cmd/compile/internal/syntax: fix syntax.Parse doc string, improved tests

1) Fix the doc string for syntax.Parse: The returned AST is
always nil if there was an error and an error handler is missing.

2) Adjust the syntax Print and Dump tests such that they print and
dump the AST even in the presence of errors.

Change-Id: If658eabdcc83f578d815070bc65d1a5f6cfaddfc
Reviewed-on: https://go-review.googlesource.com/94157
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/syntax/dumper_test.go

@@ -14,9 +14,13 @@ func TestDump(t *testing.T) {
 		t.Skip("skipping test in short mode")
 	}
 
-	ast, err := ParseFile(*src_, nil, nil, CheckBranches)
+	// provide a dummy error handler so parsing doesn't stop after first error
+	ast, err := ParseFile(*src_, func(error) {}, nil, CheckBranches)
 	if err != nil {
-		t.Fatal(err)
+		t.Error(err)
+	}
+
+	if ast != nil {
+		Fdump(os.Stdout, ast)
 	}
-	Fdump(os.Stdout, ast)
 }

src/cmd/compile/internal/syntax/nodes.go

@@ -215,8 +215,8 @@ type (
 	// Fun(ArgList[0], ArgList[1], ...)
 	CallExpr struct {
 		Fun     Expr
-		ArgList []Expr
+		ArgList []Expr // nil means no arguments
-		HasDots bool // last argument is followed by ...
+		HasDots bool   // last argument is followed by ...
 		expr
 	}
 

src/cmd/compile/internal/syntax/printer_test.go

@@ -16,12 +16,16 @@ func TestPrint(t *testing.T) {
 		t.Skip("skipping test in short mode")
 	}
 
-	ast, err := ParseFile(*src_, nil, nil, 0)
+	// provide a dummy error handler so parsing doesn't stop after first error
+	ast, err := ParseFile(*src_, func(error) {}, nil, 0)
 	if err != nil {
-		t.Fatal(err)
+		t.Error(err)
+	}
+
+	if ast != nil {
+		Fprint(os.Stdout, ast, true)
+		fmt.Println()
 	}
-	Fprint(os.Stdout, ast, true)
-	fmt.Println()
 }
 
 func TestPrintString(t *testing.T) {

src/cmd/compile/internal/syntax/syntax.go

@@ -50,12 +50,13 @@ type FilenameHandler func(name string) string
 
 // Parse parses a single Go source file from src and returns the corresponding
 // syntax tree. If there are errors, Parse will return the first error found,
-// and a possibly partially constructed syntax tree, or nil if no correct package
+// and a possibly partially constructed syntax tree, or nil.
-// clause was found. The base argument is only used for position information.
 //
 // If errh != nil, it is called with each error encountered, and Parse will
-// process as much source as possible. If errh is nil, Parse will terminate
+// process as much source as possible. In this case, the returned syntax tree
-// immediately upon encountering an error.
+// is only nil if no correct package clause was found.
+// If errh is nil, Parse will terminate immediately upon encountering the first
+// error, and the returned syntax tree is nil.
 //
 // If pragh != nil, it is called with each pragma encountered.
 //