lsp/completion: fix variadic param candidate ordering edge case

In cases like:

    var foo func(...interface{})
    var one int
    var two func() (int, int)

    foo(<>)

At <> we were preferring "two()" over "one" because we were really
excited that the multi return value function was usable. "one" was not
preferred because the expected value is interface{} (we default to
saying candidates _don't_ match interface{} to give non-type based
aspects of candidate inference a chance to shine).

Fix by applying the corresponding interface{} logic to the assignees
checking: ignore the case of completing into func(...interface{})
since all multi return value functions would match.

Fixes golang/go#46378.

Change-Id: I355daa75e067e8b14508ca50b8d3b6b727df5fec
Reviewed-on: https://go-review.googlesource.com/c/tools/+/323509
Run-TryBot: Muir Manders <muir@mnd.rs>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Trust: Peter Weinberger <pjw@google.com>

internal/lsp/source/completion/completion.go

@@ -2702,6 +2702,12 @@ func (ci *candidateInference) assigneesMatch(cand *candidate, sig *types.Signatu
 		return false
 	}
 
+	// Don't prefer completing into func(...interface{}) calls since all
+	// functions wouuld match.
+	if ci.variadicAssignees && len(ci.assignees) == 1 && isEmptyInterface(deslice(ci.assignees[0])) {
+		return false
+	}
+
 	var numberOfResultsCouldMatch bool
 	if ci.variadicAssignees {
 		numberOfResultsCouldMatch = sig.Results().Len() >= len(ci.assignees)-1

internal/lsp/testdata/multireturn/multi_return.go.in

@@ -37,3 +37,12 @@ func _() {
 	var variadic func(int, ...int)
 	variadic() //@rank(")", multiF1, multiF0),rank(")", multiF2, multiF0),rank(")", multiF3, multiF0)
 }
+
+func _() {
+	var baz func(...interface{})
+
+	var otterNap func() (int, int) //@item(multiTwo, "otterNap", "func() (int, int)", "var")
+	var one int                    //@item(multiOne, "one", "int", "var")
+
+	baz(on) //@rank(")", multiOne, multiTwo)
+}

internal/lsp/testdata/summary.txt.golden

@@ -6,7 +6,7 @@ CompletionSnippetCount = 95
 UnimportedCompletionsCount = 5
 DeepCompletionsCount = 5
 FuzzyCompletionsCount = 8
-RankedCompletionsCount = 165
+RankedCompletionsCount = 166
 CaseSensitiveCompletionsCount = 4
 DiagnosticsCount = 37
 FoldingRangesCount = 2