go/printer: don't print unnecesary commas for func type param lists

Type parameter lists are not ambiguous for function declarations in the
way that they are ambiguous for type declarations. Avoid printing an
extra comma to disambiguate.

Fixes #51548

Change-Id: I8ca2b21e271982013653b9e220f92ee74f577ba2
Reviewed-on: https://go-review.googlesource.com/c/go/+/390914
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

diff --git a/src/go/printer/nodes.go b/src/go/printer/nodes.go
index f2170dbc4f31c1ecd57660dd93867729001ab1c8..9a09d58eb2c8d6ce2a1e7bc0c2b38015f9ea37fa 100644 (file)
--- a/src/go/printer/nodes.go
+++ b/src/go/printer/nodes.go
@@ -319,9 +319,17 @@ func (p *printer) exprList(prev0 token.Pos, list []ast.Expr, depth int, mode exp
        }
 }
 
-func (p *printer) parameters(fields *ast.FieldList, isTypeParam bool) {
+type paramMode int
+
+const (
+       funcParam paramMode = iota
+       funcTParam
+       typeTParam
+)
+
+func (p *printer) parameters(fields *ast.FieldList, mode paramMode) {
        openTok, closeTok := token.LPAREN, token.RPAREN
-       if isTypeParam {
+       if mode != funcParam {
                openTok, closeTok = token.LBRACK, token.RBRACK
        }
        p.print(fields.Opening, openTok)
@@ -373,7 +381,7 @@ func (p *printer) parameters(fields *ast.FieldList, isTypeParam bool) {
                if closing := p.lineFor(fields.Closing); 0 < prevLine && prevLine < closing {
                        p.print(token.COMMA)
                        p.linebreak(closing, 0, ignore, true)
-               } else if isTypeParam && fields.NumFields() == 1 {
+               } else if mode == typeTParam && fields.NumFields() == 1 {
                        // Otherwise, if we are in a type parameter list that could be confused
                        // with the constant array length expression [P*C], print a comma so that
                        // parsing is unambiguous.
@@ -411,10 +419,10 @@ func isTypeLit(x ast.Expr) bool {
 
 func (p *printer) signature(sig *ast.FuncType) {
        if sig.TypeParams != nil {
-               p.parameters(sig.TypeParams, true)
+               p.parameters(sig.TypeParams, funcTParam)
        }
        if sig.Params != nil {
-               p.parameters(sig.Params, false)
+               p.parameters(sig.Params, funcParam)
        } else {
                p.print(token.LPAREN, token.RPAREN)
        }
@@ -428,7 +436,7 @@ func (p *printer) signature(sig *ast.FuncType) {
                        p.expr(stripParensAlways(res.List[0].Type))
                        return
                }
-               p.parameters(res, false)
+               p.parameters(res, funcParam)
        }
 }
 
@@ -1639,7 +1647,7 @@ func (p *printer) spec(spec ast.Spec, n int, doIndent bool) {
                p.setComment(s.Doc)
                p.expr(s.Name)
                if s.TypeParams != nil {
-                       p.parameters(s.TypeParams, true)
+                       p.parameters(s.TypeParams, typeTParam)
                }
                if n == 1 {
                        p.print(blank)
@@ -1829,7 +1837,7 @@ func (p *printer) funcDecl(d *ast.FuncDecl) {
        // FUNC is emitted).
        startCol := p.out.Column - len("func ")
        if d.Recv != nil {
-               p.parameters(d.Recv, false) // method: print receiver
+               p.parameters(d.Recv, funcParam) // method: print receiver
                p.print(blank)
        }
        p.expr(d.Name)

diff --git a/src/go/printer/testdata/generics.golden b/src/go/printer/testdata/generics.golden
index 4fac2c9c580b159d0a61c8e01859962e9c5d2a7e..c3a7df83725205afc4cec95cd73f57d477fbf981 100644 (file)
--- a/src/go/printer/testdata/generics.golden
+++ b/src/go/printer/testdata/generics.golden
@@ -64,3 +64,29 @@ type _ [P*T - T]struct{}
 type _[
        P *T,
 ] struct{}
+
+// equivalent test cases for potentially ambiguous type parameter lists, except
+// for function declarations there is no ambiguity (issue #51548)
+func _[P *T]()         {}
+func _[P *T, _ any]()  {}
+func _[P *T]()         {}
+func _[P *T, _ any]()  {}
+func _[P T]()          {}
+func _[P T, _ any]()   {}
+
+func _[P *struct{}]()  {}
+func _[P *struct{}]()  {}
+func _[P []int]()      {}
+
+func _[P T]()  {}
+func _[P T]()  {}
+func _[P **T]()        {}
+func _[P *T]() {}
+func _[P *T]() {}
+func _[P **T]()        {}
+func _[P *T]() {}
+
+func _[
+       P *T,
+]() {
+}

diff --git a/src/go/printer/testdata/generics.input b/src/go/printer/testdata/generics.input
index fde9d32ef04844f1b46bfb229776932f29e21f7d..66e1554f7ffb8359838be524849721a9b18c2e09 100644 (file)
--- a/src/go/printer/testdata/generics.input
+++ b/src/go/printer/testdata/generics.input
@@ -61,3 +61,28 @@ type _ [P * T - T]struct{}
 type _[
        P *T,
 ] struct{}
+
+// equivalent test cases for potentially ambiguous type parameter lists, except
+// for function declarations there is no ambiguity (issue #51548)
+func _[P *T,]() {}
+func _[P *T, _ any]() {}
+func _[P (*T),]() {}
+func _[P (*T), _ any]() {}
+func _[P (T),]() {}
+func _[P (T), _ any]() {}
+
+func _[P *struct{}] () {}
+func _[P (*struct{})] () {}
+func _[P ([]int)] () {}
+
+func _ [P(T)]() {}
+func _ [P((T))]() {}
+func _ [P * *T]() {}
+func _ [P * T]() {}
+func _ [P(*T)]() {}
+func _ [P(**T)]() {}
+func _ [P * T]() {}
+
+func _[
+       P *T,
+]() {}