SI-6083, misleading annotation error message.

When an implicit conversion causes an apparently constant
argument not to be constant, show the conversion, not the
constant.

4 files changed, 23 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 162bdd22b2..dcc2ee0f23 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -3402,7 +3402,10 @@ trait Typers extends Adaptations with Tags {
        *  floats and literals in particular) are not yet folded.
        */
       def tryConst(tr: Tree, pt: Type): Option[LiteralAnnotArg] = {
-        val const: Constant = typed(constfold(tr), EXPRmode, pt) match {
+        // The typed tree may be relevantly different than the tree `tr`,
+        // e.g. it may have encountered an implicit conversion.
+        val ttree = typed(constfold(tr), EXPRmode, pt)
+        val const: Constant = ttree match {
           case l @ Literal(c) if !l.isErroneous => c
           case tree => tree.tpe match {
             case ConstantType(c)  => c
@@ -3411,7 +3414,7 @@ trait Typers extends Adaptations with Tags {
         }
 
         if (const == null) {
-          reportAnnotationError(AnnotationNotAConstantError(tr)); None
+          reportAnnotationError(AnnotationNotAConstantError(ttree)); None
         } else if (const.value == null) {
           reportAnnotationError(AnnotationArgNullError(tr)); None
         } else
diff --git a/test/files/neg/annot-nonconst.check b/test/files/neg/annot-nonconst.check
index b43e58a0ca..5b3da7a13c 100644
--- a/test/files/neg/annot-nonconst.check
+++ b/test/files/neg/annot-nonconst.check
@@ -8,7 +8,7 @@ make your annotation visible at runtime.  If that is what
 you want, you must write the annotation class in Java.
 class Ann2(value: String) extends annotation.ClassfileAnnotation
       ^
-annot-nonconst.scala:6: error: annotation argument needs to be a constant; found: n
+annot-nonconst.scala:6: error: annotation argument needs to be a constant; found: Test.this.n
   @Length(n) def foo = "foo"
           ^
 annot-nonconst.scala:7: error: annotation argument cannot be null
diff --git a/test/files/neg/t6083.check b/test/files/neg/t6083.check
new file mode 100644
index 0000000000..c9b5ba05d3
--- /dev/null
+++ b/test/files/neg/t6083.check
@@ -0,0 +1,10 @@
+t6083.scala:6: warning: Implementation restriction: subclassing Classfile does not
+make your annotation visible at runtime.  If that is what
+you want, you must write the annotation class in Java.
+class annot(value: String) extends annotation.ClassfileAnnotation
+      ^
+t6083.scala:7: error: annotation argument needs to be a constant; found: conv.i2s(101)
+@annot(101) class C
+       ^
+one warning found
+one error found
diff --git a/test/files/neg/t6083.scala b/test/files/neg/t6083.scala
new file mode 100644
index 0000000000..1de18e6527
--- /dev/null
+++ b/test/files/neg/t6083.scala
@@ -0,0 +1,7 @@
+object conv {
+  implicit def i2s(i: Int): String = ""
+}
+import conv._
+
+class annot(value: String) extends annotation.ClassfileAnnotation
+@annot(101) class C