(name: String, age: Int)
+
+inline given Noop[User] = a => a
+
+val u = User("hello", 45)
+
+@main
+def run = println(Noop.noop(u)) // error

tests/neg/i12555c.scala

@@ -0,0 +1,4 @@
+trait Noop {
+  inline def noop: String
+}
+def test2: Unit = (??? : Noop).noop // error