fixed #2316: No longer cache entire SearchResul...

fixed #2316: No longer cache entire SearchResult when looking for implicits in the parts of the expected type. (patch by retronym -- see ticket)

A SearchResult may contain symbols local to the scope of the search that
were used as implicit parameters, so they are not safely cacheable. The
fix for #2101 does not suffice. That patch avoided bound symbols being
duplicated, but the problem is much worse.

The implicits for an expected type depend on more than just that type,
so we cannot cache them using the expected type as a key.

The neg/t2316 test illustrates this: T1 may provide two implicits, one
requires an implicit T2, another an implicit T3. If an implicit T1
is first required when only a T2 is in scope, the SearchResult will
describe the corresponding implicit. Now, if we enter an implicit value
of type T3 into scope, the search should fail (it is ambiguous), but the
cache does not take this new fact into account.

The patch replaces the erroneous aggressive caching with a more
conservative version that only caches ImplicitInfo's.

2 files changed, 50 insertions, 0 deletions

diff --git a/test/files/neg/t2316.check b/test/files/neg/t2316.check
new file mode 100644
index 0000000000..fea1745663
--- /dev/null
+++ b/test/files/neg/t2316.check
@@ -0,0 +1,7 @@
+t2316.scala:28: error: ambiguous implicit values:
+ both method T1FromT3 in object T1 of type (implicit t3: test.T3)test.T1
+ and method T1FromT2 in object T1 of type (implicit t2: test.T2)test.T1
+ match expected type test.T1
+      val t1 = requireT1
+               ^
+one error found
diff --git a/test/files/neg/t2316.scala b/test/files/neg/t2316.scala
new file mode 100644
index 0000000000..ccda9d5aac
--- /dev/null
+++ b/test/files/neg/t2316.scala
@@ -0,0 +1,43 @@
+object test {
+  case class T1(val source: String)
+
+
+  object T1 {
+    implicit def T1FromT2(implicit t2: T2): T1 = T1("implicit def T1FromT2")
+    implicit def T1FromT3(implicit t3: T3): T1 = T1("implicit def T1FromT3")
+  }
+
+  trait T2 {
+  }
+
+  object T2 {
+    implicit val t2: T2 = new T2 {}
+  }
+
+  trait T3
+
+  def requireT1(implicit t1: T1) = t1
+
+  {
+      val t1 = requireT1
+      assert(t1.source == "implicit def T1FromT2")
+  }
+
+  {
+      implicit def t3: T3 = new T3 {}
+      val t1 = requireT1
+      assert(t1.source == "implicit def T1FromT2")
+
+      // Expected a compile error here, but because T1.T1FromT2(T2.t2) was cached as a non-local implicit
+      // expression for type T1, this is not checked!
+      //
+      // (fragment of implicit-cache-error2.scala):26: error: ambiguous implicit values:
+      // both method T1FromT3 in object T1 of type (implicit t3: this.T3)this.T1
+      // and method T1FromT2 in object T1 of type (implicit t2: this.T2)this.T1
+      // match expected type this.T1
+      //    val t1 = requireT1
+      //              ^
+      // one error found
+
+  }
+}
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