go/analysis/passes/sortslice: correct diagnostic for sort.Slice(f())

Fixes golang/go#55098

Change-Id: I801279532c448bc4c9d5fce0cc3236e95fb77fdb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/431295
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Tim King <taking@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>

go/analysis/passes/sortslice/analyzer.go

@@ -52,11 +52,20 @@ func run(pass *analysis.Pass) (interface{}, error) {
 
 		arg := call.Args[0]
 		typ := pass.TypesInfo.Types[arg].Type
+
+		if tuple, ok := typ.(*types.Tuple); ok {
+			typ = tuple.At(0).Type() // special case for Slice(f(...))
+		}
+
 		switch typ.Underlying().(type) {
 		case *types.Slice, *types.Interface:
 			return
 		}
 
+		// Restore typ to the original type, we may unwrap the tuple above,
+		// typ might not be the type of arg.
+		typ = pass.TypesInfo.Types[arg].Type
+
 		var fixes []analysis.SuggestedFix
 		switch v := typ.Underlying().(type) {
 		case *types.Array:

go/analysis/passes/sortslice/testdata/src/a/a.go

@@ -6,8 +6,8 @@ import "sort"
 func IncorrectSort() {
 	i := 5
 	sortFn := func(i, j int) bool { return false }
-	sort.Slice(i, sortFn) // want "sort.Slice's argument must be a slice; is called with int"
-	sort.SliceStable(i, sortFn) // want "sort.SliceStable's argument must be a slice; is called with int"
+	sort.Slice(i, sortFn)         // want "sort.Slice's argument must be a slice; is called with int"
+	sort.SliceStable(i, sortFn)   // want "sort.SliceStable's argument must be a slice; is called with int"
 	sort.SliceIsSorted(i, sortFn) // want "sort.SliceIsSorted's argument must be a slice; is called with int"
 }
 
@@ -62,3 +62,23 @@ func UnderlyingSlice() {
 	sort.SliceStable(s, sortFn)
 	sort.SliceIsSorted(s, sortFn)
 }
+
+// FunctionResultsAsArguments passes a function which returns two values
+// that satisfy sort.Slice signature. It should not produce a diagnostic.
+func FunctionResultsAsArguments() {
+	s := []string{"a", "z", "ooo"}
+	sort.Slice(less(s))
+	sort.Slice(lessPtr(s)) // want `sort.Slice's argument must be a slice; is called with \(\*\[\]string,.*`
+}
+
+func less(s []string) ([]string, func(i, j int) bool) {
+	return s, func(i, j int) bool {
+		return s[i] < s[j]
+	}
+}
+
+func lessPtr(s []string) (*[]string, func(i, j int) bool) {
+	return &s, func(i, j int) bool {
+		return s[i] < s[j]
+	}
+}