Fix #4151: Make sure the definition of the macro is correcty formed

Author: nicolasstucki

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

@@ -508,10 +508,26 @@ class ReifyQuotes extends MacroTransformWithImplicits {
           case _: Import =>
             tree
           case tree: DefDef if tree.symbol.is(Macro) && level == 0 =>
-            markDef(tree)
-            nested(isQuote = true).transform(tree)
-              // check macro code as it if appeared in a quoted context
-            cpy.DefDef(tree)(rhs = EmptyTree)
+            tree.rhs match {
+              case InlineSplice(_) =>
+                markDef(tree)
+                nested(isQuote = true).transform(tree)
+                  // check macro code as it if appeared in a quoted context
+                cpy.DefDef(tree)(rhs = EmptyTree)
+              case _ =>
+                ctx.error(
+                  """Malformed inline macro.
+                    |
+                    |Expected the ~ to be at the top of the RHS:
+                    |  inline def foo(...): Int = ~impl(...)
+                    |or
+                    |  inline def foo(...): Int = ~{
+                    |    val x = 1
+                    |    impl(... x ...)
+                    |  }
+                  """.stripMargin, tree.rhs.pos)
+                EmptyTree
+            }
           case _ =>
             markDef(tree)
             checkLevel(mapOverTree(enteredSyms))

tests/neg/quote-macro-splice.scala

@@ -0,0 +1,27 @@
+import scala.quoted._
+
+object Test {
+
+  inline def foo1: Int = { // error
+    println()
+    ~impl(1.toExpr)
+  }
+
+  inline def foo2: Int = { // error
+    ~impl(1.toExpr)
+    ~impl(2.toExpr)
+  }
+
+  inline def foo3: Int = { // error
+    val a = 1
+    ~impl('(a))
+  }
+
+  inline def foo4: Int = { // error
+    ~impl('(1))
+    1
+  }
+
+  def impl(n: Expr[Int]): Expr[Int] = ???
+
+}