func (st *state) demangleType(isCast bool) AST {
	if len(st.str) == 0 {
		st.fail("expected type")
	}

	addSubst := true

	q := st.cvQualifiers()
	if len(q) > 0 {
		if len(st.str) == 0 {
			st.fail("expected type")
		}

		// CV-qualifiers before a function type apply to
		// 'this', so avoid adding the unqualified function
		// type to the substitution list.
		if st.str[0] == 'F' {
			addSubst = false
		}
	}

	var ret AST

	// Use correct substitution for a template parameter.
	var sub AST

	if btype, ok := builtinTypes[st.str[0]]; ok {
		ret = &BuiltinType{Name: btype}
		st.advance(1)
		if len(q) > 0 {
			ret = &TypeWithQualifiers{Base: ret, Qualifiers: q}
			st.subs.add(ret)
		}
		return ret
	}
	c := st.str[0]
	switch c {
	case 'u':
		st.advance(1)
		ret = st.sourceName()
	case 'F':
		ret = st.functionType()
	case 'N', 'Z', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
		ret = st.name()
	case 'A':
		ret = st.arrayType(isCast)
	case 'M':
		ret = st.pointerToMemberType(isCast)
	case 'T':
		ret = st.templateParam()
		if len(st.str) > 0 && st.str[0] == 'I' {
			// See the function comment to explain this.
			if !isCast {
				st.subs.add(ret)
				args := st.templateArgs()
				ret = &Template{Name: ret, Args: args}
			} else {
				ret = st.demangleCastTemplateArgs(ret, true)
			}
		}
	case 'S':
		// If this is a special substitution, then it
		// is the start of <class-enum-type>.
		var c2 byte
		if len(st.str) > 1 {
			c2 = st.str[1]
		}
		if isDigit(c2) || c2 == '_' || isUpper(c2) {
			ret = st.substitution(false)
			if len(st.str) == 0 || st.str[0] != 'I' {
				addSubst = false
			} else {
				// See the function comment to explain this.
				if _, ok := ret.(*TemplateParam); !ok || !isCast {
					args := st.templateArgs()
					ret = &Template{Name: ret, Args: args}
				} else {
					next := st.demangleCastTemplateArgs(ret, false)
					if next == ret {
						addSubst = false
					}
					ret = next
				}
			}
		} else {
			ret = st.name()
			// This substitution is not itself a
			// substitution candidate, unless template
			// arguments were added.
			if ret == subAST[c2] || ret == verboseAST[c2] {
				addSubst = false
			}
		}
	case 'O', 'P', 'R', 'C', 'G':
		st.advance(1)
		t := st.demangleType(isCast)
		switch c {
		case 'O':
			ret = &RvalueReferenceType{Base: t}
		case 'P':
			ret = &PointerType{Base: t}
		case 'R':
			ret = &ReferenceType{Base: t}
		case 'C':
			ret = &ComplexType{Base: t}
		case 'G':
			ret = &ImaginaryType{Base: t}
		}
	case 'U':
		if len(st.str) < 2 {
			st.fail("expected source name or unnamed type")
		}
		switch st.str[1] {
		case 'l':
			ret = st.closureTypeName()
			addSubst = false
		case 't':
			ret = st.unnamedTypeName()
			addSubst = false
		default:
			st.advance(1)
			n := st.sourceName()
			if len(st.str) > 0 && st.str[0] == 'I' {
				args := st.templateArgs()
				n = &Template{Name: n, Args: args}
			}
			t := st.demangleType(isCast)
			ret = &VendorQualifier{Qualifier: n, Type: t}
		}
	case 'D':
		st.advance(1)
		if len(st.str) == 0 {
			st.fail("expected D code for type")
		}
		addSubst = false
		c2 := st.str[0]
		st.advance(1)
		switch c2 {
		case 'T', 't':
			// decltype(expression)
			ret = st.expression()
			if len(st.str) == 0 || st.str[0] != 'E' {
				st.fail("expected E after expression in type")
			}
			st.advance(1)
			ret = &Decltype{Expr: ret}
			addSubst = true

		case 'p':
			t := st.demangleType(isCast)
			pack := st.findArgumentPack(t)
			ret = &PackExpansion{Base: t, Pack: pack}
			addSubst = true

		case 'a':
			ret = &Name{Name: "auto"}

		case 'f':
			ret = &BuiltinType{Name: "decimal32"}
		case 'd':
			ret = &BuiltinType{Name: "decimal64"}
		case 'e':
			ret = &BuiltinType{Name: "decimal128"}
		case 'h':
			ret = &BuiltinType{Name: "half"}
		case 's':
			ret = &BuiltinType{Name: "char16_t"}
		case 'i':
			ret = &BuiltinType{Name: "char32_t"}
		case 'n':
			ret = &BuiltinType{Name: "decltype(nullptr)"}

