go/ssa: fix bug in writeSignature on external functions

Fixes a panic in writeSignature when fn.Params is non-empty while
the function has a receiver. fn.Params is nil for non-Go
source functions (synthetic or from object files).

Change-Id: Iae3f7ce53fca05d1b154349c3b091aee015afa0b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/497155
Run-TryBot: Tim King <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

go/ssa/func.go

@@ -516,15 +516,15 @@ func (f *Function) relMethod(from *types.Package, recv types.Type) string {
 }
 
 // writeSignature writes to buf the signature sig in declaration syntax.
-func writeSignature(buf *bytes.Buffer, from *types.Package, name string, sig *types.Signature, params []*Parameter) {
+func writeSignature(buf *bytes.Buffer, from *types.Package, name string, sig *types.Signature) {
 	buf.WriteString("func ")
 	if recv := sig.Recv(); recv != nil {
 		buf.WriteString("(")
-		if n := params[0].Name(); n != "" {
-			buf.WriteString(n)
+		if name := recv.Name(); name != "" {
+			buf.WriteString(name)
 			buf.WriteString(" ")
 		}
-		types.WriteType(buf, params[0].Type(), types.RelativeTo(from))
+		types.WriteType(buf, recv.Type(), types.RelativeTo(from))
 		buf.WriteString(") ")
 	}
 	buf.WriteString(name)
@@ -599,7 +599,7 @@ func WriteFunction(buf *bytes.Buffer, f *Function) {
 			fmt.Fprintf(buf, "# % 3d:\t%s %s\n", i, l.Name(), relType(mustDeref(l.Type()), from))
 		}
 	}
-	writeSignature(buf, from, f.Name(), f.Signature, f.Params)
+	writeSignature(buf, from, f.Name(), f.Signature)
 	buf.WriteString(":\n")
 
 	if f.Blocks == nil {

go/ssa/sanity.go

@@ -8,6 +8,7 @@ package ssa
 // Currently it checks CFG invariants but little at the instruction level.
 
 import (
+	"bytes"
 	"fmt"
 	"go/types"
 	"io"
@@ -412,8 +413,10 @@ func (s *sanity) checkFunction(fn *Function) bool {
 		s.errorf("nil Prog")
 	}
 
+	var buf bytes.Buffer
 	_ = fn.String()               // must not crash
 	_ = fn.RelString(fn.relPkg()) // must not crash
+	WriteFunction(&buf, fn)       // must not crash
 
 	// All functions have a package, except delegates (which are
 	// shared across packages, or duplicated as weak symbols in a

go/ssa/ssa.go

@@ -258,8 +258,8 @@ type Node interface {
 // or method.
 //
 // If Blocks is nil, this indicates an external function for which no
-// Go source code is available.  In this case, FreeVars and Locals
-// are nil too.  Clients performing whole-program analysis must
+// Go source code is available.  In this case, FreeVars, Locals, and
+// Params are nil too.  Clients performing whole-program analysis must
 // handle external functions specially.
 //
 // Blocks contains the function's control-flow graph (CFG).