go/analysis/passes/loopclosure: avoid panic in new parallel subtest check when t.Run has single argument

The new Go 1.20 parallel subtest check in the loopclosure pass
can panic in the rare case of t.Run with a single argument:

    fn := func() (string, func(t *testing.T)) { return "", nil }
    t.Run(fn())

A real-world example:

https://github.com/go-spatial/geom/blob/3cd2f5a9a082dd4f827c9f9b69ba5d736d2dcb12/planar/planar_test.go#L118

This has been present for multiple months without seeming to
be reported, so presumably this is a rare pattern.

Avoid that panic by checking t.Run has an expected number
of arguments.

Fixes golang/go#57908

Change-Id: I5cde60edf624e16bb9f534e435bcd55e63a15647
GitHub-Last-Rev: 24e65a4
GitHub-Pull-Request: #424
Reviewed-on: https://go-review.googlesource.com/c/tools/+/462282
Run-TryBot: thepudds <[email protected]>
Reviewed-by: Russ Cox <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
gopls-CI: kokoro <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: thepudds

go/analysis/passes/loopclosure/loopclosure.go

@@ -303,6 +303,11 @@ func parallelSubtest(info *types.Info, call *ast.CallExpr) []ast.Stmt {
 		return nil
 	}
 
+	if len(call.Args) != 2 {
+		// Ignore calls such as t.Run(fn()).
+		return nil
+	}
+
 	lit, _ := call.Args[1].(*ast.FuncLit)
 	if lit == nil {
 		return nil

go/analysis/passes/loopclosure/testdata/src/subtests/subtest.go

@@ -140,6 +140,10 @@ func _(t *testing.T) {
 			t.Parallel()
 			println(test) // want "loop variable test captured by func literal"
 		})
+
+		// Check that we do not have problems when t.Run has a single argument.
+		fn := func() (string, func(t *testing.T)) { return "", nil }
+		t.Run(fn())
 	}
 }