go/types: report an error when seeing pointer designation in contracts

Pointer designations were not recognized nor supported.
Now we recognize them and report an error until type-checking
can handle them.

Change-Id: I0deed536aa03c06c3e9ec60a5c2e186e916b970f

src/go/types/NOTES

@@ -4,6 +4,8 @@ so we have a better track record. I only switched to this file in Nov 2019, henc
 ----------------------------------------------------------------------------------------------------
 TODO
 
+- implement type-checking for type parameters with pointer designation in contracts
+- figure out how to translate methods with pointer designation into interface bounds
 - review use of Contract.TParams field - it seems like it's only needed for length checks?
 - review handling of fields of instantiated generic types (do we need to make them non-parameterized,
   similar to what we did for the embedded interfaces created by contract embedding?)

src/go/types/contracts.go

@@ -33,6 +33,9 @@ func (check *Checker) contractDecl(obj *Contract, cdecl *ast.ContractSpec) {
 
 	// collect constraints
 	for _, c := range cdecl.Constraints {
+		if c.Star.IsValid() {
+			check.errorf(c.Star, "pointer designation for type parameters not yet supported (* is ignored)")
+		}
 		if c.Param != nil {
 			// If a type name is present, it must be one of the contract's type parameters.
 			pos := c.Param.Pos()

src/go/types/testdata/tmp.go2

@@ -4,6 +4,15 @@
 
 package p
 
+// TODO(gri) this should work
+contract C(T) {
+    T m()
+}
+
+func _(type T C)(x *T) {
+	x.m /* ERROR no field or method m */ ()
+}
+
 /*
 func f(type T)(T)
 

src/go/types/testdata/todos.go2

@@ -8,6 +8,11 @@
 
 package p
 
+// Pointer designation for type parameters is not yet supported.
+contract _C(T) {
+	* /* ERROR not yet supported */ T m()
+}
+
 // indexing on generic types containing type parameters in their type list
 // is not yet supported
 func _(type T interface { type T })(x T) {