		case 'F':
			accum := false
			if len(st.str) > 0 && isDigit(st.str[0]) {
				accum = true
				// We don't care about the bits.
				_ = st.number()
			}
			base := st.demangleType(isCast)
			if len(st.str) > 0 && isDigit(st.str[0]) {
				// We don't care about the bits.
				st.number()
			}
			sat := false
			if len(st.str) > 0 {
				if st.str[0] == 's' {
					sat = true
				}
				st.advance(1)
			}
			ret = &FixedType{Base: base, Accum: accum, Sat: sat}

		case 'v':
			ret = st.vectorType(isCast)
			addSubst = true

		default:
			st.fail("unrecognized D code in type")
		}

	default:
		st.fail("unrecognized type code")
	}

	if addSubst {
		if sub != nil {
			st.subs.add(sub)
		} else {
			st.subs.add(ret)
		}
	}

	if len(q) > 0 {
		if _, ok := ret.(*FunctionType); ok {
			ret = &MethodWithQualifiers{Method: ret, Qualifiers: q, RefQualifier: ""}
		} else if mwq, ok := ret.(*MethodWithQualifiers); ok {
			// Merge adjacent qualifiers.  This case
			// happens with a function with a trailing
			// ref-qualifier.
			mwq.Qualifiers = mergeQualifiers(q, mwq.Qualifiers)
		} else {
			// Merge adjacent qualifiers.  This case
			// happens with multi-dimensional array types.
			if qsub, ok := ret.(*TypeWithQualifiers); ok {
				q = mergeQualifiers(q, qsub.Qualifiers)
				ret = qsub.Base
			}
			ret = &TypeWithQualifiers{Base: ret, Qualifiers: q}
		}
		st.subs.add(ret)
	}

	return ret
}
func (st *state) demangleType(isCast bool) AST {
	if len(st.str) == 0 {
		st.fail("expected type")
	}

	addSubst := true

	q := st.cvQualifiers()
	if len(q) > 0 {
		if len(st.str) == 0 {
			st.fail("expected type")
		}

		// CV-qualifiers before a function type apply to
		// 'this', so avoid adding the unqualified function
		// type to the substitution list.
		if st.str[0] == 'F' {
			addSubst = false
		}
	}

	var ret AST

	// Use correct substitution for a template parameter.
	var sub AST

	if btype, ok := builtinTypes[st.str[0]]; ok {
		ret = &BuiltinType{Name: btype}
		st.advance(1)
		if len(q) > 0 {
			ret = &TypeWithQualifiers{Base: ret, Qualifiers: q}
			st.subs.add(ret)
		}
		return ret
	}
	c := st.str[0]
	switch c {
	case 'u':
		st.advance(1)
		ret = st.sourceName()
	case 'F':
		ret = st.functionType()
	case 'N', 'Z', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
		ret = st.name()
	case 'A':
		ret = st.arrayType(isCast)
	case 'M':
		ret = st.pointerToMemberType(isCast)
	case 'T':
		ret = st.templateParam()
		if len(st.str) > 0 && st.str[0] == 'I' {
			// See the function comment to explain this.
			if !isCast {
				st.subs.add(ret)
				args := st.templateArgs()
				ret = &Template{Name: ret, Args: args}
			} else {
				ret = st.demangleCastTemplateArgs(ret, true)
			}
		}
	case 'S':
		// If this is a special substitution, then it
		// is the start of <class-enum-type>.
		var c2 byte
		if len(st.str) > 1 {
			c2 = st.str[1]
		}
		if isDigit(c2) || c2 == '_' || isUpper(c2) {
			ret = st.substitution(false)
			if len(st.str) == 0 || st.str[0] != 'I' {
				addSubst = false
			} else {
				// See the function comment to explain this.
				if _, ok := ret.(*TemplateParam); !ok || !isCast {
					args := st.templateArgs()
					ret = &Template{Name: ret, Args: args}
				} else {
					next := st.demangleCastTemplateArgs(ret, false)
					if next == ret {
						addSubst = false
					}
					ret = next
				}
			}
		} else {
			ret = st.name()
			// This substitution is not itself a
			// substitution candidate, unless template
			// arguments were added.
			if ret == subAST[c2] || ret == verboseAST[c2] {
				addSubst = false
			}
		}
	case 'O', 'P', 'R', 'C', 'G':
		st.advance(1)
		t := st.demangleType(isCast)
		switch c {
		case 'O':
			ret = &RvalueReferenceType{Base: t}
		case 'P':
			ret = &PointerType{Base: t}
		case 'R':
			ret = &ReferenceType{Base: t}
		case 'C':
			ret = &ComplexType{Base: t}
		case 'G':
			ret = &ImaginaryType{Base: t}
		}
	case 'U':
		if len(st.str) < 2 {
			st.fail("expected source name or unnamed type")
		}
		switch st.str[1] {
		case 'l':
			ret = st.closureTypeName()
			addSubst = false
		case 't':
			ret = st.unnamedTypeName()
			addSubst = false
		default:
			st.advance(1)
			n := st.sourceName()
			if len(st.str) > 0 && st.str[0] == 'I' {
				args := st.templateArgs()
				n = &Template{Name: n, Args: args}
			}
			t := st.demangleType(isCast)
			ret = &VendorQualifier{Qualifier: n, Type: t}
		}
	case 'D':
		st.advance(1)
		if len(st.str) == 0 {
			st.fail("expected D code for type")
		}
		addSubst = false
		c2 := st.str[0]
		st.advance(1)
		switch c2 {
		case 'T', 't':
			// decltype(expression)
			ret = st.expression()
			if len(st.str) == 0 || st.str[0] != 'E' {
				st.fail("expected E after expression in type")
			}
			st.advance(1)
			ret = &Decltype{Expr: ret}
			addSubst = true

