internal/lsp/source: fix multi return value func completion

Fix a minor completion ranking issue:

    foo := func(int, int) {}
    foo(123, <>)

Previously we were preferring "foo()" at "<>" even though it can't be
used. We mistakenly thought we were completing the first param because
the *ast.CallExpr appears to only have a single param.

Change-Id: Iedbbb1870a4b9eb5d5be4ed266b8bb3e313b496b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/246697
Run-TryBot: Muir Manders <muir@mnd.rs>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/source/completion.go

@@ -1661,7 +1661,7 @@ Nodes:
 						// values, in turn getting passed in to the surrounding
 						// call. Record the assignees so we can favor function
 						// calls that return matching values.
-						if len(node.Args) <= 1 {
+						if len(node.Args) <= 1 && exprIdx == 0 {
 							for i := 0; i < sig.Params().Len(); i++ {
 								inf.assignees = append(inf.assignees, sig.Params().At(i).Type())
 							}

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

@@ -23,6 +23,7 @@ func _() {
 	f1(f)    //@rank(")", multiF1, multiF2)
 	f2(f)    //@rank(")", multiF2, multiF3),rank(")", multiF1, multiF3)
 	f2(1, f) //@rank(")", multiF1, multiF2),rank(")", multiF1, multiF0)
+	f2(1, )  //@rank(")", multiF1, multiF2),rank(")", multiF1, multiF0)
 	f2Str()  //@rank(")", multiF2Str, multiF2)
 
 	var i int

internal/lsp/testdata/lsp/summary.txt.golden

@@ -5,7 +5,7 @@ CompletionSnippetCount = 80
 UnimportedCompletionsCount = 6
 DeepCompletionsCount = 5
 FuzzyCompletionsCount = 8
-RankedCompletionsCount = 120
+RankedCompletionsCount = 122
 CaseSensitiveCompletionsCount = 4
 DiagnosticsCount = 44
 FoldingRangesCount = 2