Fix #2137: Create dummy companions for top-level objects without a real one

Previously, we sometimes ended up forcing a companion class symbol from
a previous run or from the classpath which lead to weird issues like in
`tests/pos-special/false-companion`. Even if we end up not forcing such
a symbol, its presence can still lead to issue: before this commit
incremental compilation of `dotty-compiler-bootstrapped` was broken
because we recorded a false dependency on the non-bootstrapped
`dotty-compiler` jar.

Author: smarter

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

@@ -605,6 +605,25 @@ class Namer { typer: Typer =>
           case EmptyTree =>
         }
       }
+
+      // If a top-level object has no companion class in the current run, we
+      // enter a dummy companion class symbol (`denot.isAbsent` returns true) in
+      // scope. This ensures that we never use a companion from a previous run
+      // or from the classpath. See tests/pos-special/false-companion for an
+      // example where this matters.
+      if (ctx.owner.is(PackageClass)) {
+        for (cdef @ TypeDef(moduleName, _) <- moduleDef.values) {
+          val moduleSym = ctx.denotNamed(moduleName.encode).symbol
+          if (moduleSym.isDefinedInCurrentRun) {
+            val className = moduleName.stripModuleClassSuffix.toTypeName
+            val classSym = ctx.denotNamed(className.encode).symbol
+            if (!classSym.isDefinedInCurrentRun) {
+              val absentClassSymbol = ctx.newClassSymbol(ctx.owner, className, EmptyFlags, _ => NoType)
+              enterSymbol(absentClassSymbol)
+            }
+          }
+        }
+      }
     }
 
     stats.foreach(expand)

compiler/test/dotc/tests.scala

@@ -168,6 +168,21 @@ class tests extends CompilerTest {
     compileFile(posSpecialDir, "spec-t5545/S_1")
     compileFile(posSpecialDir, "spec-t5545/S_2")
   }
+  @Test def pos_false_companion = {
+    // compile by hand in two batches, since junit lacks the infrastructure to
+    // compile files in multiple batches according to _1, _2, ... suffixes.
+    val opts = List("-priorityclasspath", defaultOutputDir) ++ defaultOptions
+
+    compileList("pos_false_companion Part 1/2", List(
+      s"$posSpecialDir/false-companion/outerFoo_1.scala",
+      s"$posSpecialDir/false-companion/outerinnerFoo_1.scala"
+    ), opts)
+    compileList("pos_false_companion Part 2/2", List(
+      // The order in which we pass these files to the compiler is important
+      s"$posSpecialDir/false-companion/00_outerinnerTest_2.scala",
+      s"$posSpecialDir/false-companion/01_outerinnerFoo_2.scala"
+    ), opts)
+  }
   @Test def pos_utf8 = compileFile(posSpecialDir, "utf8encoded", explicitUTF8)
   @Test def pos_utf16 = compileFile(posSpecialDir, "utf16encoded", explicitUTF16)
 

tests/pos-special/false-companion/00_outerinnerTest_2.scala

@@ -0,0 +1,5 @@
+package outer
+package inner
+object Test {
+  val x: Foo = new Foo
+}

tests/pos-special/false-companion/01_outerinnerFoo_2.scala

@@ -0,0 +1,5 @@
+package outer
+package inner
+object Foo {
+  // val a: Int = 1
+}

tests/pos-special/false-companion/outerFoo_1.scala

@@ -0,0 +1,2 @@
+package outer
+class Foo

tests/pos-special/false-companion/outerinnerFoo_1.scala

@@ -0,0 +1,5 @@
+package outer
+package inner
+object Foo {
+  val a: Int = 1
+}