fix #134 validate Configuration id

core/src/main/scala/sbt/librarymanagement/ConfigurationExtra.scala

@@ -110,6 +110,9 @@ private[sbt] object ConfigurationMacro {
       c,
       methodName =>
         s"""$methodName must be directly assigned to a val, such as `val Tooling = $methodName("tooling")`.""")
+    if (enclosingValName.head.isLower) {
+      c.error(c.enclosingPosition, "configuration id must be capitalized")
+    }
     val id = c.Expr[String](Literal(Constant(enclosingValName)))
     reify { Configuration.of(id.splice, name.splice) }
   }

core/src/test/scala/sbt/librarymanagement/ConfigMacroTest.scala

@@ -0,0 +1,13 @@
+package sbt.librarymanagement
+
+import sbt.librarymanagement.Configurations.config
+import org.scalatest._
+
+class ConfigMacroTest extends FunSpec with Matchers {
+  describe("Configurations.config") {
+    it("should validate the ID in compile time") {
+      """val A = config("a")""" should compile
+      """val b = config("b")""" shouldNot compile
+    }
+  }
+}