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
`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.

The added test is currently marked pending because it does not work with
JUnit (which doesn't handle separate compilation), only partest. I
didn't managed to get it to work right, and this won't be necessary once
our testing framework is overhauled by
#2125 anyway, so I'll just have to
remember to enable this test afterwards.

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/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)

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

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

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

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

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

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

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

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