cmd/go: better error for test functions with wrong signature

Check the function types before compiling the tests. Extend the same
approach taken by the type check used for TestMain function.

To keep existing behavior, wrong arguments for TestMain are ignored
instead of causing an error.

Fixes #14226.

Change-Id: I488a2555cddb273d35c1a8c4645bb5435c9eb91d
Reviewed-on: https://go-review.googlesource.com/19763
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/test.go

@@ -1206,11 +1206,11 @@ func (b *builder) notest(a *action) error {
 	return nil
 }
 
-// isTestMain tells whether fn is a TestMain(m *testing.M) function.
-func isTestMain(fn *ast.FuncDecl) bool {
-	if fn.Name.String() != "TestMain" ||
-		fn.Type.Results != nil && len(fn.Type.Results.List) > 0 ||
-		fn.Type.Params == nil ||
+// isTestFunc tells whether fn has the type of a testing function. arg
+// specifies the parameter type we look for: B, M or T.
+func isTestFunc(fn *ast.FuncDecl, arg string) bool {
+	if fn.Type.Results != nil && len(fn.Type.Results.List) > 0 ||
+		fn.Type.Params.List == nil ||
 		len(fn.Type.Params.List) != 1 ||
 		len(fn.Type.Params.List[0].Names) > 1 {
 		return false
@@ -1222,10 +1222,11 @@ func isTestMain(fn *ast.FuncDecl) bool {
 	// We can't easily check that the type is *testing.M
 	// because we don't know how testing has been imported,
 	// but at least check that it's *M or *something.M.
-	if name, ok := ptr.X.(*ast.Ident); ok && name.Name == "M" {
+	// Same applies for B and T.
+	if name, ok := ptr.X.(*ast.Ident); ok && name.Name == arg {
 		return true
 	}
-	if sel, ok := ptr.X.(*ast.SelectorExpr); ok && sel.Sel.Name == "M" {
+	if sel, ok := ptr.X.(*ast.SelectorExpr); ok && sel.Sel.Name == arg {
 		return true
 	}
 	return false
@@ -1344,16 +1345,22 @@ func (t *testFuncs) load(filename, pkg string, doImport, seen *bool) error {
 		}
 		name := n.Name.String()
 		switch {
-		case isTestMain(n):
+		case name == "TestMain" && isTestFunc(n, "M"):
 			if t.TestMain != nil {
 				return errors.New("multiple definitions of TestMain")
 			}
 			t.TestMain = &testFunc{pkg, name, ""}
 			*doImport, *seen = true, true
 		case isTest(name, "Test"):
+			if !isTestFunc(n, "T") {
+				return fmt.Errorf("wrong type for %s", name)
+			}
 			t.Tests = append(t.Tests, testFunc{pkg, name, ""})
 			*doImport, *seen = true, true
 		case isTest(name, "Benchmark"):
+			if !isTestFunc(n, "B") {
+				return fmt.Errorf("wrong type for %s", name)
+			}
 			t.Benchmarks = append(t.Benchmarks, testFunc{pkg, name, ""})
 			*doImport, *seen = true, true
 		}