cmd/compile: better handling of incorrect type switches

Don't report errors if we don't have a correct type switch
guard; instead ignore it and leave it to the type-checker
to report the error. This leads to better error messages
concentrating on the type switch guard rather than errors
around (confusing) syntactic details.

Also clean up some code setting up AssertExpr (they never
have a nil Type field) and remove some incorrect TODOs.

Fixes #24470.

Change-Id: I69512f36e0417e3b5ea9c8856768e04b19d654a8
Reviewed-on: https://go-review.googlesource.com/103615
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

3 files changed, 44 insertions, 65 deletions

diff --git a/src/cmd/compile/internal/syntax/nodes.go b/src/cmd/compile/internal/syntax/nodes.go
index 8df9fd5c4e..c1da4adf52 100644
--- a/src/cmd/compile/internal/syntax/nodes.go
+++ b/src/cmd/compile/internal/syntax/nodes.go
@@ -198,12 +198,19 @@ type (
 
 	// X.(Type)
 	AssertExpr struct {
-		X Expr
-		// TODO(gri) consider using Name{"..."} instead of nil (permits attaching of comments)
+		X    Expr
 		Type Expr
 		expr
 	}
 
+	// X.(type)
+	// Lhs := X.(type)
+	TypeSwitchGuard struct {
+		Lhs *Name // nil means no Lhs :=
+		X   Expr  // X.(type)
+		expr
+	}
+
 	Operation struct {
 		Op   Operator
 		X, Y Expr // Y == nil means unary expression
@@ -413,13 +420,6 @@ type (
 		simpleStmt
 	}
 
-	TypeSwitchGuard struct {
-		// TODO(gri) consider using Name{"..."} instead of nil (permits attaching of comments)
-		Lhs *Name // nil means no Lhs :=
-		X   Expr  // X.(type)
-		expr
-	}
-
 	CaseClause struct {
 		Cases Expr // nil means default clause
 		Body  []Stmt
diff --git a/src/cmd/compile/internal/syntax/parser.go b/src/cmd/compile/internal/syntax/parser.go
index db0fb39c8c..68d09ef697 100644
--- a/src/cmd/compile/internal/syntax/parser.go
+++ b/src/cmd/compile/internal/syntax/parser.go
@@ -907,6 +907,7 @@ loop:
 				p.next()
 				if p.got(_Type) {
 					t := new(TypeSwitchGuard)
+					// t.Lhs is filled in by parser.simpleStmt
 					t.pos = pos
 					t.X = x
 					x = t
@@ -914,7 +915,7 @@ loop:
 					t := new(AssertExpr)
 					t.pos = pos
 					t.X = x
-					t.Type = p.expr()
+					t.Type = p.type_()
 					x = t
 				}
 				p.want(_Rparen)
@@ -1584,12 +1585,12 @@ func (p *parser) bad() *BadExpr {
 var ImplicitOne = &BasicLit{Value: "1"}
 
 // SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
-func (p *parser) simpleStmt(lhs Expr, rangeOk bool) SimpleStmt {
+func (p *parser) simpleStmt(lhs Expr, keyword token) SimpleStmt {
 	if trace {
 		defer p.trace("simpleStmt")()
 	}
 
-	if rangeOk && p.tok == _Range {
+	if keyword == _For && p.tok == _Range {
 		// _Range expr
 		if debug && lhs != nil {
 			panic("invalid call of simpleStmt")
@@ -1636,51 +1637,35 @@ func (p *parser) simpleStmt(lhs Expr, rangeOk bool) SimpleStmt {
 	}
 
 	// expr_list
-	pos := p.pos()
 	switch p.tok {
-	case _Assign:
-		p.next()
-
-		if rangeOk && p.tok == _Range {
-			// expr_list '=' _Range expr
-			return p.newRangeClause(lhs, false)
+	case _Assign, _Define:
+		pos := p.pos()
+		var op Operator
+		if p.tok == _Define {
+			op = Def
 		}
-
-		// expr_list '=' expr_list
-		return p.newAssignStmt(pos, 0, lhs, p.exprList())
-
-	case _Define:
 		p.next()
 
-		if rangeOk && p.tok == _Range {
-			// expr_list ':=' range expr
-			return p.newRangeClause(lhs, true)
+		if keyword == _For && p.tok == _Range {
+			// expr_list op= _Range expr
+			return p.newRangeClause(lhs, op == Def)
 		}
 
-		// expr_list ':=' expr_list
+		// expr_list op= expr_list
 		rhs := p.exprList()
 
-		if x, ok := rhs.(*TypeSwitchGuard); ok {
-			switch lhs := lhs.(type) {
-			case *Name:
+		if x, ok := rhs.(*TypeSwitchGuard); ok && keyword == _Switch && op == Def {
+			if lhs, ok := lhs.(*Name); ok {
+				// switch … lhs := rhs.(type)
 				x.Lhs = lhs
-			case *ListExpr:
-				p.errorAt(lhs.Pos(), fmt.Sprintf("cannot assign 1 value to %d variables", len(lhs.ElemList)))
-				// make the best of what we have
-				if lhs, ok := lhs.ElemList[0].(*Name); ok {
-					x.Lhs = lhs
-				}
-			default:
-				p.errorAt(lhs.Pos(), fmt.Sprintf("invalid variable name %s in type switch", String(lhs)))
+				s := new(ExprStmt)
+				s.pos = x.Pos()
+				s.X = x
+				return s
 			}
-			s := new(ExprStmt)
-			s.pos = x.Pos()
-			s.X = x
-			return s
 		}
 
