cmd/cgo: recognized untyped Go constants as untyped constants

Fixes #28772

Change-Id: I9446d95fb73fbcbb1cd9a4d2156ebc91bc9e91cb
Reviewed-on: https://go-review.googlesource.com/c/149858
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/test/issue28545.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // Failed to add type conversion for negative constant.
+// Issue 28772: Failed to add type conversion for Go constant set to C constant.
 // No runtime test; just make sure it compiles.
 
 package cgotest
@@ -10,11 +11,16 @@ package cgotest
 /*
 #include <complex.h>
 
+#define issue28772Constant 1
+
 static void issue28545F(char **p, int n, complex double a) {}
 */
 import "C"
 
+const issue28772Constant = C.issue28772Constant
+
 func issue28545G(p **C.char) {
 	C.issue28545F(p, -1, (0))
 	C.issue28545F(p, 2+3, complex(1, 1))
+	C.issue28545F(p, issue28772Constant, (0))
 }

src/cmd/cgo/ast.go

@@ -66,6 +66,7 @@ func (f *File) ParseGo(name string, src []byte) {
 	f.Package = ast1.Name.Name
 	f.Name = make(map[string]*Name)
 	f.NamePos = make(map[*Name]token.Pos)
+	f.Consts = make(map[string]bool)
 
 	// In ast1, find the import "C" line and get any extra C preamble.
 	sawC := false
@@ -191,6 +192,18 @@ func (f *File) saveExprs(x interface{}, context astContext) {
 		}
 	case *ast.CallExpr:
 		f.saveCall(x, context)
+	case *ast.GenDecl:
+		if x.Tok == token.CONST {
+			for _, spec := range x.Specs {
+				vs := spec.(*ast.ValueSpec)
+				if vs.Type == nil {
+					for _, name := range spec.(*ast.ValueSpec).Names {
+						f.Consts[name.Name] = true
+					}
+				}
+			}
+		}
+
 	}
 }
 

src/cmd/cgo/gcc.go

@@ -1232,7 +1232,8 @@ func (p *Package) isConst(f *File, x ast.Expr) bool {
 		return x.Name == "nil" ||
 			strings.HasPrefix(x.Name, "_Ciconst_") ||
 			strings.HasPrefix(x.Name, "_Cfconst_") ||
-			strings.HasPrefix(x.Name, "_Csconst_")
+			strings.HasPrefix(x.Name, "_Csconst_") ||
+			f.Consts[x.Name]
 	case *ast.UnaryExpr:
 		return p.isConst(f, x.X)
 	case *ast.BinaryExpr:

src/cmd/cgo/main.go

@@ -62,6 +62,7 @@ type File struct {
 	Name     map[string]*Name    // map from Go name to Name
 	NamePos  map[*Name]token.Pos // map from Name to position of the first reference
 	Edit     *edit.Buffer
+	Consts   map[string]bool // untyped constants
 }
 
 func (f *File) offset(p token.Pos) int {