		case 'p':
			t := st.demangleType(isCast)
			pack := st.findArgumentPack(t)
			ret = &PackExpansion{Base: t, Pack: pack}
			addSubst = true

		case 'a':
			ret = &Name{Name: "auto"}

		case 'f':
			ret = &BuiltinType{Name: "decimal32"}
		case 'd':
			ret = &BuiltinType{Name: "decimal64"}
		case 'e':
			ret = &BuiltinType{Name: "decimal128"}
		case 'h':
			ret = &BuiltinType{Name: "half"}
		case 's':
			ret = &BuiltinType{Name: "char16_t"}
		case 'i':
			ret = &BuiltinType{Name: "char32_t"}
		case 'n':
			ret = &BuiltinType{Name: "decltype(nullptr)"}

		case 'F':
			accum := false
			if len(st.str) > 0 && isDigit(st.str[0]) {
				accum = true
				// We don't care about the bits.
				_ = st.number()
			}
			base := st.demangleType(isCast)
			if len(st.str) > 0 && isDigit(st.str[0]) {
				// We don't care about the bits.
				st.number()
			}
			sat := false
			if len(st.str) > 0 {
				if st.str[0] == 's' {
					sat = true
				}
				st.advance(1)
			}
			ret = &FixedType{Base: base, Accum: accum, Sat: sat}

		case 'v':
			ret = st.vectorType(isCast)
			addSubst = true

		default:
			st.fail("unrecognized D code in type")
		}

	default:
		st.fail("unrecognized type code")
	}

	if addSubst {
		if sub != nil {
			st.subs.add(sub)
		} else {
			st.subs.add(ret)
		}
	}

	if len(q) > 0 {
		if _, ok := ret.(*FunctionType); ok {
			ret = &MethodWithQualifiers{Method: ret, Qualifiers: q, RefQualifier: ""}
		} else if mwq, ok := ret.(*MethodWithQualifiers); ok {
			// Merge adjacent qualifiers.  This case
			// happens with a function with a trailing
			// ref-qualifier.
			mwq.Qualifiers = mergeQualifiers(q, mwq.Qualifiers)
		} else {
			// Merge adjacent qualifiers.  This case
			// happens with multi-dimensional array types.
			if qsub, ok := ret.(*TypeWithQualifiers); ok {
				q = mergeQualifiers(q, qsub.Qualifiers)
				ret = qsub.Base
			}
			ret = &TypeWithQualifiers{Base: ret, Qualifiers: q}
		}
		st.subs.add(ret)
	}

	return ret
}
// <type> ::= <builtin-type>
//        ::= <function-type>
//        ::= <class-enum-type>
//        ::= <array-type>
//        ::= <pointer-to-member-type>
//        ::= <template-param>
//        ::= <template-template-param> <template-args>
//        ::= <substitution>
//        ::= <CV-qualifiers> <type>
//        ::= P <type>
//        ::= R <type>
//        ::= O <type> (C++0x)
//        ::= C <type>
//        ::= G <type>
//        ::= U <source-name> <type>
//
// <builtin-type> ::= various one letter codes
//                ::= u <source-name>