-		as := p.newAssignStmt(pos, Def, lhs, rhs)
-		return as
+		return p.newAssignStmt(pos, op, lhs, rhs)
 
 	default:
 		p.syntaxError("expecting := or = or comma")
@@ -1820,7 +1805,7 @@ func (p *parser) header(keyword token) (init SimpleStmt, cond Expr, post SimpleS
 		if p.got(_Var) {
 			p.syntaxError(fmt.Sprintf("var declaration not allowed in %s initializer", keyword.String()))
 		}
-		init = p.simpleStmt(nil, keyword == _For)
+		init = p.simpleStmt(nil, keyword)
 		// If we have a range clause, we are done (can only happen for keyword == _For).
 		if _, ok := init.(*RangeClause); ok {
 			p.xnest = outer
@@ -1847,17 +1832,17 @@ func (p *parser) header(keyword token) (init SimpleStmt, cond Expr, post SimpleS
 					p.syntaxError("expecting for loop condition")
 					goto done
 				}
-				condStmt = p.simpleStmt(nil, false)
+				condStmt = p.simpleStmt(nil, 0 /* range not permitted */)
 			}
 			p.want(_Semi)
 			if p.tok != _Lbrace {
-				post = p.simpleStmt(nil, false)
+				post = p.simpleStmt(nil, 0 /* range not permitted */)
 				if a, _ := post.(*AssignStmt); a != nil && a.Op == Def {
 					p.syntaxErrorAt(a.Pos(), "cannot declare in post statement of for loop")
 				}
 			}
 		} else if p.tok != _Lbrace {
-			condStmt = p.simpleStmt(nil, false)
+			condStmt = p.simpleStmt(nil, keyword)
 		}
 	} else {
 		condStmt = init
@@ -2003,7 +1988,7 @@ func (p *parser) commClause() *CommClause {
 	switch p.tok {
 	case _Case:
 		p.next()
-		c.Comm = p.simpleStmt(nil, false)
+		c.Comm = p.simpleStmt(nil, 0)
 
 		// The syntax restricts the possible simple statements here to:
 		//
@@ -2049,7 +2034,7 @@ func (p *parser) stmtOrNil() Stmt {
 		if label, ok := lhs.(*Name); ok && p.tok == _Colon {
 			return p.labeledStmtOrNil(label)
 		}
-		return p.simpleStmt(lhs, false)
+		return p.simpleStmt(lhs, 0)
 	}
 
 	switch p.tok {
@@ -2068,13 +2053,13 @@ func (p *parser) stmtOrNil() Stmt {
 	case _Operator, _Star:
 		switch p.op {
 		case Add, Sub, Mul, And, Xor, Not:
-			return p.simpleStmt(nil, false) // unary operators
+			return p.simpleStmt(nil, 0) // unary operators
 		}
 
 	case _Literal, _Func, _Lparen, // operands
 		_Lbrack, _Struct, _Map, _Chan, _Interface, // composite types
 		_Arrow: // receive operator
-		return p.simpleStmt(nil, false)
+		return p.simpleStmt(nil, 0)
 
 	case _For:
 		return p.forStmt()
diff --git a/src/cmd/compile/internal/syntax/printer.go b/src/cmd/compile/internal/syntax/printer.go
index f4c2b6dde6..8ff3bfa794 100644
--- a/src/cmd/compile/internal/syntax/printer.go
+++ b/src/cmd/compile/internal/syntax/printer.go
@@ -393,13 +393,13 @@ func (p *printer) printRawNode(n Node) {
 		p.print(_Rbrack)
 
 	case *AssertExpr:
-		p.print(n.X, _Dot, _Lparen)
-		if n.Type != nil {
-			p.printNode(n.Type)
-		} else {
-			p.print(_Type)
+		p.print(n.X, _Dot, _Lparen, n.Type, _Rparen)
+
+	case *TypeSwitchGuard:
+		if n.Lhs != nil {
+			p.print(n.Lhs, blank, _Define, blank)
 		}
-		p.print(_Rparen)
+		p.print(n.X, _Dot, _Lparen, _Type, _Rparen)
 
 	case *CallExpr:
 		p.print(n.Fun, _Lparen)
@@ -557,12 +557,6 @@ func (p *printer) printRawNode(n Node) {
 		}
 		p.printSwitchBody(n.Body)
 
-	case *TypeSwitchGuard:
-		if n.Lhs != nil {
-			p.print(n.Lhs, blank, _Define, blank)
-		}
-		p.print(n.X, _Dot, _Lparen, _Type, _Rparen)
-
 	case *SelectStmt:
 		p.print(_Select, blank) // for now
 		p.printSelectBody(n.Body)