internal/lsp/source: fix false positive "..." in completions

In cases like:

    var v interface{}
    fmt.Println(<>)

Completing to "v" would insert "v..." instead of "v". This was due to
a mixup where we were checking if the variadic type "[]interface{}"
was assignable to the candidate type "interface{}" instead of the
other way around.

Fixes golang/go#38652.

Change-Id: I27c0b50bbf4b895924c8ed2c0c9dd6785e98cbe1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/229921
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

@@ -1734,8 +1734,7 @@ func (ci candidateInference) applyTypeNameModifiers(typ types.Type) types.Type {
 // matchesVariadic returns true if we are completing a variadic
 // parameter and candType is a compatible slice type.
 func (ci candidateInference) matchesVariadic(candType types.Type) bool {
-	return ci.variadicType != nil && types.AssignableTo(ci.objType, candType)
-
+	return ci.variadicType != nil && types.AssignableTo(candType, ci.objType)
 }
 
 // findSwitchStmt returns an *ast.CaseClause's corresponding *ast.SwitchStmt or

internal/lsp/testdata/lsp/primarymod/variadic/variadic.go.in

@@ -8,9 +8,10 @@ func bar() []string { //@item(vFunc, "bar", "func() []string", "func")
 
 func _() {
 	var (
-		i  int      //@item(vInt, "i", "int", "var")
-		s  string   //@item(vStr, "s", "string", "var")
-		ss []string //@item(vStrSlice, "ss", "[]string", "var")
+		i  int        //@item(vInt, "i", "int", "var")
+		s  string     //@item(vStr, "s", "string", "var")
+		ss []string   //@item(vStrSlice, "ss", "[]string", "var")
+		v interface{} //@item(vIntf, "v", "interface{}", "var")
 	)
 
 	foo()           //@rank(")", vInt, vStr),rank(")", vInt, vStrSlice)
@@ -20,6 +21,9 @@ func _() {
 
   // snippet will add the "..." for you
 	foo(123, ) //@snippet(")", vStrSlice, "ss...", "ss..."),snippet(")", vFunc, "bar()...", "bar()..."),snippet(")", vStr, "s", "s")
+
+	// don't add "..." for interface{}
+	foo(123, ) //@snippet(")", vIntf, "v", "v")
 }
 
 func qux(...func()) {}

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

@@ -1,7 +1,7 @@
 -- summary --
 CodeLensCount = 2
 CompletionsCount = 237
-CompletionSnippetCount = 75
+CompletionSnippetCount = 76
 UnimportedCompletionsCount = 6
 DeepCompletionsCount = 5
 FuzzyCompletionsCount = 8