go/types, types2: make Checker.renameTParams work on any type

This permits the rewrite of type parameters in arbitrary types,
not just tuples.

Preparation for fixing #59956.
For #59338.

Change-Id: I9ccaac1f163051cb837cae2208763cafb1d239cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/492515
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Run-TryBot: Robert Griesemer <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

src/cmd/compile/internal/types2/call.go

@@ -96,10 +96,10 @@ func (check *Checker) funcInst(tsig *Signature, pos syntax.Pos, x *operand, inst
 		}
 
 		// Rename type parameters to avoid problems with recursive instantiations.
-		// Note that NewTuple(params...) below is nil if len(params) == 0, as desired.
+		// Note that NewTuple(params...) below is (*Tuple)(nil) if len(params) == 0, as desired.
 		tparams, params2 := check.renameTParams(pos, sig.TypeParams().list(), NewTuple(params...))
 
-		targs = check.infer(pos, tparams, targs, params2, args)
+		targs = check.infer(pos, tparams, targs, params2.(*Tuple), args)
 		if targs == nil {
 			// error was already reported
 			x.mode = invalid
@@ -489,7 +489,9 @@ func (check *Checker) arguments(call *syntax.CallExpr, sig *Signature, targs []T
 			}
 		}
 		// rename type parameters to avoid problems with recursive calls
-		tparams, sigParams = check.renameTParams(call.Pos(), sig.TypeParams().list(), sigParams)
+		var tmp Type
+		tparams, tmp = check.renameTParams(call.Pos(), sig.TypeParams().list(), sigParams)
+		sigParams = tmp.(*Tuple)
 	}
 
 	// collect type parameters from generic function arguments

src/cmd/compile/internal/types2/infer.go

@@ -380,11 +380,14 @@ func (check *Checker) infer(pos syntax.Pos, tparams []*TypeParam, targs []Type,
 	return
 }
 
-// renameTParams renames the type parameters in a function signature described by its
-// type and ordinary parameters (tparams and params) such that each type parameter is
-// given a new identity. renameTParams returns the new type and ordinary parameters.
+// renameTParams renames the type parameters in the given type such that each type
+// parameter is given a new identity. renameTParams returns the new type parameters
+// and updated type. If the result type is unchanged from the argument type, none
+// of the type parameters in tparams occurred in the type.
+// If typ is a generic function, type parameters held with typ are not changed and
+// must be updated separately if desired.
 // The positions is only used for debug traces.
-func (check *Checker) renameTParams(pos syntax.Pos, tparams []*TypeParam, params *Tuple) ([]*TypeParam, *Tuple) {
+func (check *Checker) renameTParams(pos syntax.Pos, tparams []*TypeParam, typ Type) ([]*TypeParam, Type) {
 	// For the purpose of type inference we must differentiate type parameters
 	// occurring in explicit type or value function arguments from the type
 	// parameters we are solving for via unification because they may be the
@@ -413,7 +416,7 @@ func (check *Checker) renameTParams(pos syntax.Pos, tparams []*TypeParam, params
 	// Type parameter renaming turns the first example into the second
 	// example by renaming the type parameter P into P2.
 	if len(tparams) == 0 {
-		return nil, params // nothing to do
+		return nil, typ // nothing to do
 	}
 
 	tparams2 := make([]*TypeParam, len(tparams))
@@ -428,7 +431,7 @@ func (check *Checker) renameTParams(pos syntax.Pos, tparams []*TypeParam, params
 		tparams2[i].bound = check.subst(pos, tparam.bound, renameMap, nil, check.context())
 	}
 
-	return tparams2, check.subst(pos, params, renameMap, nil, check.context()).(*Tuple)
+	return tparams2, check.subst(pos, typ, renameMap, nil, check.context())
 }
 
 // typeParamsString produces a string containing all the type parameter names

src/go/types/call.go

@@ -100,10 +100,10 @@ func (check *Checker) funcInst(tsig *Signature, pos token.Pos, x *operand, ix *t
 		}
 
 		// Rename type parameters to avoid problems with recursive instantiations.
-		// Note that NewTuple(params...) below is nil if len(params) == 0, as desired.
+		// Note that NewTuple(params...) below is (*Tuple)(nil) if len(params) == 0, as desired.
 		tparams, params2 := check.renameTParams(pos, sig.TypeParams().list(), NewTuple(params...))
 
-		targs = check.infer(atPos(pos), tparams, targs, params2, args)
+		targs = check.infer(atPos(pos), tparams, targs, params2.(*Tuple), args)
 		if targs == nil {
 			// error was already reported
 			x.mode = invalid
@@ -492,7 +492,9 @@ func (check *Checker) arguments(call *ast.CallExpr, sig *Signature, targs []Type
 			}
 		}
 		// rename type parameters to avoid problems with recursive calls
-		tparams, sigParams = check.renameTParams(call.Pos(), sig.TypeParams().list(), sigParams)
+		var tmp Type
+		tparams, tmp = check.renameTParams(call.Pos(), sig.TypeParams().list(), sigParams)
+		sigParams = tmp.(*Tuple)
 	}
 
 	// collect type parameters from generic function arguments