cmd/compile/internal/types2: better errors for invalid short var decls

- rewrite Checker.shortVarDecl core loop for clarity
- match compiler error messages (#43087)
- don't allow multiple identical redeclarations (#45652)

For #43087.
For #45652.

Change-Id: I8c3329a553aa104d7853fbaea8b88049bc9b3b88
Reviewed-on: https://go-review.googlesource.com/c/go/+/312170
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/assignments.go

@@ -330,40 +330,59 @@ func (check *Checker) shortVarDecl(pos syntax.Pos, lhs, rhs []syntax.Expr) {
 	scope := check.scope
 
 	// collect lhs variables
-	var newVars []*Var
-	var lhsVars = make([]*Var, len(lhs))
+	seen := make(map[string]bool, len(lhs))
+	lhsVars := make([]*Var, len(lhs))
+	newVars := make([]*Var, 0, len(lhs))
+	hasErr := false
 	for i, lhs := range lhs {
-		var obj *Var
-		if ident, _ := lhs.(*syntax.Name); ident != nil {
+		ident, _ := lhs.(*syntax.Name)
+		if ident == nil {
+			check.useLHS(lhs)
+			check.errorf(lhs, "non-name %s on left side of :=", lhs)
+			hasErr = true
+			continue
+		}
+
+		name := ident.Value
+		if name == "_" {
+			continue
+		}
+
+		if seen[name] {
+			check.errorf(lhs, "%s repeated on left side of :=", lhs)
+			hasErr = true
+			continue
+		}
+		seen[name] = true
+
 		// Use the correct obj if the ident is redeclared. The
 		// variable's scope starts after the declaration; so we
 		// must use Scope.Lookup here and call Scope.Insert
 		// (via check.declare) later.
-			name := ident.Value
 		if alt := scope.Lookup(name); alt != nil {
+			check.recordUse(ident, alt)
 			// redeclared object must be a variable
-				if alt, _ := alt.(*Var); alt != nil {
-					obj = alt
+			if obj, _ := alt.(*Var); obj != nil {
+				lhsVars[i] = obj
 			} else {
 				check.errorf(lhs, "cannot assign to %s", lhs)
+				hasErr = true
 			}
-				check.recordUse(ident, alt)
-			} else {
-				// declare new variable, possibly a blank (_) variable
-				obj = NewVar(ident.Pos(), check.pkg, name, nil)
-				if name != "_" {
+			continue
+		}
+
+		// declare new variable
+		obj := NewVar(ident.Pos(), check.pkg, name, nil)
+		lhsVars[i] = obj
 		newVars = append(newVars, obj)
-				}
 		check.recordDef(ident, obj)
 	}
-		} else {
-			check.useLHS(lhs)
-			check.errorf(lhs, "cannot declare %s", lhs)
-		}
+
+	// create dummy variables where the lhs is invalid
+	for i, obj := range lhsVars {
 		if obj == nil {
-			obj = NewVar(lhs.Pos(), check.pkg, "_", nil) // dummy variable
+			lhsVars[i] = NewVar(lhs[i].Pos(), check.pkg, "_", nil)
 		}
-		lhsVars[i] = obj
 	}
 
 	check.initVars(lhsVars, rhs, nopos)
@@ -371,17 +390,18 @@ func (check *Checker) shortVarDecl(pos syntax.Pos, lhs, rhs []syntax.Expr) {
 	// process function literals in rhs expressions before scope changes
 	check.processDelayed(top)
 
+	if len(newVars) == 0 && !hasErr {
+		check.softErrorf(pos, "no new variables on left side of :=")
+		return
+	}
+
 	// declare new variables
-	if len(newVars) > 0 {
 	// spec: "The scope of a constant or variable identifier declared inside
 	// a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl
 	// for short variable declarations) and ends at the end of the innermost
 	// containing block."
 	scopePos := syntax.EndPos(rhs[len(rhs)-1])
 	for _, obj := range newVars {
-			check.declare(scope, nil, obj, scopePos) // recordObject already called
-		}
-	} else {
-		check.softErrorf(pos, "no new variables on left side of :=")
+		check.declare(scope, nil, obj, scopePos) // id = nil: recordDef already called
 	}
 }

src/cmd/compile/internal/types2/fixedbugs/issue43087.src

@@ -0,0 +1,43 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _() {
+	a, b, b /* ERROR b repeated on left side of := */ := 1, 2, 3
+	_ = a
+	_ = b
+}
+
+func _() {
+	a, _, _ := 1, 2, 3 // multiple _'s ok
+	_ = a
+}
+
+func _() {
+	var b int
+	a, b, b /* ERROR b repeated on left side of := */ := 1, 2, 3
+	_ = a
+	_ = b
+}
+
+func _() {
+	var a []int
+	a /* ERROR non-name .* on left side of := */ [0], b := 1, 2
+	_ = a
+	_ = b
+}
+
+func _() {
+	var a int
+	a, a /* ERROR a repeated on left side of := */ := 1, 2
+	_ = a
+}
+
+func _() {
+	var a, b int
+	a, b := /* ERROR no new variables on left side of := */ 1, 2
+	_ = a
+	_ = b
+}

src/cmd/compile/internal/types2/testdata/stmt0.src

@@ -143,11 +143,11 @@ func issue6487() {
 }
 
 func issue6766a() {
-	a, a /* ERROR redeclared */ := 1, 2
+	a, a /* ERROR a repeated on left side of := */ := 1, 2
 	_ = a
-	a, b, b /* ERROR redeclared */ := 1, 2, 3
+	a, b, b /* ERROR b repeated on left side of := */ := 1, 2, 3
 	_ = b
-	c, c /* ERROR redeclared */, b := 1, 2, 3
+	c, c /* ERROR c repeated on left side of := */, b := 1, 2, 3
 	_ = c
 	a, b := /* ERROR no new variables */ 1, 2
 }