gofmt: don't strip mandatory ()'s around composite literals in control clauses

Fixes #748.

R=rsc
CC=golang-dev
https://golang.org/cl/946043

src/pkg/go/printer/nodes.go

@@ -953,9 +953,14 @@ func (p *printer) block(s *ast.BlockStmt, indent int) {
 // TODO(gri): Decide if this should be used more broadly. The printing code
 //            knows when to insert parentheses for precedence reasons, but
 //            need to be careful to keep them around type expressions.
-func stripParens(x ast.Expr) ast.Expr {
-	if px, hasParens := x.(*ast.ParenExpr); hasParens {
-		return stripParens(px.X)
+func stripParens(x ast.Expr, inControlClause bool) ast.Expr {
+	for px, hasParens := x.(*ast.ParenExpr); hasParens; px, hasParens = x.(*ast.ParenExpr) {
+		x = px.X
+		if _, isCompositeLit := x.(*ast.CompositeLit); isCompositeLit && inControlClause {
+			// composite literals inside control clauses need parens;
+			// don't strip innermost layer
+			return px
+		}
 	}
 	return x
 }
@@ -967,7 +972,7 @@ func (p *printer) controlClause(isForStmt bool, init ast.Stmt, expr ast.Expr, po
 	if init == nil && post == nil {
 		// no semicolons required
 		if expr != nil {
-			p.expr(stripParens(expr), ignoreMultiLine)
+			p.expr(stripParens(expr, true), ignoreMultiLine)
 			needsBlank = true
 		}
 	} else {
@@ -978,7 +983,7 @@ func (p *printer) controlClause(isForStmt bool, init ast.Stmt, expr ast.Expr, po
 		}
 		p.print(token.SEMICOLON, blank)
 		if expr != nil {
-			p.expr(stripParens(expr), ignoreMultiLine)
+			p.expr(stripParens(expr, true), ignoreMultiLine)
 			needsBlank = true
 		}
 		if isForStmt {
@@ -1152,7 +1157,7 @@ func (p *printer) stmt(stmt ast.Stmt, multiLine *bool) {
 			p.expr(s.Value, multiLine)
 		}
 		p.print(blank, s.TokPos, s.Tok, blank, token.RANGE, blank)
-		p.expr(stripParens(s.X), multiLine)
+		p.expr(stripParens(s.X, true), multiLine)
 		p.print(blank)
 		p.block(s.Body, 1)
 		*multiLine = true

src/pkg/go/printer/testdata/statements.golden

@@ -144,12 +144,50 @@ func _() {
 	for x := range []int{} {
 		use(x)
 	}
-	for x := range []int{} {
+	for x := range ([]int{}) {
 		use(x)
 	}	// no parens printed
 }
 
 
+// Don't remove mandatory parentheses around composite literals in control clauses.
+func _() {
+	if x {
+	}	// no ()'s
+	if x {
+	}	// no ()'s
+	if ([]T{}) {
+	}	// ()
+	if ([]T{}) {
+	}	// ()
+	if ([]T{}) {
+	}	// ()
+
+	for x {
+	}	// no ()'s
+	for x {
+	}	// no ()'s
+	for ([]T{}) {
+	}	// ()
+	for ([]T{}) {
+	}	// ()
+	for ([]T{}) {
+	}	// ()
+
+	switch x {
+	}	// no ()'s
+	switch x {
+	}	// no ()'s
+	switch ([]T{}) {
+	}	// ()
+	switch ([]T{}) {
+	}	// ()
+
+	for _ = range ([]T{T{42}}) {
+	}	// ()
+}
+
+
 // Extra empty lines inside functions. Do respect source code line
 // breaks between statement boundaries but print at most one empty
 // line at a time.

src/pkg/go/printer/testdata/statements.input

@@ -111,6 +111,29 @@ func _() {
 }
 
 
+// Don't remove mandatory parentheses around composite literals in control clauses.
+func _() {
+	if (x) {}                // no ()'s
+	if (((x))) {}            // no ()'s
+	if ([]T{}) {}            // ()
+	if (([]T{})) {}          // ()
+	if ; (((([]T{})))) {}    // ()
+
+	for (x) {}                // no ()'s
+	for (((x))) {}            // no ()'s
+	for ([]T{}) {}            // ()
+	for (([]T{})) {}          // ()
+	for ; (((([]T{})))) ; {}  // ()
+
+	switch (x) {}                // no ()'s
+	switch (((x))) {}            // no ()'s
+	switch ([]T{}) {}            // ()
+	switch (([]T{})) {}          // ()
+
+	for _ = range ((([]T{T{42}}))) {}  // ()
+}
+
+
 // Extra empty lines inside functions. Do respect source code line
 // breaks between statement boundaries but print at most one empty
 // line at a time.