go/parser,go/types: hide API changes related to type parameters

While the dev.typeparams branch was merged, the type parameter API is
slated for go1.18. Hide these changes to the go/parser and go/types API.

This was done as follows:
 + For APIs that will probably not be needed for go1.18, simply unexport
   them.
 + For APIs that we expect to export in go1.18, prefix symbols with '_',
   so that the planned (upper-cased) symbol name is apparent.
 + For APIs that must be exported for testing, move both API and tests
   to files guarded by the go1.18 build constraint.
 + parser.ParseTypeParams is unexported and copied wherever it is
   needed.
 + The -G flag is removed from gofmt, replaced by enabling type
   parameters if built with the go1.18 build constraint.

Notably, changes related to type parameters in go/ast are currently left
exported. We're looking at the AST API separately.

The new API diff from 1.16 is:
+pkg go/ast, method (*FuncDecl) IsMethod() bool
+pkg go/ast, method (*ListExpr) End() token.Pos
+pkg go/ast, method (*ListExpr) Pos() token.Pos
+pkg go/ast, type FuncType struct, TParams *FieldList
+pkg go/ast, type ListExpr struct
+pkg go/ast, type ListExpr struct, ElemList []Expr
+pkg go/ast, type TypeSpec struct, TParams *FieldList
+pkg go/types, type Config struct, GoVersion string

Change-Id: I1baf67e26279b49092e774309a836c460979774a
Reviewed-on: https://go-review.googlesource.com/c/go/+/295929
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 7 insertions, 1 deletions

diff --git a/src/go/parser/interface.go b/src/go/parser/interface.go
index b8b312463f..123faaee77 100644
--- a/src/go/parser/interface.go
+++ b/src/go/parser/interface.go
@@ -55,8 +55,14 @@ const (
 	Trace                                          // print a trace of parsed productions
 	DeclarationErrors                              // report declaration errors
 	SpuriousErrors                                 // same as AllErrors, for backward-compatibility
-	ParseTypeParams                                // Placeholder. Will control the parsing of type parameters.
 	AllErrors         = SpuriousErrors             // report all errors (not just the first 10 on different lines)
+
+	// parseTypeParams controls the parsing of type parameters. Must be
+	// kept in sync with:
+	//  go/printer/printer_test.go
+	//  go/types/check_test.go
+	//  cmd/gofmt/gofmt.go
+	parseTypeParams = 1 << 30
 )
 
 // ParseFile parses the source code of a single Go source file and returns